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MODIFICATION OF POLYLACTIDE BIOPLASTIC USING HYPERBRANCHED
POLYMER BASED NANOSTRUCTURES

By

Rahul Bhardwaj

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
School of Packaging

2008

ABSTRACT

MODIFICATION OF POLYLACTIDE BIOPLASTIC USING HYPERBRANCHED
POLYMER BASED NANOSTRUCTURES

By
Rahul Bhardwaj

Polylactide (PLA) is the most well known renewable resource based
biodegradable polymer. The inherent brittleness and poor processability of PLA pose
considerable technical challenges and limit its range of commercial applications. The
broad objective of this research was to investigate novel pathways for polylactide
modification to enhance its mechanical and rheological properties. The focus of this work
was to tailor the architecture of a dendritic hyperbranched polymer (HBP) and study its
influence on the mechanical and rheological properties of PLA bioplastic. The
hyperbranched polymers under consideration are biodegradable aliphatic hydroxyl—
functional hyperbranched polyesters having nanoscale dimensions, unique physical
properties and high peripheral functionalities.

This work relates to identifying a new and industrially relevant research
methodology to develop PLA based nanoblends having outstanding stiffness-toughness
balance. In this approach, a hydroxyl functional hyperbranched polymer was crosslinked
in-situ with a polyanhydride (PA) in the PLA matrix during melt processing, leading to
the generation of new nanoscale hyperbranched polymer based domains in the PLA
matrix. Transmission electron microscopy and atomic force microscopy revealed the
“sea-island” morphology of PLA-crosslinked HBP blends. The domain size of a large
portion of the crosslinked HBP particles in PLA matrix was less than 100 nm. The

presence of crosslinked hyperbranched polymers exhibited more than 500% and 800%

improvement in the tensile toughness and elongation at break values of PLA, respectively,
with a minimal sacrifice of tensile strength and modulus as compared to unmodified PLA.
The toughening mechanism of PLA in the presence of crosslinked HBP particles was
comprised of shear yielding and crazing. The volume fraction of crosslinked HBP
particles and matrix ligament thickness (inter-particle distance) were found to be the
critical parameters for the toughening of PLA. The maximum average matrix ligament
thickness was 114 nm for a toughened polylactide nanoblend and correlated well with the
theoretical prediction of the matrix ligament thickness.

Fourier transform infrared spectroscopy and dynamic mechanical thermal analysis
proved the crosslinking of the HBP phase in the PLA matrix. The crosslinked HBP was
effective at hydroxyl (-OH) to anhydride molar ratios of: 2:1, 1:1 or 1:2. The glass
transition temperature values of the crosslinked HBP phase at these molar ratios were
observed to deviate from the predictions made by the Fox equation. The hydrophilic
nature of the hyperbranched polymer was altered to hydrophobic by incorporation of
polyanhydride crosslinker, as demonstrated by the increase in the contact angle with
water. Rheological studies showed that there was a network formation in the PLA matrix
after in-situ crosslinking of HBP. The HBP was found to reduce the melt viscosity of
PLA dramatically and this effect was maintained even after its in-situ crosslinking in the
PLA matrix.

Finally, the current research unwraps the new opportunities provided by the
unique physical and chemical properties of highly functional hyperbranched polymers in

generating new nanostructured multiphase polymer systems with enhanced properties.

To my Parents, and Brother Rajiv

iv

ACKNOWLEDGEMENTS

It is giving me an immense pleasure to express my gratitude to all, who have
contributed and supported me in the completion of this dissertation. First of all, I like to
express my sincere thanks to my advisor: Dr. Amar K. Mohanty for enabling me to
achieve this feat. Your constant motivation, support and confidence in my abilities have
propelled me to finish this piece of research work. Your astute comments and inputs have
been a remarkable contribution to this work. I am thankful to my committee members:
Dr. Lawrence Drzal, Dr. Susan E. Selke and Dr. Bruce Harte for their thorough reviews,
suggestions, and constructive criticism that helped immensely in improving the scope of
this dissertation. I am thankful to US National Science Foundation for providing me the
financial support for my research under NSF award DMI-0400296. I am also thankful to
college of agriculture and natural resources (CANR) and the graduate school at MSU for
awarding me the dissertation completion fellowship.

I am thankful to Dr. Manj usri Misra for her constant support and inputs during my
doctoral work. I am extremely thankful to my friends and colleagues from Dr. Mohanty’s
research group: Sanjeev Singh, Yash Parulekar, Edward Drown, Dinesh Aithani,
Mariappan Chidambarakumar, Dr. Hiroaki Miyagawa, Dr. Qiangxian Wu, Abhishek
Singh, Ajay Kathuria, and Dhiraj Hasija for their help in experiments and engaging in
several discussions on my research. I am also thankful to Alicia Pastor-Lecha, Ewa
Danielewicz, Carol Flegler, Per Askeland, Mike Rich, Brian Rook, Hazel-Ann Hosein
and Kelby Thayer for their assistance in several processing and characterization
techniques. I want to express my special thanks to my friends: Mahesh, Raj, Rajesh,

Tejveer, and Anand for their constant motivation and support.

Finally, thanks or gratitude is just not suitable word to express my appreciation to
my parents. Their unparallel support and motivation have enabled me to achieve beyond
my capabilities. I am especially thankful to the pillar of my family, my brother: Rajiv for
standing by me during my journey to this dissertation and many more things. I am
thankful to my sister, brother-in-law, and sister-in-law for providing me the
encouragement. I am also thankful to my fiance Gaytri for being a moral support to me.
Last but not the least, I am grateful to my loving niece Saachi for being a stress buster

during the ups and down of my research.

Rahul Bhardwaj
East Lansing, MI

December 18, 2007

vi

TABLE OF CONTENTS

LIST OF TABLES ................................................................................... x
LIST OF FIGURES ................................................................................ xii
KEY TO SYMBOLS OR ABBREVATIONS ............................................... xvi
Chapter 1: Introduction ........................................................................... 1
1.0 Introduction ....................................................................................... 1
1.1 Thesis Structure .................................................................................. 5
1.2 References ........................................................................................ 7
Chapter 2: Literature Review ................................................................... 10
2.0 Preparation and Structure of Polylactides ................................................... 10
2.1 Properties and Deformation of Polylactides ................................................ 15
2.2 Rheology of Polylactides ...................................................................... 21
2.3 Current Methods to Modify Polylactide Properties ........................................ 23
2.3.1 Copolymerization ........................................................................... 23
2.3.2 Plasticization ................................................................................ 24
2.3.3 Blending ..................................................................................... 27
2.3.4 Polylactide based Nanocomposites ...................................................... 34
2.4 Hyperbranched Polymers (HBP) ............................................................. 42
2.5 Toughening of Polymers ....................................................................... 46
2.5.1 Rubber Toughening ........................................................................ 49
2.5.2 Toughening Mechanism .................................................................. 50
2.5.3 Brittle to Ductile Transition (BDT) Criteria ............................................ 52
2.6 Alternate Ways of Toughening of Polymers ................................................ 54
2.6.1 Interpenetrating Networks ................................................................ 54
2.6.2 Nanostructure Blending ................................................................... 55
2.7 References ....................................................................................... 58
Chapter 3: Modification of Brittle Polylactide (PLA) by Novel Hyperbranched
Polymer based Nanostructures ......................................................... 73
3.0 Introduction .......... . ............................................................................ 73
3.1 Experimental Section ........................................................................... 75
3.1.1 Materials .................................................................................... 75
3.1.2 Reactive Extrusion ......................................................................... 76
3.1.3 Testing and Characterization ............................................................... 79
3.1.3.1 Ultracentrifugation ..................................................................... 79
3.1.3.2 Fourier Transform Infrared Microscopy (FTIR) ................................... 80
3.1.3.3 Dynamic Mechanical Thermal Analysis (DMTA) ................................ 81
3.1.3.4 Parallel Plate Rheology ............................................................... 81
3.1.3.5 Transmission Electron Microscopy (TEM) ........................................ 81

vii

3.1.3.6 Atomic Force Microscopy (AFM) ..................................................... 81

3.1.3.7 Tensile Properties ...................................................................... 82

3.1.3.8 Scanning Electron Microscopy (SEM) ............................................. 82

3.2 Results and Discussion ........................................................................ 82
3.2.1 Solvent Extraction, Morphology and Fourier Transform Infrared

Spectroscopy ................................................................................. 82

3.2.2 Dynamic Mechanical Thermal Analysis ................................................. 86
3.2.3 Rheological Evidence of Networked Interface between PLA and

Crosslinked HBP ............................................................................ 88

3.2.4 Complex Viscosity ......................................................................... 89

3.2.5 Transmission Electron Microscopy (TEM) ............................................. 91

3.2.6 Atomic Force Microscopy (AFM) ....................................................... 94

3.2.7 Tensile Properties ........................................................................... 96

3.2.8 Mode of Deformation of PLA-crosslinked HBP Blend under

Tensile Loading ............................................................................ 98

3.2.9 Scanning Electron Microscopy (SEM) ................................................ 104

3.3 References ....................................................................................... 106

Chapter 4: Effect of the Concentration of Crosslinker on the Properties of PLA/in-

situ Crosslinked HBP Blends .......................................................... 109
4.0 Introduction .................................................................................... 109
4.1 Experimental Section ......................................................................... 110
4.1.1 Materials ................................................................................... 110
4.1.2 Preparation of PLA-cHBP Blends ...................................................... 111
4.1.3 Testing and Characterization ............................................................ 112
4.1.3.1 Dynamic Mechanical Analysis ..................................................... 112
4.1.3.2 Ultracentrifugation and Fourier Transform Infrared Spectroscopy. ............ 112
4.1.3.3 Tensile Testing .......................................................................... 113
4.1.3.4 Transmission Electron Microscopy (TEM)....113
4.1.3.5 Contact Angle Measurements ...................................................... 113
4.1.3.6 Gel Permeation Chromatography (GPC) .......................................... 114
4.2 Results and Discussion ....................................................................... 114
4.2.1 Dynamic Mechanical Thermal Analysis .............................................. 114
4.2.2 Ultra-Centrifugation and Fourier Transform Infrared Spectroscopy
of Crosslinked HBP ....................................................................... 1 18
4.2.3 Tensile Properties ......................................................................... 121
4.2.4 Transmission Electron Microscopy (TEM) ........................................... 124
4.2.5 Contact Angle Measurements ........................................................... 127
4.2.6 Effect of Processing, Presence of HBP and Crosslinked HBP on
the Molecular Weight of PLA ........................................................... 129
4.3 References ...................................................................................... 131

Chapter 5: Phase Morphology and Brittle-to-Ductile Transition in PLA-crosslinked
HBP Blends ............................................................................... 133
5.0 Introduction .................................................................................... 133

viii

5.1 Experimental Section ......................................................................... 135

5.1.1 Reactive extrusion for fabrication of PLA-cHBP Blends ........................... 135
5.1.2 Testing and Characterization ............................................................ 136
5.1.2.1 Transmission Electron Microscopy (TEM) ....................................... 136
5.1.2.2 Particle Size Analysis ............................................................... 136
5.1.2.3 Density and Volume Fraction of Crosslinked HBP Particles .................. 137
5.1.2.4 Tensile Properties ..................................................................... 138
5.1.2.5 Scanning Electron Microscopy (SEM) ............................................. 138
5.2 Results and Discussions ...................................................................... 138
5.2.1 Phase morphology of PLA-crosslinked HBP Blends ................................. 138
5.2.2 Mechanical Properties and Deformation Mechanism of

PLA-crosslinked HBP Blend ............................................................ 143

5.2.3 Particle Size Analysis ..................................................................... 151
5.2.4 Brittle-to-Ductile Transition in PLA—crosslinked HBP Blend ...................... 153
5.3 References ....................................................................................... 159
Chapter 6: Conclusions and Recommendations for Future Work ..................... 161
6.1 Conclusions ..................................................................................... 161
6.2 Recommendations for Future Work ......................................................... 163
7: Appendix: Density of NaCl Salt Solutions of Different Concentrations ............ 165

ix

Table 2.1

Table 2.2

Table 2.3

Table 3.1

Table 3.2

Table 3.3

Table 4.1

Table 4.2

Table 4.3

Table 4.4

Table 5.1

Table 5.2

Table 5.3

LIST OF TABLES

Comparison of mechanical and thermo-mechanical properties of PLAs
with conventional polymers; here Mv — molecular weight
(Adopted after ref. no. 3, 13-17) ................................................ 16

Properties of PLA and some other biopolymers. (After ref. no. 2, 18-
23) ................................................................................... 17

Typical differences in the characteristics of a linear and dendritic
polymer (After ref. 130) .......................................................... 43

Processing parameters of neat PLA and its blend with
hyperbranched polymer (HBP), in-situ crosslinked HBP

and polyanhydride (PA) ......................................................... 79
The solution behavior of PLA and its blends after dissolving

in methylene dichloride .......................................................... 83
Mechanical properties of neat PLA and its blends ........................... 97
Processing parameters of neat PLA and its blends .......................... 111

Estimation of amount of crosslinked hyperbranched polymer
(using equation 4.1) formed in various PLA blends after
in-situ crosslinking of hyperbranched polymer with different
concentration of

polyanhydride ........................................................................ 1 19
Effects of HBP/PA ratio on the tensile properties

of PLA-CHBP blends ............................................................ 122
Contact angle of various materials with water .............................. 128

Details of various PLA based compositions and their processing
parameters ........................................................................ 135

Effects of amount of crosslinked hyperbranched polymer
of OH /anhydride ratio (1:1) on the tensile properties
of PLA-CHBP Blends ............................................................ I44

Particle size characteristics of crosslinked HBP particles
in PLA-cHBP blends ............................................................ 151

Table 5.4 Comparison of experimental matrix ligament thickness with theoretical
values for toughened nanoblends; here type A: PLA/HBP/PA
(88/9.7/2.3. type B: PLA/HBP/PA (92/6.4/1.6) .............................. 157

xi

Figure 2.1
Figure 2.2

Figure 2.3

Figure 2.4

Figure 2.5

Figure 2.6

Figure 2.7

Figure 2.8

Figure 2.9

Figure 3.1

Figure 3.2

Figure 3.3

Figure 3.4

Figure 3.5

LIST OF FIGURES

General chemical structure of PLA ............................................. 10
Stereochemical structures: A: Lactic acid; B: Lactides ...................... 11

The schematic of formation of a PLA prepolymer, lactide and
conversion of lactide in PLA. (redrawn afier ref. no.7) ...................... 12

Conversion of lactide in to PLA via coordination insertion mechanism
in presence of tin octoate, Sn (Oct)2, here R= growing polymer chain
(redrawn after ref. no.7) .......................................................... 12

A typical stress-strain curve of poly (L-lactide), PLLA obtained at a
crosshead speed of 15.4 mm/min at ambient conditions (25°C) ............ 18

Schematic representation of the difference in the morphology

of dendrimer (monodisperse) and Hyperbranched polymer

(polydisperse). Redrawn after ref. 133.

Here (0 ) represents central core and (IE. ) represents end groups .......... 44

Schematic of Matrix Ligament Thickness. (Redrawn after ref. 162) ...... 52

Schematic of stress volume sphere around a rubber particle. Here
shaded area represents a rubber particle. (Redrawn after ref. 146) ......... 53

Schematic representation of method of preparation of conventional
blends and nanostructure blends. (Redrawn after Ref. 178).... . . . . . . . . . . ....5 7

(A) Chemical structure of the chief constituent of hyperbranched
polymer (HBP). (B) Chemical structure of polyanhydride (PA) ........... 77

Schematic illustrations of in-situ crosslinking of

hyperbranched polymer (HBP) in PLA melt with

the help of a polyanhydride (PA). (After ref. 25) .............................. 78
SEM photomicrographs of extracted crosslinked HBP particles ............ 84
FTIR spectrum: A: Neat PLA; B: HBP-PA particles (Extracted),

C: HBP-PA particles (Outside); D: Hyperbranched polymer (HBP);

E: Polyanhydn'de (PA) ............................................................ 84
Temperature dependence of the loss modulus (E") of: (A) neat PLA,

(B) PLA/HBP (92/08), and (C) PLA/HBP/PA (92/5.4/2.6) blends. . . . . 87

xii

Figure 3.6

Figure 3.7

Figure 3.8

Figure 3.9

Figure 3.10

Figure 3.11

Figure 3.12

Figure 3.13:

Figure 3.14

Figure 3.15

Figure 3.16

Figure 4.1

Figure 4.2

Behavior of storage modulus, G’ of neat PLA and its blend as

a function of oscillatory frequency (w) ......................................... 88
Variation of complex viscosity, 7] *, of neat PLA and its blend as

fimction of oscillatory frequency ((1)) .......................................... 90

Transmission electron micrograph of PLA having

8 wt.% of pristine HBP. (Scale bar: 50 nm) ................................... 92
Transmission electron micrographs of PLA having crosslinked HBP
3.8A: Scale bar: 200 nm; 3.88: Scale bar: 100 nm ........................... 93
Tapping mode AF M phase image of PLA/HBP/PA(92/5.4/2.6)blend
3.10A: Scan size: 10 um; 3.10B: Scan size: 5 pm ..................................... 95
Stress-strain curves obtained at a crosshead speed of 15.4 mm/min ....... 96

Tensile specimens after testing to failure at 15.4 mm/min under
uniaxial tension ................................................................... 99

TEM photomicrograph of deformed region of neat PLA.
The arrow indicates the tensile load direction ( Scale bar: 2pm) ......... 100

TEM photomicrograph of deformed region of modified PLA;

3.14A: Extension of crazes; 3.14 (scale bar: 2pm);

3148: Extension of crosslinked HBP.

The arrow indicates the tensile load direction.(scale bar: 500nm) ........ 101

Schematic illustration of different stages of mechanical deformation
process of PLA in presence of crosslinked HBP particles .................. 103

Scanning electron micrographs of tensile fractured surfaces:
3.16A: Neat PLA; 3.16B: PLA/HBP (92/08);
3.16C: PLA/HBP/PA (92/5.4/2.6) ............................................. 105

Effect of OH to anhydride molar ratio on the

glass transition temperature of crosslinked HBP phase in PLA blends
where, A: OH/anhydride (4:1); B: OH/anhydride (3:1);

C: OH/anhydride (2:1); D: OH/anhydride (1:1) and

E: OH/anhydride (1:2) ......................................................... 115

Temperature dependency of loss modulus of neat PLA and its
blends with HBP and in- situ crosslinked HBP: (A) Neat PLA;
(B) PLA/HBP (92/08); (C) PLA/HBP/PA (92:7.1:0.9)

OH to anhydride 2:1; (D) PLA/HBP/PA (92:7.1 10.9)

OH to anhydride 1:1; (E): PLA/HBP/PA (92:7.1:0.9)

xiii

Figure 4.3

Figure 4.4

Figure 4.5

Figure 4.6

Figure 4.7

Figure 4.8

Figure 5.1:

Figure 5.2

Figure 5.3

Figure 5.4

Figure 5.5

OH to anhydride 1:2 ............................................................. 1 17

FTIR spectrum: A: Polyanhydride; B: HBP, C:
Extracted HBP-PA (2: l ); D: Extracted HBP-PA (1 : 1);
E: Extracted HBP-PA (1 :2); F : Neat PLA .................................... 120

Effect of weight of crosslinked HBP particles on the
elongation at break value of PLA-cHBP blends ............................. 123

TEM photo-micrographs (Scale bar: 500 nm)
A: PLA/HBP/PA (9227551045) OH to anhydride, 4:1
B: PLA/HBP/PA (92:7.4:0.6) OH to anhydride, 3:1 ........................ 125

TEM photo-micrographs (Scale bar: 500 nm)
A: PLA/HBP/PA (92:7.1:0.9) OH to anhydride, 2:1
B: PLA/HBP/PA (9216.4:126) OH to anhydride, 1:1 ........................ 126

TEM photomicrograph (Scale bar: 500 nm )of PLA/HBP/PA
(92/5.4/2.6) ........................................................................ 127

Molecular weight and polydispersity of PLA in

various processing conditions, and in presence of HBP and crosslinked
HBP: A: Neat PLA; B: PLA (3 min.); C: PLA (10min.); D: PLA/HBP
(92/08); E: PLA/HBP/PA (92:7.1:0.9) OH to anhydride 2:1; F:
PLA/HBP/PA (92:7.1:0.9) OH to anhydride 1:1; G: PLA/HBP/PA
(92:7.1:0.9) OH to anhydride 1:2 ............................................. 129

Transmission Electron Photomicrograph (Scale bar: 500nm);
A: (PLA/HBP/PA(98/1.6/0.4)
B: PLA/HBP/PA(96/3.2/0.8) ................................................... 139

Transmission Electron Photomicrograph ( Scale bar: 500 nm)
A: PLA/HBP/PA (92/6.4/1.6)
B: PLA/HBP/PA (88/9.7/2.3) .................................................................. 140

TEM photomicrograph ( Scale bar: 500 nm) of
PLA/HBP/PA (84/12.9/3.1) .................................................... 141

Stress —— strain curves of neat PLA and various PLA-cHBP blends
obtained at a strain rate of 15.4 mm/mm min; A: Neat PLA; B:
PLA/HBP/PA (98/1 .6/O.4); C: PLA/HBP/PA (96/32/08); D:
PLA/HBP/PA (92/6.4/1.6); E: PLA/HBP/PA (88/97/23); F:
PLA/HBP/PA (84/12.9/3. 1) .................................................... 145

Effect of concentration of crosslinked hyperbranched polymer on the
tensile toughness of neat PLA and PLA-cHBP blends ..................... 146

xiv

Figure 5.6

Figure 5.7

Figure 5.8

Figure 5.9

Figure 5.10

Figure 5.11

SEM photomicrograph of tensile fractured samples

A: Neat PLA

B: (PLA/HBP/PA(98/1.6/0.4)

C: PLA/HBP/PA(96/3.2/0.8) ................................................................... 149

SEM photomicrograph of tensile fractured samples
A: PLA/HBP/PA (92/6.4/1.6)
B: PLA/HBP/PA (88/9.7/2.3)

C: PLA/HBP/PA (84/12.9/3.1) ................................................ 150
Particle size distributions of crosslinked HBP particles.

(A) PLA/HBP/PA (92/6.4/ 1.6); (B) PLA/HBP/PA (88/9.7/2.3) .......... 152
Schematic of Matrix Ligament Thickness. Redrawn after ref. 7 ......... 153

Effect of volume fraction on the inter-particle distance of
crosslinked HBP particles in PLA-cHBP Blends ........................... 155

Effect of inter-particle distance on the tensile toughness of
PLA-cHBP blends ............................................................... 156

XV

KEY TO SYMBOLS OR ABBREVATIONS

Symbols Key

Tg Glass Transition Temperature
Tm Melt Temperature
T Temperature

i Standard deviation
® Registered

°C Degrees Celcius

°F Degrees Fahrenheit
°K Degrees Kelvin

A Angstrom

cc cubic centimeter

m meter

mm millimeter

nm nanometer

um micron meter

8 Epsilon

II Pi

p Density

xvi

MPa

GPa

Hz

Pa

DMA

GPC

TEM

SEM

AFM

PLA

HBP

cHBP

PA

BDT

LDPE

LLDPE

HDPE

HIPS

ABS

Sununafion

Volume fraction

Frequency

Megapascal

Gigapascal

Hertz

Joules

Pascal

Dynamic mechanical analysis
Gel Permeation Chromatography
Transmission Electron Microscopy
Scanning Electron Microscopy
Atomic Force Microscopy
Polylactide or Poly(lactic acid)
Hyperbranched Polymers
Crosslinked Hyperbranched Polymer
Polyanhydride

Brittle to Ductile Transition

Low density polyethylene

Linear low-density polyethylene
High density polyethylene
High-impact Polystyrene

Acrylonitrile butadiene styrene

xvii

PS

PBAT

PBS

PCL

PEO

PHA

PHB

PHBV

PU

US

Polystyrene

Polybutylene adipate-co-terephthalate
Poly(butylene succinate)
Poly(a-caprolactone)

Polyethylene Oxide
Polyhydroxyalkanoate
Polyhydroxybutyrate
Polyhydroxybutyrate-co-valerate
Polyurethanes

United States

xviii

Chapter 1: Introduction

1.0 Introduction

Synthetic polymers such as polyethylene (PE), polypropylene (PP), polystyrene
(PS), polyethylene terephthalate (PET) and polyvinyl chloride (PVC) share a major
chunk of commodity plasticsm. The non-renewability, staggering waste problem,
emissions of greenhouse gases during production and incineration of synthetic polymers
are making a serious dent on the versatility of these polymers. In the last couple of
decades, there was a paradigm shift in developing biobased polymers, blends and
composites as potential candidates to replace or substitute conventional polymers for real
world applications. Biomass/ agricultural feedstocks offer an ample opportunity for
utilization of abundant carbon-neutral renewable resources for the production of
bioenergy and biomaterials, which can facilitate a transition from nonrenewable carbon
sources to renewable bioresourcesm. A popular approach involves the use of biological
processes to produce chemical commodities from agricultural feedstocks.
The biodegradable polymers especially obtained from renewable resources have
shown much promise to overcome the problems associated with conventional
polymers. The global biodegradable plastic market was more than 100 million pounds

in 2005 and is likely to increase to 200 million pounds by 2010”]

. The green polymers
like polylactides, PLA; polyhydroxyalkanoates, PHAs; starch plastics; biobased poly
(trimethylene terephthalate), and functionalized vegetable/plant oil-based resins are
some examples of biologically derived materials that are moving rapidly towards

mainstream applications [4'6].

Polylactides are the leading biodegradable polymers, which are looked upon as
sustainable alternative to petroleum based plastics. The primary interest in these polymers
originated due to the production of their precursor, i.e. lactic acid, from renewable
resources such as corn, sugar beet, sugarcane, cellulosic waste, and rice starchm”. The
polymerization of lactic acid was first introduced by Carothers et al. in 1932“”. But the
major breakthrough in pilot production of PLA was achieved by Cargill in 1992.
Currently NatureWorks LLC owned by Cargill is independently producing PLA from
corn based feedstock in its plant at Blair, Nebraska, USAIBI. NatureWorks, with a PLA
production capacity of 140,000 t per annum, is the single largest producer of PLA
bioplasticml. The other producers of PLA are Toyota (Japan), Biomer (Germany) and
Hycail (N etherlands)[15 '17]. PLA has been used in biomedical applications for many years
but its use as a commercial commodity thermoplastic was pursued only in the last two
decades. Wal-Mart, the world's largest retailer, started using PLA in packaging
applications in the beginning of November 2005““. The current applications of PLA
bioplastic are in packaging, textiles and electronics.

The inherently poor properties of PLA such as poor impact strength, low
elongation at break, poor melt strength, low heat deflection temperature (HDT), narrow
processing window and low thermal stability are impeding its large scale commercial
applications. Modification of these properties of PLA is necessary for developing it as a
commodity plastic. The properties of PLA heavily depend on the stereochemical makeup
of its repeat unit. Polylactide is commonly prepared through the ring-opening
polymerization of a cyclic dimer of lactic acid known as lactide. The lactide units are

known to exist in three-stereochemical forms i.e. L-lactides, D-Iactides and iii/meso—

lactidesIlgI. The choice of lactide dimer and catalyst can provide PLA with a wide range
of backbones such as block, alternating or random. PLA made entirely from L—lactide is
called as poly (L-lactide), PLLA, and is the most common form of polylactides. High
molecular weight PLA is a stiff, colorless and glossy thermoplastic having properties
similar to polystyrene (PS) and can be processed by injection molding, fiber spinning,
thermoforming and film castingm]. Previously, PLA was only modified for biomedical
applications but there is a strong increase in the number of research papers and patent
applications pertaining to the modification of properties of PLA bioplastics for
commodity applications. Several methods such as copolymerization, plasticization,
blending, and clay based nanocomposites have been mentioned in the literature to
improve the properties of polylactides. These methods are referenced later in related
sections of this dissertation. The improvement in the toughness and flexibility of PLA is
targeted through copolymerization, plasticization and blending with other tough
polymers. Heat deflection temperature, gas barrier and stiffness of polylactides are
improved by clay based nanocomposites.

In the midst of these approaches, nanostructure polymer blends having a minority
polymer phase with dimensions less than 100 nm, have shown a lot of promise because of
their enhanced thermo-mechanical properties, optical transparency and toughness in
comparison to conventional polymer blends. Reactive blending, utilizing the concepts of
in-situ polymerization, graft and block copolymerization lead to creation of nanostructure
blendsm‘ 2”. So far the nanostructure blending is restricted to a handful of petroleum
based polymers such as polyamide (PA), polypropylene (PP), and poly(vinylidene

fluoride) (PVDF)I20'22].

The emerging hyperbranched polymer (HBP) can help in designing and
manipulating new nanoscale morphologies for polymer modification. HBP falls in the
category of dendritic polymers, which are gaining attention due to their unique structures
and properties. Hyperbranched polymers encompass highly branched nanoscopic
structures having high peripheral functionalitym]. They are polydisperse and can be
prepared using one-pot synthesis, unlike dendrimers, making the former less costly. The
role of hyperbranched polymers is reported as a processing aid, branching agent and
compatibilizer for conventional therrnoplastics [24%]. But the unique physical properties
and high peripheral functionality of HBP offer lot of other opportunities for polymer
modifications. The HBP can play a role of novel building blocks for generating new
nanostructures inside a polymer matrix, ranging from core-shell to highly networked
morphologies. There is a great scope to tailor the architecture of hyperbranched polymers
and study their influence on the properties of the host polymer.

In this work, hyperbranched polymers (HBP) are investigated as potential
modifiers for PLA bioplastics. The effect of hyperbranched polymer on mechanical and
rheological properties of PLA is investigated. A reactive extrusion technique is used to
generate new structures based on hyperbranched polymer inside a polylactide matrix. An
in-situ crosslinking reaction of HBP is carried out with help of a polyanhydride to
generate highly networked particles in a continuous phase of polylactide. The toughening
effect of crosslinked HBP particles is investigated. The size, volume fraction and matrix

ligament thickness are correlated with the mode of deformation of PLA.

1.1 Thesis Structure

This dissertation is divided into 6 chapters. A brief overview of all remaining chapters is
described as follows

Chapter 2 of this dissertation is a review of literature pertaining to polylactide
bioplastic, its structure, properties and current methods of its modification.
Hyperbranched polymers, their structure and properties are also discussed. A literature
review of toughening of polymers, toughening theories and parameters that determine the
brittle to ductile transition in multiphase polymer systems are discussed. Alternate
toughening methodologies such as interpenetrating networks and. nanostructure blending
are also discussed.

Chapter 3 describes a new polymer modification technique, in which a hydroxyl
fimctional hyperbranched polymer is in-situ crosslinked in PLA matrix during its melt
processing. New nano size HBP based particles are generated, which have a profound
toughening effect on PLA without sacrificing much of its modulus and strength. The
pristine form of HBP was not effective in improving the ductility of PLA. The
toughening mechanism of PLA is investigated. The rheological studies revealed
reduction in the melt viscosity of PLA in presence of pristine as well as crosslinked HBP
particles.

Chapter 4 incorporates the study of effects of crosslinking of HBP with different
amounts of polyanhydride and its effect on the mechanical properties and morphology of
PLA-cHBP blend. The Fox equation is applied to correlate the experimental glass

transition temperature of crosslinked HBP phase in PLA matrix. The optimized hydroxyl

(OH) to anhydride molar ratio for effective crosslinking of HBP is identified by this
study.

Chapter 5 investigates the effects of volume fraction of crosslinked HBP on the
toughening of PLA. An understanding of phase morphology of crosslinked HBP particles
and its relation with brittle to ductile transition in modified PLA was established. The
matrix ligament thickness parameter (distance between two neighboring particles) is
investigated as a critical parameter for toughening of PLA. The experimental matrix
ligament thickness is correlated with theoretical values.

Chapter 6 discusses the summary of results, conclusions and recommendations for

future work.

1.2 References

10.

11.

Amass, W., Amass, A., Tighe, B., A Review of Biodegradable PolymerszUses,
Current Developments in the Synthesis and Characterization of Biodegradable
Polyesters, Blends of Biodegradable Polymers and Recent Advances in
Biodegradation Studies. Polymer International, 1998. 47: p. 89-144.

Ragauskas, A. J., Williams, C. K., Davison, B. H., Britovsek, G., Caimey, J .,
Eckert, C. A., Frederick Jr., W. J., Hallett, J. P., Leak, D. J., Liotta, C. L.,
Mielenz, J. R., Murphy, R., Templer, R., Tschaplinski, T., The Path Forward for
Biofuels and Biomaterials. Science, 2006. 311: p. 484-489.

Plastic News February 27, 2006 [cited March 2, 2007]; Available from:
http://www.plasticsnews.com/subscriber/fyi.html?id=1 140814250

 

Gross, R. A., Kalra, B., Biodegradable polymers for the environment. Science,
2002. 297(5582): p. 803-807.

Carole, T. M., Pellegrino, J., Paster, M. D., Opportunities in the Industrial
Biobased Products Industry. Applied Biochemistry and Biotechnology, 2004. 113-
116: p. 871-885.

Kurian, I. V., Sorona Polymer: Present Status and Future Perspectives. Natural
Fibers, Biopolymers, and Biocomposites, ed. A.K. Mohanty, Misra, M., Drzal, L.
T. 2005, Boca Raton: CRC Press.

Ray E. Drumright, Patrick R. Gruber, and and David E. Henton, Polylactic Acid
Technology. Advance Materials, 2000. 12(23): p. 1841-1846.

Goksungur, Y., Guvenc, U., Batch and continuous production of lactic acid from
beet molasses by Lactobacillus delbrueckii [PO 3202. Journal of Chemical
Technology and Biotechnology, 1997. 69(4): p. 399-404.

Timbuntam, W., Sriroth, K., Tokiwa, Y., Lactic acid production from sugar-cane
juice by a newly isolated Lactobacillus sp. Biotechnology Letters, 2006. 28(11): p.
811-814.

Shen, X. L., Xia, L. M., Lactic acid production from cellulosic waste by
immobilized cells of Lactobacillus delbrueckii. World Journal of Microbiology &
Biotechnology, 2006. 22(11): p. 1109-1114.

Fukushima, K., Sogo, K., Miura, S., Kimura, Y., Production of D-lactic acid by
bacterial fermentation of rice starch. Macromolecular Bioscience, 2004. 4(11): p.
1021-1027.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

Holten, C. H. , Lactic Acid Properties and Chemistry of Lactic Acid and
Derivatives. Verlag Chemie, Germany, 1971.

http://www.naturew0rkslIC.com/About-NatureWOrks-LLC.aspx (accessed on
February 28, 2007).

http://www.european-bioflastics.org/indcx.phjfiidfl41 (Accessed On March 5,
2007)

 

Biotechnology and Afforestation Businesses, Environmental Aspects,
httpj/wwwtoyotaco.ip/en/environmental rep/06/download/pdf/e report06 p39_
941 .pdf. (Accessed on March 1, 2007).

 

http://www.bionier.tle/, (Accessed on February 25, 2007).

 

http://wwwhvcaIl.com/ (Accessed on February 27, 2007).

 

Esposito, F. , Retailers face tough resin options. Plastics News, October 24, 2005.

Garlotta, Donald, A Literature Review of Poly(Lactic Acid). Journal of Polymers
and the Environment,, 2002. 9(2): p. 63-84.

Hu, G. H., Cartier, H., Reactive Extrusion: Toward Nanoblends. Macromolecules,
1999. 32: p. 4713-4718.

Ruzette, A. V., Leibler, L., Block copolymers in tomorrow’s plastics. Nature
Materials, 2005. 4: p. 19-31.

Shimizu, H., Li, Y., Kaito, A., and Sano, H. , Formation of Nanostructured
PVDF/PAll Blends Using High-Shear Processing. Macromolecules, 2005. 38: p.
7880-7883.

Seiler, M. , Dendritic Polymers—Interdisciplinary Research and Emerging
Applications from Unique Structural Properties. Chem. Eng. T echnol, 2002. 25(3).

Hong, Y., Coombs, S.J., Cooper-White, J .J ., Mackay, M.E., Hawker, C.J.,
Malmstro'm, E., Rehnberg, N. , Film blowing of linear low-density polyethylene
blended with a novel hyperbranched polymer processing aid. Polymer, 2000. 41:
p. 7705-7713.

Jannerfeldt, G., Boogh, L., Manson, J.A.E. , Tailored interfacial properties for
immiscible polymers by hyperbranched polymers. Polymer, 2000. 41: p. 7627-
7634.

26. Kil, S.B., Augros, Y. . Leterrier, Y., Manson, J .A.E. , Rheological Properties of
Hyperbranched Polymer/Poly(ethylene Terephthalate) Reactive Blends. Polymer
Engineering and Science, 2003. 43(2).

Chapter 2: Literature Review

2.0 Preparation and Structure of Polylactides

The chemical makeup of polylactides (PLA) (Figure 2.1) consists of repeating
units of lactic acid monomer. Lactic acid (CH3CHOHCOOH) exists in two
stereochemical enantiomers, i.e. L-lactic acid and D-Iactic acid, due to the presence of a
chiral carbon center (Figure 2.2). The lactic acid is predominantly produced from
carbohydrate fermentation using various strains of the genus lactobacilli, which
exclusively produce lactic acidm. The lactic acid is converted into polylactides by three
predominant routes of polymerization; condensation/coupling, azeotropic dehydration

condensation and ring opening polymerization (ROP)[ZI

. The cyclic compounds known
as lactides are the main precursors of polylactide formation. Lactides are known to exist
in three stereochemical forms, i.e. L-lactide, D-lactide and meso/DL-lactide (Figure 2.2)I3I.

The corresponding polylactides are called poly(L-lactide), PLLA; poly(D-lactide), PDLA;

and poly(DL-lactide), PDLLA, respectively.

CH3 O

E0 —CH— C

Figure 2.1." General chemical
structure of PLA.

The ring opening polymerization of lactides is the most general approach for

production of high molecular weight polylactides. A schematic of lactide formation and

its polymerization are presented in Figure 2.3”]. The possible routes of lactide

10

polymerization are cationic, anionic and coordination reaction mechanism. Catalyst
systems based on tin, aluminum, zinc and lanthanides are used for polymerization of
lactides. Tin compounds, especially tin(lI) bis-2-ethylhexanoic acid (tin octoate) have
been preferred for the bulk polymerization due to their solubility in molten lactide, high

catalytic activity and low rate of racemizationm. A generalized mechanism of

O i
HO\C)I\OH HO\CI/I\OH
H1 I'CHs I "'H
CH3
L-lactic acid D-lactic acid
A
O O O

L-lactide DL-lactide D-lactide

B

Figure 2.2: Stereochemical structures: A.“ Lactic acid; 8: Lactides

11

coordination insertion polymerization of lactide to polylactides is shown in Figure 2.4”].
Cargill Dow LLC, currently known as NatureWorks LLC has developed an
environmentally friendly method for production of polylactides from lactides in the melt
state rather than in solutionm. The properties of polylactides strongly depend on the

stereochemical makeup of the lactide units. The stereochemistry of lactides depends on

O
O O O

Lactic acid High molecular weight PLA
Ring opening
1 Condensation polymerization
-H20 (ROP)
O
Depolymerization $0
HOLII/O OWLO OaLII/OH OW/L’x

O

Prepolymer Lactide

(Mn~ 5000)

Figure 2.3: The schematic of formation of a PLA prepolymer, lactide
and conversion of lactide into PLA (redrawn after ref. no. 7)

O
O ROH )KKZ O’II\|/ ’IOI\./
0N: O —-' 0 .—RO OTry/L0H
NO Sn(Oct) /I\r0 H 2&1???" A O‘H JT Sn(Oct)
O 2 O\\ O SQ O R 2
n

R0 0 .
Sln, I Sn(Oct)2
(0°02 (OCI)2

Figure 2. 4: Conversion of lactide in to PLA via coordination insertion mechanism in

presence of tin octoate, Sn (Oct) 2, here R= growing polymer chain (redrawn after ref
no. 7)

12

the feed ratio and stereochemical nature of the lactic acid monomer, temperature and
catalyst. In turn the stereochemistry of the lactides will determine the stereochemical
makeup of the resulting polymer. The choice of lactide dimer and catalyst can provide
PLA with a wide range of backbones such as block, alternating or random copolymerI‘I’ 5].
The stereochemical composition of the polylactide has a strong influence on the melting
point, rate and extent of crystallization of the polymer.

PLA is a semi—crystalline polymer and its mechanical properties depend on the
details of crystalline and amorphous regions. The degree of crystallinity, size and
arrangement of crystallites has profound effects on the physical and mechanical
properties of the polymer. The crystal structure of PLA plays a significant role in
determining its physical properties, so it becomes important to discuss the crystal
morphology of PLA. PLA can have three crystal structures (a, B and y forms), depending
on the crystallization conditions and drawing. The (1 form was determined for both
optically pure PLLA and PDLA as reported by De Santis and KovacsIbI. The crystals
were thought to be left handed 107 helix for PLLA and right handed 103 helix for PDLA.
The crystal lattice was an orthorhombic unit-cell having constant values i.e. a =1.06 nm,
b = 0.610 nm and c = 2.88 nm. The ratio of a/b suggested hexagonal close packing of
helices. Puiaggali et aim studied the formation of a frustrated crystal structure of PLLA
upon stretching or stroking. Here stroking meant shearing of the PLA film on a glass
slide with the help of a razor blade. The frustrated structure consisted of three-fold

helices in a trigonal unit cell. The structure accommodated the random up-down

orientation of neighboring chains associated with rapid crystallization conditions.

13

Hoogsteen et al.[8I reported the or and B crystal form of the solution spun poly (L-
lactide), PLLA fibers. The chain conformation of a and B were left handed 10/3 and 3/1
helices respectively. The conformational energies and packing of these two structures
were nearly the same, therefore both crystal modifications can co-exist. The a form was
comprised of a lamellar folded chain morphology while the B form corresponded to a
fibrillar morphology. The transformation from a to B form was possible at high draw
temperature and draw ratio. Takahashi et al.I9] studied the effect of uniaxial tensile
drawing on the crystal transformation of the PLLA film. The highly oriented PLLA film
having a crystal form was prepared and subjected to various draw ratios (DR), draw
temperatures (Td) and draw stresses. Initially the crystals were oriented in the draw
direction and progressively transformed into B form on further drawing. The properties of
PLLA based fibers were found to depend on the draw ratio, crystallinity and molecular
weight of the PLLA resin. The solution spun fibers showed better tensile strength caused
by low entrapment of entangled chain in solution-spun fibers as compared to melt-spun
fibers. The tensile strength of the fiber was also correlated with drawing temperatures,
which in turn had effects on the two-crystal modification of PLLAI'OI. The temperature
dependent variation in the lamellar structure was studied by Cho et aim]. The crystal
thickness and long spacing, i.e. distance between the center of adjacent crystallites were
minimum at 120°C and increased for both lower and higher crystallization temperatures.
Kishore et aim] studied the isothermal melting of PLLA. It was reported that the lamellar
thickening occurred during PLA crystallization, with an increase in the annealing time
leading to an increase in the melting point and heat of fusion. The high activation energy

(828 KJ/mol) obtained from plotting the logarithm of the melting rate versus the

14

reciprocal temperature, suggested the complex nature of melting and thickening. A
proper explanation was not given for this phenomenon. PLA crystal morphology and its
crystallization kinetics have been extensively researched but there is a scarcity of
literature that correlates the crystal morphology of PLA with its bulk properties. This is
probably due to the deformation mechanism of PLA, which is not entirely dependent on
the crystalline structure of PLA. The lack of conformational flexibility of PLA and low
entanglement density has a major effect on the deformation mechanism and the bulk
properties of PLA (discussed in the next section). The correlation of these parameters
with the crystalline structure of PLA can provide some interesting information about the

bulk property behavior of PLA.

2.] Properties and Deformation of Polylactides

PLA possesses several good physical properties that make it suitable for
commercial applications. PLA has good crease-retention and crimp properties, excellent
grease and oil resistance, easy low-temperature heat sealability, and good barrier to
flavors and aromasm. A comparison of properties of two different types of PLAs with
some conventional polymers is given in Table 2.1[3’ ”"71. Both types of PLA exhibited
good tensile and flexural properties. The low elongation at break, poor impact strength
and low heat deflection temperature are some of the inferior properties of PLA. PLA has
very similar physical properties as polystyrene (PS), such as tensile modulus and tensile
strength. Auras et al.“4I studied PLA properties with respect to its packaging applications.
It was reported that PLA has good heat sealability below its melting point, and lower
C02, 0; and water permeability coefficients than polystyrene (PS) but higher than

poly(ethylene terephthalate), PET. Other biobased as well as petroleum derived

15

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biopolymers are also emerging parallel to PLA. A comparison of properties of PLA with
some of the existing biopolymers is given in Table 22” '8'23]. PLA has higher strength
and modulus but has lower elongation at break and heat distortion temperature than other
biopolymers. The inherent brittleness of PLA is one of the chief culprits for its limited
commercial applications. A typical stress-strain curve of poly (L-lactide) is given in
Figure 2.5. The curve suggests that PLA had poor plastic behavior and failed in a brittle
fashion after exhibiting little strain softening.

The brittleness of PLA is a poorly understood phenomenon. The deformation of a
polymer glass is controlled by the entanglement density and chain stiffnessml. A polymer
is considered as glassy at a temperature, when it exhibits a glass transition at least 10

20°C above that temperature. So, PLA (Tg ~58°C)[3] falls in the category of polymer

 

 

80—
60-
a?
D.
E
g 40-
e
5
20-
l
o . , . 1
o 4 8

Strain (°/o)

Figure 2.5: A typical stress-strain curve of poly (L-lactide). PLLA obtained at a
crosshead speed of 15.4 mm/min at ambient conditions (250C).

18

glasses at ambient conditions. Tonelli et alps] studied the conformational statistics of
randomly oriented poly (L-lactic acid). The characteristic ratio, Coo, a measure of stiffness
of a chain, was 2.0 for polylactides. This value of C00 suggest that PLLA will deform in
ductile manner rather than brittle but the stress-strain behavior suggested the brittle nature
of PLLA. Later, Joziasse et 211.1261 studied the chain stiffness of poly(lactide)s and
reported higher values of C.0 for polylactides. The characteristic ratio, Coo, increased with
an increase in the stereo-regularity of stereo-copolymers of polylactides, while it
decreased for stereo-impure polylactide. This decrease in characteristic ratio for stereo-
copolymer of PLA was correlated with a decrease in the glass transition temperature of
stereo-copolymer with an increase in D-lactide mol.%. The characteristic ratio was 11.8
for isotactic PLLA while it was 9.5 for racemic PDLA. The higher stiffness of PLA chain
was explained by geometric isomerism of the ester group of a PLA chain. The ester group
in PLA is planer and trans due to the partial double bond character of the skeletal C-O
bond. The presence of a smaller valence angle at the skeletal oxygen enhances steric
interaction, hindering rotation around the O-C“l bondm]. In another study, the higher

chain stiffness of isotactic PLLA was explained by the presence of helical sequenceslzg].

”91 studied the chain entanglement, drawability and mechanical

Grijpma et a1.
properties of polylactides. They calculated the molecular weight between entanglements
and correlated it with the mechanical properties and drawability of poly (lactide). The
characteristic ratio C00, was found to have values, Cco = 11.7 and Cc,0 = 9.1 for poly(L-
lactide) and L- and D-lactide copolymer, respectively. The high value of characteristic

ratio and low entanglement density were deemed responsible for the brittle failure of the

amorphous polylactide. The molecular weight between entanglements was 10x103, which

1.9

corresponded to a value of characteristic ratio, C3512. This suggested the stiffness of the
poly(L-lactide) chain. Grijpma et 31.130] revealed the crazing phenomenon in poly (D,L-
1actide) responsible for its brittle behavior. In conventional polymers, polystyrene (PS) is
a well known craze forming polymerl3 1]. The crazes are the dense array of polymer fibrils
that are separated by the voids. The craze volume fraction plays a crucial role in
determining the propensity of a craze resulting in a catastrophic failure. The amorphous
polylactide has a glass transition temperature of ~55°C and fracture of amorphous
polylactides was found to occur through crazing mechanismm]. The poly(L-lactide) and
LfD lactide copolymers behaved as glass at room temperature.

Polylactides are primarily used for medical applications including drug delivery
devices and hard tissue scaffolds. The rate of mass loss and degradation of mechanical
properties are very critical in these two cases. The effect of crystallinity on deformation
mechanism and bulk properties of polylactides was studied by Renouf-Glauser et 311331
The effect of microstructures on the deformation mechanism of PLA under load was
investigated. The amorphous PLA deformed with formation of craze structures. The dry
and hydrated PLA samples showed similar deformation through craze formation, while
degraded samples showed intermediate behavior. In annealed PLA sample, hydration
dramatically changed the bulk properties such as tensile strength, modulus and elongation
at break due to the plasticization effect of water. But in crystalline PLA, the deformation
occurred by crystal mediated deformation having contributions from cavitation and
fibrillated shear process. It was concluded that crystallinity has a huge effect on the bulk
and microscopic properties of PLA, more significantly than it did on the micro

mechanism of deformation of PLA.

20

The basic mechanical properties of six different molecular weight amorphous
polylactides containing various amount of initial meso-lactide, lO-40%, was studied by
Witzkem]. The tensile, flexural and Izod impact properties of polylactides were evaluated
and compared with high-flow unmodified polystyrene. The unmodified amorphous
poly(L-meso-lactide) was a stiff, brittle and strong plastic having properties quite similar
to polystyrene (PS). The tensile modulus for amorphous poly(L-co-meso-lactide) was
between 549-587 kpsi and was found to increase with molecular weight. The tensile yield
strength of amorphous poly(L-co-meso-lactide) decreased with initial meso-lactide
fraction i.e. 7700-7800 psi for 10% meso-, and 6600-6800 psi for 34-40% meso-lactide.
Flexural strength also decreased with increasing meso-lactide fraction. The flexural
modulus, elongation at break and notched Izod impact strength for amorphous poly(L-co-

meso-lactide) were independent of initial meso fraction.

2.2 Rheology of Polylactides

The melt rheology of a polymer is an essential element of polymer processing.
Processing operations such as fiber spinning, extrusion blown film, thermoforming, and
cast films are markedly affected by the rheological properties of a polymer. Dorgan et
al.[35’39] extensively studied the melt rheology of polylactic acid. The melt rheology of
PLA was difficult to determine due to the thermal degradation of polylactides. The
effects of architectural modification of polylactide on its rheological properties were
studied”). The branching of polylactides was pointed out as a good method to control the
elasticity and viscosity of this polymer. It was found that branched PLA exhibited higher

zero shear viscosity than the linear counterpart. This behavior was caused by the longer

relaxation of the polymer chain in the terminal region for the branched polylactide. It was

21

also found that the branched PLA showed strong shear thinning, resulting in a lower
viscosity at a higher shear rate. Melt rheology of high L-content of poly(lactic acid) was
studied by Palade et. al.[40]. The dynamic, shear and transient viscosities of PLA were
measured. The polylactide having high L-content could be drawn to large Hencky strain
without breaking and showed strain hardening properties. The plateau modulus value for
PLA was 5 x105 Pa. The entanglement molecular weight was approximately 9000 g/mol.
An additive, tris (nonylphenyl) phosphite (TNPP) acted as a secondary antioxidant and
was found to improve the melt stability of PLA by avoiding its degradation.

Yamane et all/“1 studied the effect of addition of a small amount of poly (D-lactic
acid), PDLA on the melt rheology of poly (L-lactic acid), PLLA. The addition of PDLA
produced stereocomplexation in PLLA and acted as a cross-linking point in PLLA
resulting in the introduction of long chain branching, leading to an increase in the
molecular weight. The zero shear viscosity of PLLA was increased with the addition of
PDLA. The addition of high and low molecular weight PDLA caused strong strain
hardening character to the PLLA melt. In a study done by Ramkumar et alml, the
rheological properties such as complex viscosity, 1]; first normal stress coefficient, w; loss
modulus, G" ; and storage modulus, G, were higher for D,L -PLA than L-PLA. The melt
elasticity was found to be higher for D,L-PLA as compared to L-PLA and elasticity
extends the linear visco-elasticity region to long relaxation time. The entanglement
density of D,L-PLA was slightly higher than L-PLA. It was also reported that polylactic
acid (PLA) is difficult to test for rheological properties due to its degradationm]. The
primary reason was the presence of a labile CH proton in the proximity of the carbonyl

group, which makes it thermally sensitive. The abstraction of this labile proton was

22

augmented at higher temperature leading to a free radical chain scission reaction. The
ultimate result was a decrease in the molecular weight due to the breakdown of the

polymer chains.

2.3 Current Methods to Modify Polylactide Properties

There are several methods mentioned in the literature to modify the properties of
polylactides, such as copolymerization, plasticization, blending, clay based
nanocomposite. This section discusses these methods and their effect on structure-

property relationships of PLA.

2.3.1 Copolymerization

The basic development of the PLA copolymer began for biomedical applications.
Crystalline poly(L-lactic acid) is used in biodegradable osteosynthesis and tissue
scaffolds, both in animals and humans[43 1. A drawback of this polymer is a poor balance
between strength retention and loss of mass. Its high crystalline nature also slows down
its resorption. The crystalline debris may even lead to inflammatory reactions. This kind
of complication and requirements for tougher polylactides compelled researchers to
modify the crystalline and brittle nature of poly(L-lactic acid). Stereocopolymerization of
L-lactide with D-lactide and meso-lactide is reported as a method to control the
crystallinity and physical properties of polylactidesm]. Stereocopolymers of L-lactide
with D-lactide having comonomer weight ratios varying from 85/15 to 15/85 were
amorphous in nature. Grijpma et alml studied the effect of copolymerization of L-
lactide with lactones and cyclic carbonates on the mechanical properties and crystallinity.

It was found that lactones and cyclic carbonates reduced the crystallinity of the material.

23

The entanglement density and the interconnectivity of physical networks influenced the
toughness of polylactide.) The tensile and impact properties of poly(D,L-lactide) were
improved when it was oriented at a temperature close to its glass transition
temperaturelw]. The orientation helped in reducing the crazing behavior and caused
ductile energy dissipation. The copolymers of PLA with polyethylene oxide (PEO),

polycaprolactone (PCL) and polyethylene glycol (PEG) are also reported [4449].

2.3.2 Plasticization

Plasticization is a common method to improve the flexibility and processability of
a stiff and thermally unstable polymer. Low molecular weight compounds having good
solubility with the host polymer are generally used as plasticizers. The plasticization of
polylactide has been extensively researched to overcome its brittleness and to widen its
area of applicationso'oo. Labrecque et 31.150] studied the effect of citrate esters on the
thermal, mechanical and degradation behavior of PLA based extruded films. Triethyl
citrate, tributyl citrate, acetyl triethyl citrate, and acetyl tributyl citrate were used as
plasticizers. All of these plasticizers were found to decrease the glass transition
temperature and improve the elongation at break of PLA. The citrate plasticizers were
more effective in improving the elongation at break when present in excess of 10 wt.%.
There was ~ 50% drop in tensile yield strength when the plasticizer concentration was
around 10 wt.%. The tensile strength at yield of plasticized PLA was less than 10 MPa
when plasticized with 20- 30 wt.% of plasticizers. The tensile yield strength of neat PLA
was 51.7 MPa. The loss of citrate plasticizers during extrusion was also observed due to
their high volatility. The enzymatic degradation of the plasticized PLA was increased

with addition of low molecular weight citrate esters as compared with the unplasticized

24

PLAN]. Jacobsen et al.[511 used poly (ethylene glycol), glucose monoesters and partial
fatty acid esters for plasticizing poly(lactide), PLA. This research reported the
improvement in elongation at break and impact resistance when poly(ethylene glycol)
was used as plasticizer, while in case of glucose monoesters and partial fatty acid esters,
elongation at break of polylactide improved but impact resistance was decreased. Stress
whitening was observed in samples which contained 10 wt% of glucose monoesters and
fatty acid esters. Glycerol, citrate ester, polyethylene glycol (PEG) of different molecular
weight, PEG monolaurate and oligomeric lactic acid have also been reported as
plasticizers for PLAISZ]. These plasticizers were found to decrease the glass transition
temperature of PLA and improve its elongation at break. The maximum elongation at
break, i.e. 200%, was exhibited by polylactide based composition plasticized with 20% of
oligomeric lactic acid (OLA). But there was about 28 to 65% decrease in the modulus
depending upon the type and amount of plasticizer used. There was enhancement in the
percent crystallinity of polylactide on plasticization. This trend was explained by the fact
that the plasticization of thermoplastic facilitates increased chain mobility, leading to the
increase in crystallizing ability of a polymer. The crystallization temperature of
plasticized PLA also decreased due to the lowering of glass transition temperature and
increase in the PLA chain mobility. Ljungberg et al.[53'57] have extensively studied the
effects of different plasticizers on thermo—mechanical properties, aging and film extrusion
of poly(lactic acid). The poly(lactic acid) was plasticized with monomeric and oligomeric
plasticizers such as tributyl citrate (TBC), diethyl bishydroxymethyl malonoate (DBM),
TBC-oligoesters, DBM-oligoesters and DBM-oligoestermidesl53]. All of these plasticizers

were found to improve the flexibility of poly(lactic acid). But the physical aging of

25

plasticized PLA revealed that PLA plasticized with oligomeric plasticizer exhibited better
morphological stability than with monomeric plasticizers. This research showed that the
use of high molecular weight plasticizers can help in eliminating the problems such as
phase separation, physical aging and migration in plasticized PLA films. In another

[54], triacetine, tributyl citrate, triethyl citrate, acetyl tributyl citrate and acetyl

study
triethyl citrate were investigated as potential plasticizers for polylactic acid. The
evaluation of dynamic mechanical and thermal properties showed that triacetine and
tributyl citrate were the most effective plasticizers as they exhibited the largest
depression in the glass transition temperature of PLA. Both these plasticizers have a
solubility parameter difference of < 1 (Jcm'3)“2 with PLA. It was found that phase
separation occurred when these plasticizers were present in excess of 25 wt.%. Phase
separation also occurred when the plasticized PLA was heat treated at 50°C. Increase in
the crystallinity of plasticized PLA was deemed responsible for the plasticizers’ phase
separation. Tributyl citrate oligomers were synthesized and used as plasticizers for
polylactide[55]. These oligomers were able to plasticize the PLA and improved its
flexibility. But there was partial phase separation when the plasticized PLA was aged
over 4 months. The morphological stability of plasticized PLA was enhanced when the
plasticizer concentration was 10-15 wt.%. Oligomeric malonate esteramides were

synthesized and reported as efficient plasticizers for polylactidefim

. The oligomer
synthesis was based on a plasticizer, diethyl bishydroxymethyl malonate (DBM). The
benefits of DBM-based oligomers were intended to avoid the phase separation and to

obtain long term phase stability of plasticized PLA based film. It was also proposed that

the presence of polar amide groups in oligomeric malonate esteramides would able to

26

interact with the PLA chains and improve the compatibility between the plasticizer and
PLA. But again the problem of plasticizer phase separation was reported when the
plasticized PLA was aged above the glass transition temperature of the blend. It was
found that the cold crystallization of plasticized PLA was absent when it was aged at
ambient conditions and the plasticized PLA maintained its flexibility. Ljungberg et 211.157]
studied the film extrusion and weldability of poly(lactic acid) plasticized with triacetine
and tributyl citrate. It was found that storage of plasticized film caused an increase in
crystallinity and reduced the constant heat (CH) welding capability of the film. Hu et
al.[58'601 studied the crystallization, phase separation and aging of poly(lactic acid)
plasticized with poly(ethylene glycol), PEG. The crystallization of slowly cooled

PLA/PEG blend suggested that the morphology of PLA/PEG blend consisted of large

spherulites dispersed in a homogenous matrix.

2.3.3 Blending

Polymer blending is considered as the most benign modification technique in the
field of polymer science and engineering. Two or more different polymers are blended to
obtain synergism of properties in the resultant blend compared to the individual
polymers. Blends may be miscible or immiscible depending on the chemical nature and
compatibility of the two polymers. Since, PLA is a biodegradable polymer, the natural
choice is to blend it with some other biodegradable tough polymers. Biodegradable
blends of PLA with various polyhydroxyalkanoates (PHAs), poly(e-caprolactone) (PCL),
poly(butylene succinate) (PBS), and poly(butylene adipate terephthalate) (PBAT), are
reported in the literatureél‘gz. PLA has also been blended with polyethylene (PE)83'86, and

87-89

rubber in an attempt to improve its toughness.

27

Blends of PLA with polyhydroxybutyrate (PHB) have been investigated for
miscibility, crystallization and morphology of the blendslm]. It was found that PLA and
PHB were immiscible in an amorphous state and exhibited two glass transition
temperatures corresponding to the respective components. There was some evidence of
miscibility when samples were prepared at higher temperatures. There was
transesterification between PLA and PHB leading to the formation a PLA-PHB
copolymer, which enhanced the compatibility between the PLA and PHB. Other work[°2]
reported that low molecular weight PLA was miscible with PHB over the whole
composition range, while high molecular weight PLA showed phase separation when
blended with PHB. Ohkoshi et al.[°3] reported the miscibility of PLA with atactic PHB
having molecular weight (Mw) ranging from 9400 to 140,000. The result showed that
PLLA was miscible with atactic PHB having low molecular weight. The compatibility
between PLA and PHB blends was very much dependent on the molecular weight of the
components.

Park et al.[°4] studied the uniaxial drawing and mechanical properties of blends of
poly(L-lactic acid) (PLLA) with two kinds of PHB having different molecular weights.
The blends were prepared through a solvent casting method. The blends were immiscible
at all compositions. The drawing of the blends was carried out at two temperatures, one
at the Tg of PHB, i.e. 2°C and other at the Tg of PLA, i.e. 60°C. It was observed that the
PLA domains in the PHB rich phase was unstretched when PHB had a low molecular
weight, but the PLA was able to orient when it was present as a dispersed phase in the

continuous phase of ultrahigh molecular weight PHB. PLLA was blended with a PHB

based copolymer poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), in order to

28

improve the toughness of PLLA[°5]. The blends were incompatible and showed different
glass transition temperatures by differential scanning calorimetry (DSC) and dynamic
mechanical analysis (DMA). There was an elastic deformation of the blendpnder tensile
loading with the addition of PHBV.

The properties of PLA, especially brittleness and low impact strength, were
improved by blending it with medium chain length poly(3-hydroxy alkanoates) (Mcl—
PHA) and epoxidized poly(3-hydroxyalkanoates) (ePHA)[°°]. Mcl-PHAs had a side alkyl
chain of C6-C14 carbons. Mcl-PHAs had higher rubber like properties besides being
biocompatible. It was hypothized that the blend of PLA and Mel-PHA would behave
similarly to polystyrene (PS) / acrylonitrile-butadiene-styrene (ABS) blend. The blends
of Mel-PHA and ePHA with PLA were immiscible. The Mcl-PHA and ePHA were
present as a dispersed phase in the PLA matrix and enhanced the impact toughness of the
blend by a mechanism similar to rubber toughening. The ePHA was more effective in
improving the toughness of PLA due to the presence of epoxy groups, which interacted
with the PLA phase. The domain size of ePHA in PLA matrix was 0.1-1um. Polylactic
acid was also blended with another branched bacterial polyester copolymer comprised of
3-hydroxybutyrate (3HB) and other 3-hydroxyalkanoate unit (3HA) having alkyl side
chains of C3 or more carbon atomslm]. These polymers are known as the NodaxTM
copolymers. The toughness of PLA was substantially increased when blended with 20
wt.% of this PHA copolymer. The PHA was not able to crystallize at this concentration
and existed as a rubbery amorphous phase in the PLA matrix. The finely dispersed phase
of the PHA copolymer helped in the dissipation of energy and improved the toughness of

the blend.

29

PLA has been blended with other non-renewable resource based biodegradable
polymers for improving its toughness. Blends of PLA with poly (e-caprolactone)
(PCL)[°°’7°], polybutylene succinate (PBS)[7°’7°], poly(butylene succinate adipate)
(PBSA)[°°', and poly(butylene adipate terephthalate) (PBAT)[°” have been reported.
BASF, a Germany company, has recently developed a PLA based biodegradable blend
called Ecoviomw. The blend consists of 45 percent PLA and the rest is BASF’s existing
biodegradable plastic Ecoflex®, which is an aliphatic—aromatic polyester derived from
petrochemicals. The target applications for Ecovio® include flexible plastic bags and

“’81 studied the structure and mechanical

other packaging applications. Broz et a1.
properties of PLA-PCL blends. The tensile modulus, strain at failure, and ultimate tensile
strength could be tuned through varying the blend composition. A percolation model was
used to explain the observed mechanical properties. The percolation threshold in
mechanical properties was achieved around the PLA mass fraction of 50%. The blends
were immiscible but some adhesion was predicted when the blend had a PCL rich phase.

In another study, Semba et al.[°°]

studied the effect of crosslinking on the mechanical
properties of PLA-PCL blends. The aim of this research was to improve the compatibility
between PLA-PCL blends and ultimately improve the tensile strain of the blends.
Dicumyl peroxide (DCP) was used as a crosslinking agent for the PLA-PCL blends. It
was observed that the blend showed a yield point and ductile behavior when a small
concentration of DCP was added. The optimum blend composition was 70 wt.% PLA and
30 wt.% PCL with a DCP concentration of 0.3 phr. The improved compatibility between

PLA and PCL was originated through the formation of crosslinks initiated by the

peroxide along with the interactions of the ester groups. The dispersed PCL phase in a

30

continuous phase of PLA extended under tensile loading, thus delaying the failure of the
blend. Wang et al.[7°] studied the properties of a reactive blend of PLA with PCL using a
branching and coupling agent.

Chen et al.[7°] studied the role of twice functionalized organoclay (TFC) as a
compatibilizer in an immiscible PLA-PBS blend. Poly(butylene succinate) (PBS) is a
petroleum-derived tough biodegradable polymer. TFC was prepared by treating an
organo-clay i.e. Cloisite 25A, with (glycidoxypropyl) trimethoxy silane. Effect of
different amounts of TFC on the compatibility of a PLA-PBS blend having 75/25 weight
ratio was studied. It was observed that when the TFC content was 5 wt.% or more in the
PLA-PBS blend, it acted as a compatibilizer via formation of a PLA-silicate-PBS hybrid
and helped in reducing the domain size of the dispersed PBS phase. Tensile modulus and
elongation at break of PLA-PBS blends were found to increase in the presence of TFC.
Park et al.[77] studied the phase behavior and morphology of PLA-PBS blends. The
blends were classified as semicrystalline/semicrystalline blends due to the presence of
two distinct melting peaks in the DSC therrnograms of blends over the entire composition
range. The interaction parameter between PLA and PBS obtained from the Flory—
Huggins equation showed a value of -0.15. Small-angle X-ray scattering revealed that the
PBS component was expelled out of the interlameller region leading to the decrease of
the long period and amorphous layer thickness of PLA. A significant crystallization-
induced phase separation was observed when PBS weight content was greater than 40
wt.%. In another study, Park et al.[78] used synchrotron wide angle X—ray scattering
(WAXS) and small angle X-ray scattering (SAXS) techniques to study the crystallization

behavior and morphological changes of PLA-PBS blends during heating. The

31

synchrotron WAXS exhibited absence of co-crystallization between PLA and PBS; they
crystallized independently of each other. The lamella formation in the blend followed the
dual lamellar stack model, which was indicative of formation of PLA and PBS lamellae
at different locations.

Blending of polylactide (PLA) with polyethylene (PE) is reported in the

[83'8"]. PLA-LDPE blends are immiscible in nature and require compatibilizers

literature
for toughness improvement of PLA. Synthesis of polyethylene- poly(L-lactide) diblock
copolymer (PE-b-PLLA) and its role in compatibilizing the PLA-LDPE blend was
studied83 1. The role of the microstructure of PLA in compatible blends with linear low-
density polyethyelene (LLDPE) is also reportedlm]. The polylactidefpolyethylene (PLLA-
PE) block copolymers were used as compatibilizers. It was found that amorphous PLA
was toughened only in the presence of PLLA-PE block copolymer, while the toughness
of semicrystalline PLA was obtained without the compatibilizer. The PLLA-PE
copolymer compatibilizer was effective when the individual polymers had a molecular
weight above their entanglement molecular weight. The impact strength of PLLA was
improved from 20 to 660 J/m when blended with 20 wt.% of LLDPE in the presence of 5
wt.% of PLLA-PE block copolymer compatibilizers. Anderson et al.[85] studied the
_ interfacial properties and dispersed phase properties of LDPE on the toughening of
PLLA. It was observed that using a block copolymer with varying ability to crystallize
could alter the interfacial adhesion. For the lowest modulus of PE dispersed phase, the
block copolymer that gave the strongest adhesion was effective in toughening the PLA,

while the block copolymer that gave the weakest interfacial adhesion was instrumental in

toughening of PLA with the stiffest PE. On other hand, in blends of PLA with

32

intermediately stiff PE, the copolymer causing intermediate adhesion was most effective
in improving the toughness of PLA. The immiscible blend of poly (L-lactide) and low
density polyethylene was reactively compatibilized with glycidyl methacrylate (GMA)
grafted PE (PE-GMA)[°°]. The compatibility was believed to originate from the in-situ
reaction of epoxy groups of PE-GMA with the —OH end groups of PLA. PE-GMA
significantly reduced the domain size of the LDPE dispersed phase and improved the
elongation at break for the PLA —LDPE blend.

Rubber toughening of PLA has also been used to overcome its brittleness[°7'8°].
PLLA was blended with poly(cis-l,4-isoprene) (PIP), a major ingredient of natural

rubberW].

The PLLA-PIP blend was incompatible and the blend compatibility was
improved by grafting a vinyl acetate monomer on the PIP (PIP-g-PVAc). Although there
was phase separation in this system but the decrease in the Tg of PLLA with increase in
graft copolymer content indicated the mixing of the PVA moiety of PIP-g-PVA with
PLLA. There was an increase in the toughness of the resulting blend compared to neat
PLLA. A model polyisoprene (PD-polylactide (PLA) copolymer was synthesized to
obtain a toughened PLA[°°'. A PI-OH prepolymer was synthesized with living anionic
polymerization followed by the coordination insertion ring opening polymerization with
lactides to produce PI-PLA diblock copolymer. Differential scanning calorimetry (DSC)
and small angle X-ray scattering (SAXS) suggested that the block copolymers were
micro-phase separated. In another relevant study, polybutadiene-polylactide diblock
copolymers were also synthesized [89]. The study focused on the kinetics and mechanism

of synthesis of the diblock copolymer. The mechanical properties of these copolymers

were not reported.

33

2.3.4 Polylactide based N anocomposites

Inclusion of nano-structures in polymeric matrices is looked upon as a unique
approach to create revolutionary material combinations. In polymer nanocomposites, the
commonly used reinforcements are nanoclay, cellulose nanowhiskers, graphite
nanoparticles and carbon nanotubes. Polymer-clay nanocomposites have been the most
extensively studied. Clay is a silicate mineral consisting of octahedral sheets of aluminum
or magnesium hydroxide sandwiched between two tetrahedral sheets of silicon
hydroxideml. The individual clay platelet has a thickness of 1 nm and lateral dimension
of 100-1000 nm. The exceptionally high aspect ratio and nanoscale dimensions of clay
enables the polymer-clay nanocomposites to exhibit improved mechanical, thermal and
barrier properties without the loss of toughness and optical transparencym]. “Green”
nanocomposites. having either a renewable or nonrenewable resource based
biodegradable polymer matrix are the next generation of materialsm]. Polylactide based
nanocomposites having organo-clay as reinforcement are the most extensively
researchedm'l 23 1.

Ogata et al.[°3] reported the first PLA-clay nanocomposite in 1997. The purpose of
this research was to develop environmentally friendly benign nanocomposites. A PLA-
clay blend was prepared using chloroform as a solvent. The montmorillonite clay was
modified with distearyldimethylammonium chloride (DSAC). The paper reported
inadequate dispersion of clay in the PLA matrix by this method, which was evidenced by
presence of clay tectoids, which consist of a stack of several monolayers of clay platelets.

There was only a small improvement in the Young’s modulus of PLA-clay

nanocomposites. This result was attributed to an absence of intercalation geometry in

34

1 [94403]

PLA-clay nanocomposites. Sinha Ray et a extensively studied the fabrication,

characterization and properties of PLA-layered silicate nanocomposites. In one of the

[94], montmorillonite (MMT) clay modified With trimethyl

research efforts
octadecylammonium cation was used as organically modified clay for a PLA
nanocomposite fabrication. The modified clay was termed C13- MMT. There was a
significant improvement in flexural modulus, storage modulus, heat distortion
temperature and a decrease in oxygen permeability of neat PLA after incorporation of
Clg-MMT clay. The biodegradation of neat PLA was enhanced in the presence of C18-
MMT. The increase in biodegradation was attributed to the absorption of water by clay
from the compost, which triggered the heterogeneous hydrolysis of PLA.

The first successful preparation of intercalated PLA-layered silicate
nanocomposites was reported by Sinha Ray et 31.195]. The paper reported the melt
extrusion of PLA and Clg-MMT clay to prepare the nanocomposite. The role of oligo-
(e—caprolactone) (o-PCL) as a compatibilizer in a PLA/clay nanocomposite was also
investigated. The presence of a compatibilizer (0.2 to 3 wt.%) improved the mechanical
properties of nanocomposite to a great extent. The hydroxyl functional compatibilizers
helped in generation of a flocculated clay structure by enhancing hydroxylated edge-edge
interaction of the silicate layers. The PLA/layered silicate nanocomposites exhibited
improved material properties in both solid and melt states as compared to the matrix
without clay. Sinha Ray et al.”°4] also studied the melt rheology of PLA-clay
nanocomposites. The nanocomposites had clay loadings of 2, 3 and 4.8 wt.% of C13-

MMT clay. The nanocomposites showed pseudo-solid like behavior, revealed by the

smaller terminal region slope values of storage moduli, G'(Qt)), and loss modului,

35

G"(o)) as compared to neat PLA. The nanocomposite also exhibited shear independency
of G'(o)) and G"((t)).values in lower frequency (awn) regime. This behavior occurred due
to the formation of a spatially-linked structure of clay platelets in PLA matrix which
caused the preventive relaxation of PLA chain occurring from the physical jamming
polymer chains. The flow activation energy and complex viscosity of PLA increased in
the presence of MMT clay. The important observation of this research was the tendency
of PLA-clay nanocomposites to show strain-induced hardening behavior under uniaxial
elongation flow. This property of PLA-clay nanocomposite was exploited in fabrication
of foam using carbon dioxide (C02) as a blowing agent. The PLA nanocomposite showed
smaller cell size and larger cell density as compared to the neat PLA foam. The
nucleating effect of clay was instrumental in the formation of uniform cells. The same

[105] also reported a separate research article on PLA based nanocellular

research group
foam prepared by a batch process using supercritical C02 as blowing agent. The
nanocomposites of PLA were also foamed by using a mixture of carbon dioxide (C02)
and nitrogen (N2) as blowing agent in a batch foaming process“°°]. PLA nanocomposite
foams revealed reduced cell size and increased cell density at very low organoclay
content. The cell size of foams decreased while both cell density and foam density
increased with an increase in the organoclay content.

Maiti et al.“°7]

examined the effect of chain length of organic modifiers in three
different types of clay: smectite, montmorillonite (MMT), and mica, in PLA based
nanocomposites. In PLA-smectite nanocomposites, increase in the chain length of

phosphonium ion based organic modifier had a profound effect on the miscibility of

organo-clay. There was a decrease in the glass transition temperature of PLA when the

36

chain length of the modifier increased from C3 to C16. The melting point of PLA also
decreased with an increase in the chain length of the modifier. This suggested that C16
salt had better miscibility with PLA. Properties of PLA-nanocomposites having smectite,
montmorillonite (MMT), and mica were also compared. The clays were modified with
C16 phosphonium salt. The increase in the storage modulus was greater for smectite
nanocomposite than MMT or mica nanocomposite. The better dispersion of smectite clay
instead of its lower aspect ratio than MMT and mica, was responsible for this behavior.
Smectite nanocomposites also showed better gas barrier properties than the MMT or mica
systems. The results indicated the better interaction of smectite clay with PLA caused a
stiffer local environment, which helped in improving the modulus and barrier. The effect
of clay as a nucleating agent in PLA nanocomposites was researched by Nam and
coworkersmg]. The spherulites in neat PLA and PLA/C-18 MMT showed negative
birefringence but the ordering of spherulites for nanocomposites was less than that of
neat PLA due to the dispersion of clay platelets inside the spherulites. The crystallization
rate of PLA was increased in the presence of clay particles and the researchers concluded
that dispersed clay particles acted as nucleating agents for PLA.

Shibata et al.“°°] studied the thermal and mechanical properties of
nanocomposites prepared from plasticized polylactide and organo-modified
montmorillonites. The plasticized PLA systems contained diglycerine tetracetate (PL-
710) and ethylene glycol oligomeric plasticizers. The organo-clays were modified with
ammonium salt of octadecylamine (ODA-M) and poly(ethylene glycol) stearylarnine (
PGS-M). The incorporation of ODA-M clay in PLA and PLA plasticized with PL-710

resulted in an improvement in tensile strength and modulus as compared to PGS-M clay.

37

PLA plasticized with 10 wt.% PL-710 resulted in a remarkable improvement in
elongation at break on incorporation of 3 wt.% of ODA- M clay, while only plasticized
system showed no improvement in the said property. The paper did not provide a
scientific explanation of this result. In other research[1 1°], PLA-organically modified clay
based nanocomposites were prepared via a solution technique. A lactide emulsion was
used to improve the compatibility between the clay and PLA matrix. The storage
modulus of the nanocomposite was improved but decreased once the concentrations of
silicate were increased. The presence of a large dimension of porosity in PLA-clay
nanocomposites having higher silicate content was suggested as a possible reason for this
behavior.

The twice functionalization of clay was reported as an effective method of
obtaining a exfoliated geometry of clay in a PLA matrix leading to the high mechanical
properties of the nanocompositem”. A commercial organo-clay (Cloisite® 25A) was
functionalized by reacting with (glycidoxypropyl) trimethoxysilane. The resulting clay
called twice functionalized clay (TFC), had improved organophillicity. The epoxy group
of the silane in TFC reacted with the end groups of PLA and resulted in better
compatibility between PLA and TFC. The improvement in tensile modulus, strength and
elongation at break for PLLA-TFC nanocomposite was superior to that of PLLA-clay
nanocomposites. The effect of addition of polycaprolactone (PCL) on the mechanical
properties of a PLA-clay nanocomposite was studied [1 '2]. The three different molecular
weights of PCL used were 10K, 40K and 70-100K. The addition of PCL to PLA-clay

nanocomposites improved the tensile strength and elongation at break. It was also

38

reported that PCL was not helpful in obtaining exfoliated clay geometries in PLA-
organoclay (Cloisite® 25A) nanocomposites.

Pluta and coworkers[1 13'1”]

extensively studied the role of organoclays on the
physico-chemical properties, aging behavior of plasticized PLA and the effect of
processing cycle time on the properties of PLA-clay nanocomposites. Different weight
contents of three clays: Cloisite® 20A, Cloisite® 25A and Cloisite® 30B were
incorporated in a PLA based composition plasticized with 20 wt.% of polyethylene
glycol (PEG)[”3]. The size exclusion chromatography revealed a decrease in the
molecular weight with an increase in the filler concentration. This research reported the
decreasing affinity of clay towards PLA in order of Cloisite® 3OB, Cloisite® 20A, and
Cloisite® 25A, respectively. In an extension to this work, Pluta et 211.014] studied the
effect of aging periods (1 and 4 years) on the physical and chemical properties of
plasticized PLA nanocomposites. The plasticized PLA and plasticized PLA based
nanocomposites were unstable and showed deplasticization after aging. The degradation
was more pronounced in the plasticized PLA, where PEG was phase separated and
diffused out with aging. The clay helped in restricting the diffusion and was noticeable
when the affinity between clay (Cloisite® 20A and 30B) and PLA was good. Pluta et
al.“'51 also reported the formation of intercalated and exfoliated geometries with an
increase in the compounding time. Rheological studies showed pseudo-solid like
behavior of PLA nanocomposites. Thermal stability of PLA was found to increase with
nano-clay addition. PLA-clay nanocomposite cast film was prepared via a masterbatch
methodll 1°]. An increase in tensile modulus and elongation at break was reported for cast

film having 5 wt.% of organically modified montmorillonite clay. Paul et al.[”7' ”81

39

studied nanocomposites prepared from plasticized PLA. The thermal and morphological
properties of the nanocomposites of plasticized PLA having 20 wt.% of polyethylene
glycol (PEG 1000) were studiedlm]. The thermal stability of the nanocomposite was
reported to be highest when organically modified montmorillonite clay was modified
with bis-(Z-hydroxyethyl) methyl (hydrogenated tallow alkyl) ammonium cation. The
result was related to the better affinity of this clay with PLA owing to the presence of
polar interactions. In another study by Paul et al.[“°], both intercalated and exfoliated
PLLA/organo modified montmorillonite nanocomposites were synthesized by in-situ ring
opening polymerization of L-lactide. Mechanical properties of these nanocomposites
were not reported.

Thellen et al.[”9] first reported the fabrication of an extrusion blown film of
plasticized PLA based nanocomposites. The effect of processing parameters (screw
speeds) on the barrier, thermal, mechanical, and biodegradation properties of the
nanocomposites were evaluated and compared to that of neat polymer. There was 48 and
50% improvement in the oxygen barrier and water vapor barrier of the nanocomposite
films, respectively, compared to the neat films. The Young’s modulus of the
nanocomposite was improved by 20%, while the elongation at break was unaffected on
nanoclay addition. Tensile strength of the nanocomposites did not change significantly.
Thermal stability of the nanocomposite film was enhanced as compared to neat PLA.
Tanoue et al. “2°! reported the use of polyethylene glycol (PEG) of different molecular
weights as plasticizers for PLA nanocomposites. PEG was not helpful in obtaining an

1[121, 122]

exfoliated clay geometry in the PLA matrix. Chang et a studied the thermo-

mechanical properties, morphologies, gas permeability and optical translucency of PLA

40

nanocomposite films. Solution intercalation processing was used for the dispersion of two
different kind of clays in PLAN”. The organo-clays were montmorillonite modified with
hexadecylamine (Cl6—MMT) and fluorinated-mica modified with hexadecylamine (Cl6-
Mica). One of the important findings of this work was the unaffected optical translucency
of nanocomposite film containing organoclay content up to 6 wt%, while the films
containing 8 wt% organoclays were slightly cloudier. In another study, Chang et awn]
reported the use of hexadecylamine—montmorillonite (Cl6—MMT), dodecyltrimethyl
ammonium bromide—montmorillonite (DTA-MMT), and Cloisite®25A organoclays in the
preparation of PLA based nanocomposites. An increase in the tensile strength and gas
barrier was reported for the nanocomposites.

Gorassi et al.[m]

evaluated the transport properties of PLA based nanocomposites
in term of sorption (S) and the zero-concentration diffusion coefficient (D-0). These
properties were studied for both exfoliated and intercalated clay geometries in PLA
nanocomposites prepared with montmorillonite clays (l, 3, 5, and 10 wt.%) modified
with bis-(2-hydroxyethyl)methyl (tallow-alkyl) ammonium cation. The amount of a
solvent absorbed i.e. obtained from the linear part of the sorption curve, was lower in the
intercalated nanocomposites than the exfoliated nanocomposite at all clay contents. In
intercalated samples, the thermodynamic diffusion parameters, D-O, decreased by one
order of magnitude with increasing the clay content from 0 to 10 wt%. The exfoliated
nanocomposite showed strong lowering in the D-0 value by about two orders of
magnitude with respect to all the other samples (unfilled and intercalated). It was found

that the exfoliation of the clay platelets was the most vital factor to improve the barrier

properties of the material to water vapor. In other researchlm], oxygen barrier properties

41

of nanocomposites of ethylene-vinyl alcohol copolymer (EVOH) and PLA were studied
under dry and humid conditions. Organically modified kaolinite and montmorillonite
clays were used for PLA nanocomposite fabrication. The oxygen barrier improved in
both nanocomposites but the increase was smaller than that of EVOH nanocomposite.
The insufficient compatibility of PLA with these clays was one major reason behind
these results. The higher sorption of solvent and slower diffusion kinetics of
nanocomposite was predicted as suitable properties for applications such as active
packaging. In an effort to develop PLA based bionanocomposites, PLA-cellulose

. . 12 -128
nanowhrsker nanocomposues have been explored recently[ 5 1.

2.4 Hyperbranched Polymers (HBP)

The polymer science is dominated by the two major molecular architectures linear
and crosslinked. The molecular architecture of a polymer has a marked effect on its
physical properties. In recent years, macromolecules having dendritic topology are
gaining lot of interest due to their nanostructure and unique physical properties.
Dendrimers and dendritic polymers such as hyperbranched polymers are at the forefront
of this emerging new class of macromoleculesllzg]. Some of the characteristic differences
in properties of a linear and dendritic architecture are shown in Table 231130]. The term
dendrimers is derived from Greek word dendron (tree) and meros (part). The dendrimers
are three dimensional, uniforms, monodisperse macromolecules having tree like
structures. Their general construction is comprised of a central core or focal point, a well-
defined radially symmetrical layer of repeating units, often termed as generation and

endgroups, which are also called as terminal or peripheral groups.

42

Table 2.3: Typical differences in the characteristics of a linear and dendritic
polymer. (After ref 130)

 

Linear

Dendritic

 

 

Random Coil Configuration

Semicrystalline/crystalline

materials: higher glass

transition temperature

3. Lower solubility- decrease
with molecular weight

4. Entanglement directed
rheological properties: shear
sensitivity

5. Intrinsic viscosity follows
logarithmic- Increase with
MM

6. Mobility based on reptation:
segmental and molecular
mobility

7. Anisotropic electronic

conductivity

Nu—a

 

. Predictable shape changes as a

function of th. and core —
robust spheroid; breathing
deformability

. Non-crystalline, amorphous

materials: lower glass
transition temperatures
Increased solubility- Increases

with th.
. Newtonian type rheology- no

shear sensitivity, considerably
lower viscosity

Exhibits viscosity maximum
and minimmn plateau with
th.- low viscosity

Mobility involves whole
dendrimer as the kinetic flow
unit— virtually no reptation
Isotropic electronic
conductivity.

 

 

“29‘ '3" '32] carried out extensive work in the synthesis of dendrimers

Tomalia et al.
and reported the nanoscale dimension of dendrimer (hydrodynamic radii), which
increases with the increase in the generation of dendrimers. Hyperbranched polymers
(HBP) are extremely branched globular macromolecules that fall in the category of
dendritic polymers. They are considered as mutant offspring’s of the dendrimers. They
are also dendritic in nature but are polydisperse and have irregularity in branching and
structure['3 3 1. Figure 2.6“341 describes the basic difference in the structure of a dendrimer

and a hyperbranched polymer. The major advantage of hyperbranched polymers over

dendrimers lies in their one-pot synthesis, which make their synthesis simple and cost

43

PA

A

  

A

5
A

4

    

V

Dendrimer Hyperbranched polymer

Figure 2. 6: Schematic representation of the difference in the morphology
of dendrimer (monodisperse) and Hyperbranched polymer @olydisperse). Redrawn
after ref 13 4. Here (.) represents central core and ( B) represents end groups.

effective. On the other hand the synthesis of dendrimers is marred by the
protection/deprotection, addition chemistry and purification at every step, making their
synthesis extremely costly and time consuming. The hyperbranched polymers possess
unique properties like dendrimers in spite of their imperfect molecular architecture. The
principal reason for this analogous behavior of HBP is their extensively branched
structure and high peripheral functionality. So, the hyperbranched polymers are
considered as economical alternates to dendrimers in various application areas. The
dendritic and hyperbranched structures can be synthesized in three different ways['3 5 1:

1. Divergent Approach

2. Mixed Reactivity Approach

3. Convergent approach

In the divergent approach, a multifunctional core is reacted with an equivalent

amount of chain extenders depending upon the functional group on the core and reactive

44

group on the chain extender. Usually an A2B type of chain extender is reacted with
central core resulting in molecule having twice the fimctionality of the core. The similar
chain extender can further be used to increase the branching and molecular weight. In the
mixed reactivity approach, different chain extenders having selective chemistry can be
used to form dendritic structures. For example, in AZB chain extender, the functionality
B can react with a central core having functionality C but functionality A cannot react
with B or C. Similarly another chain extender C2D can be used for further growth. In this
case the functionality D has reactivity with A but C cannot react with either A or D. In
the convergent approach, the first step is to produce branches and then connect them to
the central core.

The target applications of hyperbranched polymers are as rheological modifiers,
additives in process engineering and processing of thennoplastics, lubricants, foams,
surface modifiers, toughener for epoxy based resins, plasticizers, and viscosity modifiers
for paintsl133 1. Some of known examples of hyperbranched polymers are polyphenylenes,
aromatic and aliphatic polyesters, polyethers, polyamides, polyurethanes, polycarbonates
and polyesteramidesl13 °]. Hyperbranched polymers especially hydroxyl functional
aliphatic polyesters, consist of a polyalcohol core from which branches extend and form a
core-shell structure. The core shell structure has abundant hydroxyl groups at the
periphery. Perstorp Polyols, a leading company in the manufacturing of hyperbranched
hydroxyl functional aliphatic polyesters, use an initial core made up of pentaerythritol or

trimethylol propane and uses dimethylol propionic acid as a chain extendermsl. This

results in generation of hydroxyl groups on the periphery of these molecules. The number

45

of OH groups on the surface will depend on the generation of the hyperbranched
polymer. Some properties of hyperbranched polymers are:

0 High reactivity due to peripheral functionality

- Nonlinear relationship of viscosity with molecular weight

0 High thermal stability

- Variable glass transition temperature

Hyperbranched polymers (HBP) have attracted attention as toughness modifiers for
thermosets (e.g. epoxy resins) instead of commercial tougheners such as rubbers and

[137-139]

thermoplastics Their role as compatibilizers and matrix modifiers in polymer

$140442]. The current use of hyperbranched

blends and composites have also been reporte
polymers as modifiers for linear thermoplastics is not that widespread and is even rare in
bioplastics. In the research relevant to polylactides, Wong et al.“43] reported a significant
increase in the interlaminar fracture toughness of PLA based biocomposites in the
presence of a hyperbranched polymer. In other research, a dendritic hyperbranched

polymer improved the tensile strength and elongation at break of PLAH‘M]. It also acted as

a nucleating agent and improved the crystallization rate and crystallinity of PLA.

2.5 Toughening of Polymers

The term “toughness” denotes the absorption of energy during deformation of a
material, which ultimately leads to fracture. Several applications of polymeric material
require that these polymers should have the ability to deform and to sustain loads in
various loading modes and should deform in a non-catastrophic way under sudden
impact. Toughening of glassy polymers is more frequently achieved by means of

incorporation of a dispersed rubbery phase. A polymer is considered glassy at

46

temperatures 10 to 20°C below its glass transition temperature (TQM). A polymer glass
usually fails by crazing, yielding or by combination of crazing and yielding. Polymers are
classified in two main types: brittle (Type 1) and pseudoductile ( Type 10”“. The brittle
polymers usually fail by crazing and have low crack initiation and crack propagation
energy. The examples are polystyrene (PS), styreneacrylonitrile (SAN) and
polymethylmethacrylate (PMMA). While pseudoductile polymers have propensity to
fails by mean of yielding and usually have high crack initiation energy but low crack
propagation energy. The examples are polyethylene terephthalate (PET), nylon and
polycarbonate (PC).

Wum] showed that brittle/ductile behavior of a polymer glass depends on the two chain

parameters i.e. entanglement density and characteristic ratio.

Entanglement density, V8, and characteristic ratio, C 00 , a measure of chain stiffness

is given by following equationsm]

, V _ Pa
Entanglement densrty, e “ — ............................................. 2.1
M6
- - - . 2 2
Characteristic ratio. C00 2 Ltm[Ro /nl ] .................................. 2.2
n —) 00

Here pa is amorphous mass density and Me is molecular weight of entangled

strand, R02 is the mean square end-to-end distance of an unperturbed chain, It is number

of statistical skeletal units, and l " rs the mean square length of a statistical skeletal unit.

47

The molecular criteria for craze /yield behavior is shown to depend on the
competition between crazing and yielding. When the crazing stress is lower than the yield
stress, a polymer fails by crazing and it fails in a ductile mode if the condition is vice
versa. The mixed deformation occurs when values of crazing and yield stress are
comparable. An equation which gives the molecular criteria of brittle/ductile behavior is

given by equation 2.3”“.

Here 0‘2 is the crazing stress and a), is the yield stress.

So, polymer having a low entanglement density, V6, and higher characteristic
ratio, Coo will tend to craze rather than yield. On other side, a polymer having higher

V6 and lower Coo will tend to yield rather than craze. Although these criteria for

craze/yield behavior is originated strictly for amorphous polymers but it is also known. to
be applicable for semi-crystalline polymer glasses that don’t contain exceedingly high
crystallinity. In these polymers amorphous phase is dominating in determining the
behavior of the polymer.

A possible route for modification of brittle polymers is to promote large number
of local energy absorption process by incorporating a rubbery dispersed phase. It is now
well established that thermoplastics can be toughened by adding a suitable amount of

[145

rubber 1. In these multiphase systems the rubbery second phase initiates the energy

absorbing process leading to the enhancement of toughness of the polymer. Several

48

brittle glassy polymers such as polystyrene (PS), polymethyl methacrylate (PMMA),
polycarbonate (PC), and polyvinyl chloride (PVC) have been toughened by elastomeric
modificationmm. Such elastomeric modification approaches is also being extended to
semi-crystalline polymers such as polypropylene (PP), polyamide (PA) and other brittle
thermoplastics and thermosetts. Besides this modification several other approaches are
also available to improve the toughness of brittle polymers. The common approaches are:
1. Rubber toughening

2. Blending with tough polymers

3. Particle filled toughening

Rubber toughening is more relevant to this research and is discussed here.

2.5.1 Rubber Toughening

The existence of rubber toughened plastic has been known for several years and
has been researched extensively. The addition of small amount of suitable rubbery
material (~10-25%) can provide the enhancement in the toughness of a brittle polymer by
several orders of magnitude. The most prominent example is the use of butadiene rubber
in polystyrene (PS) and styrene acrylonitrile (SAN) that lead to development of high
impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene (ABS) rubber. Other
brittle amorphous polymers toughened with rubber particle are PMMA and PC. Among
semicrystalline polymers, polypropylene possesses poor impact strength. The impact
properties of polypropylene have been improved by the incorporation of rubbery
polymers such as natural rubber, and ethylene propylene diene monomer (EPDM) rubber

[147]

49

2.5.2 Toughening Mechanism

There are several theories developed for the toughening mechanism of a two-phase
polymer system in which the rubbery particles form the dispersed phase. The commonly
known theories are:

l. Shear yielding

2. Cavitation

3. Multiple crazing

4. Mixed combination of shear yielding, multiple crazing and cavitation.

These theories have been widely applied for toughened amorphous polymer based
systems. In case of semicrystalline polymers, the toughening mechanism in the presence
of rubbery particles becomes complicated due to the interference of crystalline region.
The common phenomena behind these toughening theories are critical ligament
thickness, voiding, cavitation and damage competition.

Shear Yielding

The shear yielding phenomenon corresponds to plastic deformation in the
localized zones in a polymer matrix. In unfilled polymers, shear yielding usually appears
in the form of shear bands but in the case of impact modified systems, usually diffuse
regions of plastic deformation appears in addition to the shear bands. Shear yielding is
one of common toughening mechanism in several rubber modified blends “48’ ‘49]. In the
case of multiphase systems, dispersed-phase particles play the role of stress-concentrators
which cause three-dimensional stress concentration. This induces the volume-expanding
processes of cavitation, interfacial debonding, matrix crazing, and so forth. These

mechanisms provide enough opportunity for the matrix around the particles to produce

50

shear yielding. The stress field caused by neighboring particles will overlap at a certain
interparticle distance and the shear-yielding region in the matrix will be expanded further
[150].
Cavitation

Dispersing rubbery heterogeneity in a polymer matrix led to creation of several
stress centers around the rubbery particles. Stress is usually more at the equatorial plane
of dispersed-phase particles under the application of an outside force. So, the interfacial
debonding between the matrix and particle will occur first at this place, leading to the
formation of microvoids. In another case, if the interfacial adhesion between the particle
and matrix is strong, cavitation in the dispersed rubber particle phase occur, owing to the
difference in Young’s modulus and Poisson’s ratio between the dispersed-phase rubber
particles and the plastics matrix[15 I]. This helps in relieving the triaxial stress state in the
matrix, these microvoids/cavities also absorb deformation or fracture energy to produce
the brittle to ductile transition. This concept of microvoid toughening in polymers has
been studied both theoretically and experimentallylIS 2'15 4].
Multiple Crazing

It is well known that several glassy thermoplastics deform in brittle fashion by

“554571. Crazes are microvoids interconnected with the help of craze

formation of crazes
fibrils. The fibrils can sustain considerable plastic strain but the localized nature of crazes
cause high stress on them, and instead of causing energy dissipation, they tend to break
down, causing the failure of the polymer in a catastrophic manner. In other aspects of

crazes, toughness of a glassy polymer can be enhanced by several orders of magnitude by

incorporating rubbery particles that promote a large density of crazes['58"°°]. There is a

51

stress distribution by creation of numerous crazes, which prevent the catastrophic failure
of a single craze. The rubber particles acts as passive stress concentrators and tend to
initiate crazes around their equatorial region due to difference in the shear modulus of
matrix and rubber particles. A certain degree of adhesion between the matrix and the

rubbery phase is necessary in order to avoid premature craze failure.
2.5.3 Brittle to Ductile Transition (BDT) Criteria

Concept of Matrix Ligament Thickness and Percolation Model

[24’ '61’163] studied the relationship between the chain structure,

Wu and coworkers
phase morphology and toughness in polymers and blends. Wu studied the impact
strength and rubber content of Nylon- 6/EPDM blends. They derived a brittle to ductile
transition (BDT) master curve and proposed a concept of the matrix ligament
thickness, 1:, which is defined as the nearest distance of the matrix between two

neighboring rubber particles (Figure 2.7)['63 1, as a critical parameter for toughening of a

multi-phase system. When the average ligament thickness is smaller than the critical

‘l
v

L

iHT Hi

Figure 2.7: Schematic of Matrix Ligament Thickness.
(Redrawn after ref. 163)

52

ligament thickness, 1, the blend will be tough; when greater, it will be brittle. In other
words, the BDT will occur at r. This is because if L is smaller than t, a plane—strain to
plane—stress transition would occur and the ligament would shear-yield, and the blend
would be tough. On the other hand, if L is greater than t, such transition would not occur,
and the matrix ligament would fail in a brittle fashion.

“64° ‘65] proposed a percolation theory for BDT transition. It

Margolina and Wu
was proposed the interconnectivity of thin ligaments is needed for facilitating the ductile
failure of the matrix by the mechanism of shear yielding. They also put forward the
concept of stress volume spheres, in which a percolation in BDT will occur if the stress
volume spheres associated with rubber particles will overlap with each other. For this
phenomenon to occur the distance between the particles should be below a critical
ligament thickness. The schematic of stress volume sphere surrounding a rubber particle

is given in Figure 2.8[147]. Here d is the diameter of the particle. S is the stress volume of

the rubber particle and 1: is the critical ligament thickness.

 

 

d S=d+13

<——>

 

 

 

Figure 2. 8: Schematic of stress volume sphere around a rubber
particle. Here, shaded area represents a rubber particle.
(Redrawn afler ref 1 4 7)

53

2.6 Alternate Ways of Toughening of Polymers
2.6.1 Interpenetrating Networks

Interpenetrating networks are novel blends composed of crosslinked polymers.
Usually two crosslinked polymer systems are more or less intimately mixed without
having covalent bonds between them. There are two popular techniques for producing
IPN’s “°°]. In the first type, a polymer of type A is crosslinked and then swollen with a
monomer of polymer of type B. This is followed by the in-situ polymerization and
crosslinking of polymer B.

In the second approach, linear polymers, prepolymers, or monomers of the two
types of polymer, along with their respective crosslinking agents, are taken in some liquid
form, e.g. bulk (melt), solution, or dispersion. This is accompanied by the simultaneous
polymerization and crosslinking of the two polymers. The normal blending or mixing of
polymers is usually affected by the problem of phase separation. The well known reason
for this behavior is the large chain length of polymer, which imposes restrictions on the
large gain of entropy upon mixing of polymers. However, In the case of IPN the phase
separation can be prevented by formation of an interpenetrating network. In a perfect
IPN, the only way by which the phase separation can occur, is through breaking of
covalent bond. But IPN are also affected by the phase separation, which usually occur
well before the crosslinking of polymers take place especially in incompatible polymers.
On the other hand, compatible polymers can form perfect LPNs circumventing phase
separation completely.

IPN’s are classified in several types depending on the mode of their

[167]

preparation such as sequential IPN, simultaneous interpenetrating network (SIN) latex

54

IPN, gradient IPN, thermoplastic IPN, Semi-IPN. The interpenetrating networks have
been developed for toughening of thermosetslwg’lm. Among bioplastics, PLLA was
successfully toughened by creating a pseudo-semi-interpenetrating network with
fullerene end-capped poly( ethylene oxide)[m' '73]. There was 100 times increase in the

strain energy of PLLA for this blend.

2.6.2 Nanostructure Blending

Blends of different polymers can exhibit enhanced mechanical, optical and
electro-optical properties, which are not attainable with a single polymer, especially if the
blend morphology is formed at submicrometer or nanometer scales. Fabrication and
designing of thermodynamically stable polymer blends having structures at
submicrometer/nanometer scales poses significant scientific and industrial challenges.
These nanostructured materials present a unique combination of properties, which are
impossible to achieve with classical blends. For polymer blends processed under typical
extrusion conditions, the particle size of the dispersed phase is rarely below 0.1 u (or 100
nm), irrespective of the compatibilization method employed. Polyethylene and polyamide
have been melt blended to obtain new co-continuous morphologies at nanometer

[174

scale 1. The principle interest in the nanostructured polymer blends is because of the

enhanced properties of such materials such as:

- Improved transparency
° Superior toughness
° High heat resistance

55

° Remarkable creep resistance

° Superior stiffness-toughness balance
0 Effective at low concentration of tough polymer
° Retain properties after annealing

In a conventional blending, the micron scale particle of a minority polymer is
dispersed in continuous matrix of other polymer. While, the principle aim of
nanostructure blending is to obtain nanoscale dimension (less than 100 nm) of the
dispersed phase. There are several approaches mentioned for preparation of nanostructure

blends. The most common approaches are: “75'1771

1. Reactive Extrusion ( in situ copolymerization, graft copolymerization)
2. Block copolymerization

3. High shear processing

The nanostructured blends mentioned in the literature arelm'm]:

1. Poly(vinylidene fluoride) (PVDF) and polyamide ll (PAl l.) blends
2. Polypropylene (PP) and polyamide-6 (PA-6) blends
3. Polyethylene (PE)- polyamide (PA) blends
A graft copolymer having affinity for both polymers is normally used as
compatibilizers. When the compatibilizer present above a critical concentration, they tend
to reduce the domain size of minority polymer. A schematic representation of difference

of micro blend from nanoblend is shown in Figure 2.9“781.

56

 

 

 

 

+l——»

Polymer B/or

Conventional
Blending

 

 

Reactive Blending

 

 

 

 

 

Monomer of B or
both

Block Copolymerization

V

 

 

High Shear Processing

 

 

57

 

~lum

.' '00..

...o.oo.'o

0.. . ..
'0 o .

o 'o
000. .0.

-

~1pm

Figure 2. 9: Schematic representation of methods of preparation of conventional
blends and nanostructure blends. (Redrawn after Ref 178)

2.7 References

10.

ll.

Hartmann, M. H., High Molecular Weight Polylactic Acid Polymers. Biopolyrners
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Garlotta, Donald, A Literature Review of Poly(Lactic Acid). Journal of Polymers
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Chamberlain, B. M., Cheng, M., Moore, D. R., Ovitt, T. M., Lobkovsky, E. B.,
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Dow technical datasheet of STYRON 615APR General Purpose Polystyrene
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[cited 03/02/ 2007]; Available from:
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Bhardwaj, R., Mohanty, A. K., Drzal, L. T., Misra, M., Novel ‘Green
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Bionolle biodegradable aliphatic polyesters, Techanical data sheet, Showa
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72

Chapter 3: Modification of Brittle Polylactide (PLA) by Novel Hyperbranched
Polymer based N anostructures

3.0 Introduction

There is an increased emphasis on the enhancement of material properties by
means of structures engineered at the nanometer length scales. The inherently high
surface area to volume ratio of nanometer sized materials plays a key role in enhancing
the desired properties. Recently, nanostructure polymer blends having a minority polymer
phase with nanoscale dimensions, offered much promise because of the enhanced
therrno-mechanical properties, optical transparency and. toughness in comparison to
conventional polymer blends. Reactive blending, utilizing the concept of in-situ
polymerization, graft and block copolymerization leads to creation of nanostructure
blendsn‘zl. So far the nanostructure blending is restricted to a handful of petroleum based
polymers, i.e. polyamide (PA), polypropylene (PP), and poly(vinylidene fluoride)
(PVDF)“'3]. In the midst of these polymers, polylactide (PLA) or polylactic acid is
emerging as promising thermoplastic polyester due to its renewable resource based origin
along with its biodegradability and biocompatibilityl‘”. PLA can exist in three stereo-
chemical forrns: poly(L-lactide) (PLLA), poly(D-lactide) (PDLA), and poly(DL—lactide)
(PDLLA)[5]. The inherent brittleness of PLA has been a major bottleneck for its large-
scale commercial applications. Numerous approaches such as plasticization, block
copolymerization, blending with tough polymers, and rubber toughening have been
adopted to improve the toughness of brittle polylactide bioplastic[°'9]. The major

drawbacks of these methods are the substantial decreases in the strength and modulus of

73

the toughened polylactide. So, a polylactide based material having good stiffness-

toughness balance along with high biobased polylactide content is still elusive.

Previous research suggested that the brittleness of PLA is due to low
entanglement density (V8) and the high value of characteristic ratio (Cm), a measure of

chain stiffnesslm'm. The enantiomerically pure PLAs, such as PLLA and PDLA as well
as amorphous PLA deform in brittle fashion due to the formation of crazesll3 1. Crazes are
microcracks bridged by small fibrils and often lead to the catastrophic failure of a
polymer. In conventional polymers, polystyrene (PS) is a classical example of a craze
forming polymerm]. Micron scale rubber particles are usually dispersed in the PS matrix
to enhance its toughness. In conventional rubber toughening, some of the major
drawbacks are incompatibility between rubber particles and the polymer phase,

requirement of high shear processing and increased melt viscosity.

In the midst of polymers having conventional molecular architecture such as
linear, branched and crosslinked, hyperbranched polymers (HBP) are emerging additives,
which fall in the category of dendritic polymers and are gaining attention due to their
unique structures and properties. Hyperbranched polymers encompass highly branched
nanoscopic structures having high peripheral functionalitiesl15 1. They are polydisperse
and can be prepared in a one-pot synthesis unlike dendrimers, making the former less
costly. Reported roles of hyperbranched polymers include processing aids, branching
agents, compatibilizers and tougheners for conventional thermoplastics and thennosets“°'

19]. However, the unique physical properties and high peripheral functionalities of HBP

offer many other pathways for polymer modification. HBP can play a role of novel

74

building blocks for generating new nanostructures inside a polymer matrix ranging from

[2°], crosslinking of

core-shell to highly networked morphologies. In our recent findings
hydroxyl functional hyperbranched polyester in PLA matrix is demonstrated as a new

method to overcome its brittleness, and the modified PLA showed promise in various

applications.

In this chapter, we report the creation of a novel polylactide (PLA) based
nanoblend by in-situ generation of new hyperbranched polymer (HBP) based
nanostructures in the matrix polymer prepared via an environmentally friendly melt
processing technique. Fourier transform infrared (FTIR), low temperature dynamic
mechanical thermal analysis (DMTA), parallel plate rheology, transmission electron
microscopy (TEM), atomic force microscopy (AFM), tensile testing and scanning

electron microscopy (SEM) were used to characterize the blends.
3.1 Experimental Section
3.1.] Materials

Poly(lactic acid), PLA (BIOMER® L9000) (Mol. Wt.: 188 kDa from GPC) was
obtained from Biomer, Germany. The PLA was the L-forrn of poly(lactic acid). The
hyperbranched polymer (HBP), a biodegradable dendritic polymer having the trade name
BOLTORN® H2004[2”, mo]. wt.-3200 g/mol was procured from Perstorp, Sweden.
BOLTORN®H2004 has 6 primary hydroxyl groups, with a hydroxyl value of 110-135 mg
KOH/g and maximum acid number of 7 mg KOH/g. Polyanhydride, trade name PA-18

(LV) having an average molecular weight ~ 22500 g/mol was obtained from Chevron-

75

Phillips, USA. Here, LV stands for low viscosity. The PA is a linear polyanhydride resin

having a 1:1 mole ratio of l-octadecene and maleic anhydride.
3.1.2 Reactive Extrusion

The PLA was dried for 4 h at 40°C in a vacuum oven before processing. The HBP
was dried for 4 h at 80°C in a convection oven. The blends were prepared by melt mixing
in a microcompounder/microextruder (DSM Research, Netherlands). The instrument is a
co-rotating twin-screw microcompounder having a screw length of 150 mm, L/D of 18,
and barrel volume of 15 cm3. The test specimens were prepared with the help of a mini-
injection molder. mini-injection molder. All compositions are reported on a weight basis.
Initially, PLA was melted in the microcompounder. The hyperbranched polymer (HBP)
was added, followed by addition of an appropriate amount of polyanhydride (PA). The
chemical structures of the polyanhydride (PA) and the chief constituent of HBP are
depicted in Figure 3_1[22. 23]. The hyperbranched polymer was a second generation of
dendritic aliphatic polyester having alkyl and hydroxyls group at the periphery. It was
synthesized from an ABZ type monomer and a B4 type core. The hyperbranched
polyester was based on 2,2-bis-methylolpropionic acid (bis-MPA) as a chain extender
with a pentaerythritol derivative core. There were 6 primary hydroxyl groups (—OH)
groups at the periphery of this HBP. The glass transition temperature (T8) of the HBP was
around —40°C. A multifunctional polyanhydride (PA) was selected as a cross-linking
agent for hyperbranched polymer. PA was a copolymer of octadecene and maleic
anhydride having 1:1 molar ratio. The hydroxyl functionality of HBP was exploited to
cause interparticle cross-linking of HBP to obtain new highly networked morphologies

inside the PLA matrix. The reactivity of hydroxyl (—OH) functional compounds with

76

anhydride groups containing copolymer is well knownm]. The molar ratio of hydroxyl (-
OH) to anhydride group was 1:2. The molar ratio was calculated on the basis of hydroxyl

and anhydride equivalent weight of hyperbranched polymer (HBP) and polyanhydride

OR HO
O O
o 0 OR
to

OR H3 Here R= CO(CH2)nCH3 [n= .6 or 81

(A)

 

(3)

Figure 3.1: (A) Chemical structure of the chief constituent of hyperbranched
polymer (HBP). (B) Chemical structure of polyanhydride (PA).

77

[25] schematically depicts the in-situ crosslinking of HBP

(PA), respectively. Figure 3.2
with PA in PLA matrix performed during melt processing of PLA. The details of
processing parameters and material types are given in Table 3.1. The processing cycle
time of different blends were obtained with the help of variation of extruder force with
cycle time. Here, the cycle time represent the duration of time used for the mixing of
polymers after material feeding and before removal of material from the
microcompounder. In the case of neat PLA, the extruder force increased until the PLA
pellets melted and then dropped with the cycle time. For, PLA/HBP (92/08) blend, the
force dropped remarkably due to the lubricating effect of HBP, which resulted in the

decrease in the melt viscosity of PLA. The addition of polyanhydride (PA) in the

PLA/HBP system caused the extruder force to increase with cycle time until it stabilized

OH + mm 0 O

OH
OH Polyanhydride

Hyperbranched (PA)

Polymer (HBP)
Reaction site of
WO‘

Reactive Blending ' ‘5‘ ’jg
«6‘53.-
{WK/L:
‘ k’lf/

Figure 3.2: Schematic illustrations of in-situ crosslinking of hyperbranched
polymer (HBP) in PLA melt with the help of a polyanhydride (PA). (After ref 25)

OH

 
   

OH

HO

 

HBP based
crosslinked
OH particles

 

78

around 10 minutes of cycle time. This cycle time represented the time for which PLA,
HBP and PA were mixed in microcompounder at a screw speed of 100 rpm. The overall
composition for this blend was PLA/I-IBP/PA (92/5.4/2.6). The increase in the force was
attributed to the crosslinking reaction of HBP with PA in the PLA matrix. A control
sample of PLA/PA (97.3/2.7) blend was also fabricated at the same processing conditions
as that of PLA/HBP/PA (92/5.4/2.6) blend. This blend had a similar ratio of PLA to PA
concentration as the PLA/HBP/PA blend. The purpose of this blend was to investigate the

possible interaction of PA with PLA and its effect on the resulting properties of PLA.

Table 3.1: Processing parameters of neat PLA and its blend with
hyperbranched polymer (HBP), in-situ crosslinked HBP and polyanhydride (PA)

 

 

 

 

 

 

Processing temperature (°C) at

. three zones of mini-extruder Cycle Screw Mold

Materlal System .
(wt %/wt %) trme Speed Temp.

' ' Top center Bottom Melt (Minutes) (rpm) (°C)
Neat PLA 185 185 185 178 3 100 50
PLA/HBP 185 185 185 178 3 100 50
(92/08)
PLA/HBP/PA 185 185 185 178 10 100 50
(92/5.4/2.6)
PLA/PA 185 185 185 178 10 100 50
(973/27)

 

 

 

 

 

 

 

 

 

 

3.1.3 Testing and Characterization

3.1.3.1 Ultracentrifugation

79

Solvent extraction and ultracentrifugation technique was used to determine the
insoluble component in neat PLA and its blends. 0.5 gram of processed blends was
dissolved in 30 ml of methylene dichloride (CHzClz) (Jade Scientific) and stirred for 4 hrs
at room temperature. The solutions were observed for their physical appearance. The
insoluble fraction of the PLA/HBP/PA (92/5.4/2.6) blend was extracted with repeated
cycles of centrifugation (3000 rpm, 30 minutes) and solvent wash. The weight fraction of
the insoluble crosslinked HBP particles formed by the reaction of HBP and PA was
calculated using following equation:

Wins
WHBP + WPA

 

WCHBP = ................................. 3.1

Where,

WcHBP= Weight fraction of crosslinked HBP particles.
Wins = Final weight of insoluble fraction after extraction and drying.
WHBP = Initial weight of HBP in PLA composition.

Wp A = Initial weight of PA in PLA composition.

3.1.3.2 Fourier Transform Infrared Microscopy (FTIR)

The chemical composition of the insoluble fraction of crosslinked HBP and other
materials was evaluated with the help of Perkin Elmer system 2000 FTIR spectrometer

having an ATR assembly.

80

3.1.3.3 Dynamic Mechanical Thermal Analysis (DMTA)

The dynamic mechanical thermal analysis (DMTA) of the neat polymers and their
blends was performed with DMA Q800, TA Instruments, DE. The test was carried out at
a heating rate of 3°C/min from -90 to 150°C in the single cantilever mode. The drive

amplitude was 15 um, while, the oscillating frequency was 1 Hz.

3.1.3.4 Parallel Plate Rheology

Rheological studies of neat PLA and its blends were conducted with an ARES
parallel plate rheometer in dynamic strain frequency sweep mode. The plates had 25 mm
diameter. The gap between plates was 0.5 mm during testing. Testing was conducted in
the frequency range of 0.1 to 100 rad/sec at a strain rate of 1% and at a testing

temperature of 175°C.

3.1.3.5 Transmission Electron Microscopy ('1‘ EM)

A transmission electron microscope (TEM) (JEOL 100 CX) was used to analyze
the morphology of PLA based blends at an accelerating voltage of 100 kV. Microtomed
ultra-thin film specimens with thickness of less than 100 nm were used for TEM
observation. The microtomy was carried out at room temperature using a diamond blade
on a RMC microtome station. The PLA based blend samples were stained with ruthenium

tetroxide (RuO4) before examination.

3.1.3.6 Atomic Force Microscopy (AFM)

Atomic force microscopy (AFM) of microtomed surfaces of modified PLA was
performed on an AF M microscope (Digital Instrument MultiMode SPM with Nanoscope

IV controller, Digital Instruments, NY) in tapping mode. Scan sizes of 10 and 5 pm at a

81

scan rate of 0.5044 Hz were used for AFM analysis. Contrast enhancement of pictures

was also performed.

3.1.3.7 Tensile Properties

Tensile properties of neat PLA and its blends were measured using a United
Calibration Corp SFM 20 testing machine as per ASTM D638. Specimens having a gage

length of 2.54 mm were tested at a strain rate of 15.4 mm/mm-min.

3.1.3.8 Scanning Electron Microscopy (SEM)

A scanning electron microscope, JEOL (model J SM-6400) was used to evaluate
the morphology of and extracted hyperbranched polymers and tensile-fractured surfaces
of the neat PLA and its blends. An accelerating voltage of 15kV was used to produce the
SEM photomicrographs. The samples were sputter coated with gold before SEM

analysis.

3.2 Results and Discussion

3.2.1 Solvent Extraction, Morphology and Fourier Transform Infrared

Spectroscopy

The physical observation of solutions of PLA and its blends in methylene
dichloride are depicted in Table 3.2. All of the solutions were clear in physical
appearance except that of the PLA/HBP/PA (92/5.4/2.6) blend, which was turbid in
appearance. The turbidity was attributed to the scattering of light by the suspended

particles in continuous PLA solution. Methylene dichloride is a reported as a good

82

Table 3.2: The solution behavior of PLA and its
blends after dissolving in methylene dichloride.

 

 

 

 

 

Material
Composrtrons Appearance
(wt. %/wt. %)
Neat PLA Clear
PLA/HBP Clear
(92/08)
PLA/HBP/PA -
(92:5.4:2.6) Turbrd
PLA/PA Clear
(97.3/2.7)

 

solvent for PLAIN. So it was likely that the particles were the reaction product of HBP
and PA. The reaction of HBP and PA could form a crosslinked network due to their

multi-functionality and make them insoluble in a solvent. The insoluble mass was dried
and characterized with SEM and FTIR. The scanning electron microscopy (Figure 3.3) of
extracted crosslinked HBP revealed particle morphology of the extracted insoluble
fraction. The particles were fused with each other after solvent removal. Most of these
particles had dimensions less than 100 nm. The photomicrograph also revealed a large
size distribution of crosslinked particles. In order to confirm the reaction of HBP with
PA, HBP was allowed to react with polyanhydride (PA) in a laboratory flask in the
absence of PLA for 10 minutes at 185°C. The molar ratio of OH to anhydride was 1:2.
There was gelation and formation of a solid mass as a result of the reaction between HBP
and PA. Figure 3.4 represents the FTIR spectrum of PLA, HBP-PA particles (extracted),

HBP-PA particles (reacted in absence of PLA), neat HBP and PA. The FTIR spectrum of

83

 

as? i E 5;: 1 nm

Figure 3.3: SEM photomicrographs of extracted crosslinked HBP particles
from the PLA matrix.

% Transmittance
l
U

 

 

 

l ' I ' I ' I ' l ' l ' l
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm'1)

Figure 3.4: FT IR spectrum: A: Neat PLA; B: HBP-PA particles (Extracted),
C: HBP-PA particles (Outside); D: Hyperbranched polymer (HBP); E:
polyanhydride (PA)

84

extracted particles showed a spectrum having similar peaks to that of HBP and PA. There
was strong reduction in the broadness and intensity of OH peaks (3600-3500cm'l) in the
spectra of extracted particles. On other hand, the FTIR spectra of neat HBP showed
strong absorption in this region due to presence of primary hydroxyl groups in its
structure. The broadness in the peak for HBP was due to the presence of inter— and intra-
molecular hydrogen bonding between HBP molecules. The interpretation of hydroxyl
peaks suggested that the —OH groups of HBP had been reacted with anhydride groups of
PA and formed monoester. Two characteristic anhydride frequency bands at 1855 and
1777 cm'1 in extracted HBP-PA particles attributed to the coupled vibration of symmetric
and asymmetric stretching of two >C=O groups in the anhydride group of the
polyanhydride. These bands attributed to the presence of unreacted anhydride groups in
the extracted particles. The molar ratio of OH to anhydride was 1:2 in the PLA/HBP/PA
blend so it was quite expected that there would be unreacted anhydride groups. The
characteristic anhydride frequency bands of extracted particle matched perfectly with that
of polyanhydride (PA). The fingerprint region (< 1500 cm'l) of the spectra of extracted
particles was in coherence with that of HBP. There were no characteristic bands of PLA
in the spectra of extracted particles. This suggested that the main reaction occurred
between HBP and PA in the PLA matrix, while the PLA chains were physically
entangled in the network of HBP and PA. PLA has firnctional groups (-OH and —COOH)
only at the chain ends, which might not able to react with OH and anhydride group of
HBP and PA, respectively. A further confirmation of the reaction of HBP with PA in the
PLA matrix was performed by comparing the FTIR spectra of reaction product of HBP

and PA in absence of PLA. The FTIR spectra of extracted particles were in complete

85

agreement with that of particle prepared outside. This result confirmed that
polyanhydride had successfully caused the crosslinking of HBP in the PLA matrix. The
weight fraction of extracted crosslinked HBP particles was calculated as per equation 3.1
and was ~0.89 i.e. 89%. So there was high reactivity of HBP with PA in PLA matrix. The
presence of some unreacted HBP and PA was primarily due to steric hindrance created by

the large amount of PLA media.

3.2.2 Dynamic Mechanical Thermal Analysis

Low temperature dynamic mechanical thermal analysis (DMTA) was conducted
to observe the crosslinking of HBP, phase separation and molecular interaction of PLA
with HBP and crosslinked HBP (cHBP). The temperature dependence of the loss
modulus (E ") of neat PLA, PLA/HBP (92/08) blend and PLA/HBP/PA (92/5.4/2.6) is
shown in Figure 3.5. In the PLA/HBP (92/08) blend, there were two loss modulus (E ")
relaxation peaks at -40 °C and 58°C respectively, corresponding to the glass transition
temperatures (Tg) of HBP and PLA, respectively. There was slight depression in the Tg of
PLA in the presence of HBP. This suggested that there was some molecular interaction
between PLA and HBP but this system was dominated by phase separation. Phase
separation has also been a major problem in most of the plasticized PLA systemsm]. The
creation of free volume in plasticized PLA enhances its molecular mobility facilitating
the increase in its crystallinity, which result in the expulsion of plasticizer molecules from
the amorphous region.

On the other hand, the PLA/HBP/PA (92/5.4/2.6) composition revealed two peaks
at -31 and 58°C representing the glass transition temperature (Tg) of crosslinked HBP

(cHBP) and PLA respectively. There was a shift in glass transition temperature from — 40

86

 

 

 

3.0-
‘ /A)
2.5-
J B
53 () (C)
2 2.0-
in Cw M...
3 1.5—
,k.
1.0- /
. U
0.5-
I ' I ' I T r . t . I
-100 -50 0 50 100 150

Temperature, 0C

Figure 3.5: Temperature dependence of the loss modulus (E") of:
(A) neat PLA, (B) PLA/HBP (92/08), and (C) PLA/HBP/PA (92/5. 4/2. 6) blends.

°C for HBP in PLA/HBP blend to —31 °C for cHBP in PLA/HBP/PA blend. This
suggested that polyanhydride (PA) had selectively caused the crosslinking of HBP in
PLA matrix. The increase in the glass transition temperature of crosslinked HBP was
attributed to the restricted segmental mobility and increase in the molecular weight of
HBP after cross linking. It is known that cross-linking helps in reducing the macro-phase
separation in polymer blendslzg]. The present system could be considered as a pseudo-
semi-interpenetrating network system. Here, in the presence of a linear PLA chain, the
hyperbranched polymer was crosslinked and thus leads to the formation of highly
branched networked domains in the PLA matrix. The growing network of HBP during in-
situ reaction could cause physical interlocking with PLA chains and thus helped in

87

reducing the macro-phase separation of HBP from the PLA phase. It is likely that
physical interlocking of the PLA chain with the growing network of HBP occurred at the
phase boundaries between PLA and crosslinked HBP only, primarily due to the limited

miscibility of PLA with HBP.

3.2.3 Rheological Evidence of Networked Interface between PLA and Crosslinked
HBP

The variation of storage modulus, G' with small-amplitude oscillatory frequency
for the neat PLA and its blends is given in Figure 3.6. There was a distinct formation of a
rubbery plateau of highest length in the case of PLA/HBP/PA (92/5.4/2.6) blend unlike
that of neat PLA, PLA/HBP (92/08) and PLA/PA (97.3/2.7) blend in lower frequency

region. The rubbery plateau is determined by the presence of crosslinks and entanglement

 

0 Neat PLA
o PLA/HBP (92/08) Dace:
OUDZAAAQQ
10000 0 PLA/HBP/PA (92/5.4/2.6) DUDE-339999
A PLA/PA (97.3/2.7) CCU (\Aaoooo
ODD:AA:OOO
A OD Daégooo
53 1000 We
:—/ DOD é§88
On 00802355
U) 000033300
3. 100 .
:3 c5338
8 O 56%58
2 10 cos 58980
5)!) 00009333198ng 5
CU QDD AA
H
O
+a
m I
0.1. ..s....., . ...... . -......,
0.1 1 10 100
freq (rad/s)

Figure 3. 6: Behavior of storage modulus, G ' of neat PLA and its blend as a
function of oscillatory frequency ((0)

88

th“'-§!fl=l

in a polymerm]. When observing the lowest frequency region of all curves, it is found
that the PLA/HBP/PA (92/5.4/2.6) blend had the highest value of plateau modulus. Such
increase in plateau modulus is caused by the presence of crosslinks or entangled networks
in a polymer meltm]. So it is believed that the presence of crosslinked HBP network
caused the upward shift in the plateau modulus. On the other hand, the in-situ
crosslinking of HBP with PA in PLA melt caused the formation of crosslinked particles,
which had tendency to form an entangled network with PLA chain preferably at the
interface. The terminal slope of the G’-c0 curve approached zero for PLA/HBP/PA
(92/5.4/2.6) system reflecting its pseudo-solid like behavior. The relaxation and motion
of PLA chains, which was a major fraction in the blend, would have a prominent effect
on rheological properties in the low shear regime. The longer relaxation of PLA chain in
the presence of the HBP-PA particles caused the frequency independence of the storage
modulus in lower shear regime. This behavior also strongly indicated towards formation
of a physical network of PLA chains with HBP/PA particles during their in-situ reaction

in PLA matrix.

3.2.4 Complex Viscosity

Figure 3.7 represents the dependency of the complex viscosity on the oscillatory
frequency for neat PLA, PLA/HBP (92/08) and PLA/HBP/PA (92/5.4/2.6) blends
obtained at 175°C. The neat PLA tended to show shear thinning behavior with increase in
the shear rate. In a previous study, shear thinning behavior of PLA is also reportedm].
There was strong reduction in the complex viscosity of PLA with the addition of

hyperbranched polymer. Kim et al.”” showed that blends of hyperbranched

polyphenylene and polystyrene exhibit a reduced viscosity at high temperatures as

89

10000

 

 

 

E1000
c'cs :
e, l
E‘ 100
g l
S l ‘ Neat PLA
:5 10 l *—-— PLA/HBP (92/08)
3 . PLA/HBP/PA(92/5.4/2.6)
O
Q 1
0.1 la -, , e.
0'1 1 10 100

freq (rad/sec)

Figure 3. 7: Variation of complex viscosity, 17 *, of neat PLA and its blend as
function of oscillatory frequency (0)).

compared to pure polystyrene. In a recent studyml

, a strong decrease in the melt viscosity
of polylactide is reported in the presence of a biodegradable hyperbranched poly(ester
amide). The reduction in melt viscosity of polylactide is attributed to the globular shape
of HBP molecules, which don’t entangle with each other. Their lubrication effect can
separate the PLA chain and introduce large free volume in the PLA. This effect can lower
the frictional forces between the PLA chains and they can slip past each other leading to
the reduction in its melt viscosity. It was interesting to note that in-situ crosslinking of
HBP phase in the PLA matrix did not diminish the viscosity modifier effect of HBP as

the networked structure of crosslinked HBP maintained a good amount of free volume

between the PLA chains.

90

3.2.5 Transmission Electron Microscopy (TEM)

Transmission electron microscopy (TEM) was carried out to evaluate the
morphology and dispersion of pristine and crosslinked HBP particles in the PLA matrix.
The ultra microtomed samples of PLA/HBP (92/08) and PLA/HBP/PA (92/5.4/2.6) blend
were stained with ruthenium tetroxide (RuO4) before TEM observation. Ruthenium
tetroxide selectively stained the ether moiety present in the hyperbranched polymer and
appeared darker than the PLA phase in the TEM photomicrographs. The TEM
photomicrograph (Figure 3.8) of PLA /HBP (92/08) blend revealed the nanoscopic
dimension of HBP (4—6 nm) but the particles were not well distributed in the PLA matrix
and rather appeared as agglomeration. This was attributed to the limited miscibility of
HBP with PLA and presence of strong intermolecular hydrogen bonding between HBP
molecules due to their high peripheral hydroxyl functionalities. Figure 3.9A and 3.9B
revealed the nanoscale phase separated sea-island morphology of the PLA/HBP/PA
(92/5.4/2.6) blend, in which crosslinked HBP particles (dark patches) had heterogeneous
size distribution having dimensions ranging from 50-100 nm. The multi-functionalities of
HBP and PA and random effective collisions of HBP molecules with PA molecules
during their reaction in the large sea of PLA are the possible reasons behind the random
size distribution. The heterogeneity in the morphologies of crosslinked HBP particles
would also arise from the polydispersity of pristine HBP itself and uncontrolled
crosslinking reactions. The variation in stain intensity of crosslinked HBP particles also
indicated the random crosslinking of HBP particles. The phase boundaries of all of the
crosslinked particles were diffused and devoid of any sharp demarcation, which is an

indication of formation of a network between PLA and crosslinked HBP chains. Such

91

networked structures at an interface would increase the compatibility between the two

phases.

 

Figure 3. 8: Transmission electron micrographs of PLA having 8 wt.% of
pristine HBP. (Scale bar: 50 nm).

92

 

Figure 3. 9: Transmission electron micrographs of PLA having crosslinked
HBP. 3. 9A: Scale bar: 200 nm; 3. 93: Scale bar: 100 nm.

93

3.2.6 Atomic Force Microscopy (AFM)

Further confirmation of the formation of HBP based nanoscale particles in
modified PLA was performed by tapping mode atomic force microscopy (TM-AFM)
analysis. There was clear evidence of formation of nanoscale morphologies in the
PLA/HBP/PA (92/5.4/2.6) blend in the phase images (Figure 3.10A & B). The difference
in the viscoelastic behavior of two phases present in the sample caused a phase lag, which
led to an appearance of contrast in the AFM image[3 3 1. The lighter area corresponds to the
soft crosslinked HBP phase while the darker area was due to the stiff PLA phase. There
was heterogeneity in the dispersed phase morphology that correlated well with the TEM
images. The dispersed phase dimensions from AFM were in the range of 80-200 nm. The
difference in the dispersed phase size from AFM and TEM micrographs was obvious as
the tip radii, scan rate and scan size affect the resolution in AFM. Another major
difference was the appearance of lesser number of particles in AFM pictures than TEM
pictures. The AFM was carried out in tapping mode over a smooth surface of modified
PLA. It is possible that some particles were not being exposed to the surface of the
scanned area. On other hand, a thin cross section (~70 nm) of modified PLA was stained
with ruthenium tetroxide and looked at under the electron microscope. So, it was possible
to spot these stained crosslinked HBP particles more easily. So, the TEM can estimate the
larger population of particles and their sizes more accurately than AFM for this sample.
The purpose of characterizing modified PLA with AFM was to validate the two-phase
morphology of modified PLA and the softer nature of crosslinked HBP particles. The

presence of depressions around crosslinked HBP particles in the AFM images was a

94

microtomy artifact. The knife cut the softer crosslinked HBP particle deeper as compared

to the surrounding stiff PLA matrix and therefore left a depression.

 

0 2.5 5-0 mm
Figure 3.10: Tapping mode AFM phase image
of PLA/HBP/PA (92/5.4/2.6) blend 3.10A:

Scan size: 10 ,um; 3. JOB: Scan size: 5 pm

95

3.2.7 Tensile Properties

Stress-strain curves of neat PLA, PLA/HBP (92/08) and PLA/HBP/PA
(92/5.4/2.6) blends are shown in Figure 3.11, while the detail of the measured tensile
properties is provided in Table 3.3. The PLA and PLA/HBP (92/08) blends underwent
strain softening and deformed in brittle fashion. On other hand, PLA/HBP/PA
(92/5.4/2.6) blend showed initial strain softening after yielding and then underwent
considerable cold drawing. The stress-strain curve after the yield point showed a

combination of strain softening and cold drawing. In this region there was competition

80-

‘ (A)
70 -
(B)

60 q (C)
50 -

40-

 

Stress (MPa)

30-

 

 

 

20

10

 

O l ' I ' l ' I ' l ' I ' I

1 0 20 30 40 50 60
Strain (%)

C

Figure 3.11: Stress-strain curves obtained at a cross head speed of 15.4
mm/min: (A) neat PLA; (B) PLA/HBP (92/08); (C) PLA/HBP/PA (92/5. 4/2. 6)

96

Table 3.3: Mechanical properties of neat PLA and its blends

 

 
  
 

 

 

 

 

Properties Tensile Tensile Strength Elongation Toughness
Modulus at Yield (MPa) at Break 3
(GPa) (%) (MJ/m )
Materials
(wt/wt.)
PLA
3.6 10.07 76.5 $1.45 5.] i1 .7 2.6 $0.9
(100/00)
PLA/HBP
3.4 i005 64.2 3:08 5.1 :l:l .0 1.6 $0.7
(92/08)
PLA/HBP/PA
2.8 $0.2 63.9 :t1.7 48.3 326.3 17.4 $1.5
(92/5.4/2.6)
PLA/PA
2.6i0.l 76.2i0.5 3.3i0.2 l.5i0.6
(97.3/2.7)

 

between PLA chain orientation and crack formation. Hence, there was drop in stress with
increasing strain. After 15% of strain, cold drawing dominated with a constant stress.
This suggested that there was large energy dissipation occurred in the presence of
crosslinked HBP particles in the PLA/HBP/PA (92/5.4/2.6) blend. The toughness,
calculated as the area under the stress-strain curve, declined in the PLA/HBP (92/08)
blend as compared to the neat PLA, while it increased dramatically from 2.6 MJ/m3 for
neat PLA to 17.4 MJ/m3 for the PLA/HBP/PA (92/5.4/2.6) blend. This was quite an
unusual result, as the modified PLA maintained a quite high tensile modulus value of 2.8

GPa. The result was very significant in obtaining a biobased material having remarkable

97

stiffness—toughness balance. The tensile strength of modified PLA decreased to 63.9 MPa
from 76.5 MPa for neat PLA. These result suggested that the nanoscale crosslinked HBP
particles behaved like rubber particles and were effective in improving the toughness of
PLA at low concentration (8 wt.%) with a minimal sacrifice of tensile strength and

modulus.

3.2.8 Mode of Deformation of PLA-crosslinked HBP Blend under Tensile Loading

In such a multi-phase system having a dispersed rubbery phase, inelastic
deformation occurs via the phenomena of shear yielding, multiple crazing and cavitation
in the softer dispersed phasem]. PLA is an example of a semi-crystalline glassy polymer
and undergoes brittle deformation via craze formations“3 1. The craze volume fraction (V f)

plays a crucial role in determining the propensity of a craze for catastrophic failure. The

true stress 0,, acting on the craze fibrils, is inversely proportional to the volume fraction

of craze, which is given by the equation 3.2114].

0'

 

at = 00 zlaoo .............................. 3.2

v
f
Here or, is the applied stress and it is the craze extension ratio. So increasing the volume

fraction, v f , of craze can reduce the true stress on an individual craze.

Dispersing an elastic second phase and plasticization with low molecular weight
molecules can provide control over nucleation and growth of crazes and can increase
craze plasticity in glassy polymers leading to a decrease in the brittleness[35’38]. In the

PLA/HBP/PA (92/5.4/2.6) blend, the nanoscale rubbery crosslinked HBP (Tg ~ -30°C)

particles were formed in a continuous PLA matrix. During tensile testing, there was

98

stress-whitening in the sub-fracture region in PLA/HBP (92/08) and PLA/HBP/PA
(92/5.4/2.6) blends (Figure 3.12). The stress whitening was more pronounced in PLA
samples having crosslinked HBP (cHBP) particles i.e. the PLA/HBP/PA (92/5.4/2.6)
blend. There was possibility of the creation of numerous tiny stable crazes nucleated by
crosslinked HBP (cHBP) in the PLA matrix. The stress-whitening occurred due to the

scattering of light by these tiny crazes. Such intense stress whitening was absent in the

Neal PLA I’I,.;'\I"I'l BI’ PLA/l l BPIPA
(OZ/08) (OZ/5.40.6)

 

Figure 3.12: Tensile specimen after testing to failure at 15.4 mm/min
under uniaxial tension.

99

neat PLA sample (Figure 3.12). The neat PLA deformed by craze formation, the crazes
were several microns long and formed normal to the direction of tensile loading (Figure
3.13). The catastrophic deformations of these crazes were the primary reason for the
brittle failure of unmodified PLA. The phenomenon of matrix crazing and shear yielding
were likely the possible energy dissipation mechanisms behind the toughening of
modified PLA bioplastic. It is also possible that the tiny crosslinked HBP particles can be
incorporated into craze fibrils and can help in the extension of these fibrils under tensile

loading.

 

 

Figure 3.13: T EM photomicrograph of deformed region of neat PLA.

The arrow indicates the tensile load direction( Scale bar: 2pm)

100

 

Figure 3.14: T EM photomicrograph of deformed region of modified
PLA; 3.14 A: Extension of crazes; 3.14 (scale bar: 2,1.0n); 3143:
Extension of crosslinked HBP. The arrow indicates the tensile load
direction. (scale bar: 500nm)

101

The TEM photomicrograph of a deformed region of modified PLA revealed large
extension of crazes and deformation of crosslinked HBP in the tensile direction (Figure
3.14). These deformation modes would have contributed strongly towards energy
dissipation in the modified PLA. The PLA/HBP (92/08) blend also exhibited stress
whitening but the sample was failed in a brittle manner. The stability of crazes initiated
by a rubbery phase in a brittle polymer depends on the level of adhesion between the
polymer matrix and dispersed particles. The crazes nucleated by HBP were not stable due
to the incompatibility of PLA with HBP. In the PLA/HBP (92/08) blend, pristine HBP
molecules have strong intermolecular hydrogen bonding due to their abundant hydroxyl
groups, which seems to promoted their aggregation in the PLA matrix and resulted in
macroscopic phase separation and inferior properties.

The deformation in the toughened PLA can be divided in following stages:

Stage 1: Stress concentration: The dispersed rubbery crosslinked hyperbranched
particles in PLA matrix acted as stress concentrators due to their different visco-elastic
behavior. Primarily, the crosslinked HBP particles have lower modulus than the PLA
matrix at ambient temperature due to their low glass transition temperature. This stress
concentration can help in reliving a triaxial stress state in the neighboring PLA matrix.

Stage II: Matrix Crazing/shear yielding: In this stage, the crosslinked HBP can initiate
multiple crazes in the matrix leading to reduction in the stress on the single craze in PLA
matrix. At the same time the thin ligament between two particles can shear yield and can
eliminate the formation of large crazes. There was no observation of cavitation in the

crosslinked particles as seen in TEM of deformed regions of modified PLA. It was

102

observed that the crosslinked particles tend to extend in the tensile direction. By this
mechanism they can absorb considerable strain energy.

Stage III: Necking: After reaching at a critical strain, the stress fields associated with
crosslinked HBP particles can overlap, causing the yield strength to decrease and
facilitating the cold drawing by formation of neck until the sample fails.

The above mentioned mechanical deformation stages were previous reported by Kim
et all”) in PMMA nanocomposite nanofibers prepared from electron spinning process. A
schematic illustration of the mechanical deformation process of PLA in the presence of

crosslinked HBP particles is shown in Figure 3.15.

(111)
(11) 4*

 

 

 

 

 

Tensile Direction

 

 

 

 

 

 

 

 

 

 

Increasing Strain

A
V

Figure 3.15: Schematic illustration of different stages of
mechanical deformation process of PLA in presence of crosslinked
HBP particles.

103

3.2.9 Scanning Electron Microscopy (SEM)

Figure 3.16 represents the scanning electron micrographs of the tensile fractured
surfaces of neat PLA, PLA/HBP (92/08) and PLA/HBP/PA (92/5.4/2.6) blends. The
surface of neat PLA was extremely flat, indicating the brittle failure of PLA under tensile
loading. The tensile fractured surface of PLA/HBP (92/08) revealed inhomogeniety,
formation of voids and absence of ductile tearing. This was an indication of phase
separation, incompatibility between the PLA and HBP, and brittle failure under tensile
loading. The in-situ crosslinking of HBP in PLA matrix dramatically changed its surface
characteristics after deformation. The tensile fractured surface of PLA/ HBP/PA
(92/5.4/2.6) blend exhibited considerable ductile tearing, surface roughness and surface
integrity. The increased surface area of fractured surface of PLA/ HBP/PA (92/5.4/2.6)
blend suggested that the crack paths were highly bifurcated and crack propagation
absorbed considerable strain energy before failure. This led to a conclusion that the in-
situ crosslinking of HBP was instrumental in improving the compatibility between PLA

and HBP phases and resulted in significant toughness enhancement of PLA bioplastic.

104

 

r. L 9 ‘ r >1 .

3.16C M . . 1
Figure 3.16: Scanning electron micrographs of
tensile fractured surfaces: 3.16A: Neat PLA;
3.16B: PLA/HBP (92/08); 3.16C: PLA/HBP/PA
(92/5. 4/2. 6).

105

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108

Chapter 4: Effect of the Concentration of Crosslinker on the Properties of PLA/in-
situ Crosslinked HBP Blends

4.0 Introduction

Dispersing rubbery particles in polymeric matrices is a popular approach for
improving the toughness of brittle polymers. Rubber, being an excellently elastic and
tough material, is the most effective toughening agent for amorphous as well as semi-

2]. Polylactide can be amorphous or crystalline

crystalline brittle thermoplastics“
depending upon the stereochemistry of its repeat unit. PLA behaves as a glassy and brittle
polymer at ambient conditions due to its high glass transition temperature. Rubber
toughening of PLA is mentioned in the literature to overcome its brittleness”). In chapter
3, we reported a new modification technique to overcome the brittleness of polylactide
without a substantial sacrifice in strength and modulus of the materialm. In this
approach, rubbery nanoscale domains of crosslinked hyperbranched polymer were in-situ
created during the melt processing of PLA. The technique is analogous to rubber
modification, where micron or submicron scale particle are usually dispersed in a brittle
polymer.

The well established energy dissipating mechanisms of rubber toughened
polymers are shear yielding, multiple crazing and cavitation in rubber particles. Wuls] put
forward the concept of matrix ligament thickness (surface to surface distance between
two neighboring rubber particles) as an important parameter for the brittle to ductile

transition in rubber toughened blends. A smaller average matrix ligament thickness than a

critical value, will make a blend tough, while it will be brittle in the opposite case. Phase

109

morphology, polydispersity, particle size, flocculation and particle volume fraction have
a direct effect on the matrix ligament thickness[5 1.

In this chapter, in-situ crosslinking of hyperbranched polymer is carried out by
using different molar ratio of polyanhydride in the PLA matrix. The effect of extent of
crosslinking of hyperbranched polymer on the mechanical properties of PLA—crosslinked
HBP (cHBP) blends is studied. The size, polydispersity and distribution of crosslinked
hyperbranched polymer in PLA matrix are evaluated. Contact angle measurements of
PLA, HBP and crosslinked HBP are correlated with the dispersion of particles. The
effects of processing, presence of HBP and cHBP on the molecular weight of PLA are

also determined.

4.1 Experimental Section
4.1.] Materials

Poly(lactic acid), PLA (BIOMER® L9000) (Mol. Wt.: 188 kDa from GPC) was
obtained from Biomer, Germany. The PLA was L-form of poly(lactic acid). The
hyperbranched polymer (HBP), a dendritic polymer having the trade name BOLTORN®
H2004, mol. wt.-3200 g/mol was procured from Perstorp, Sweden. BOLTORN®H2004
has 6 primary hydroxyl groups, with a hydroxyl value of 110-135 mgKOH/g and
maximum acid number of 7 mg KOH/g. Polyanhydride, trade name PA-18 (LV) having
an average molecular weight ~22500 gm/mol was obtained from Chevron-Phillips, USA.
The PA is a linear polyanhydride resin having 1:1 mole ratio of l-octadecene and maleic

anhydride.

110

4.1.2 Preparation of PLA-cHBP Blends

In this part of research, HBP was crosslinked with varying molar ratios of
polyanhydride. The detail of processing parameters is given in Table 4.1. The PLA was
dried for 4 hours at 40°C in a vacuum oven before processing. The HBP was dried for 4
hours at 80°C in a convection oven. The blends were prepared by melt mixing in a

microcompounder/micro-extruder (DSM Research, Netherlands). The instrument is a co-

Table 4.]: Processing parameters of neat PLA and its blends

 

Processrng temperature ( C) 0f Cycle Screw Mold

Material Type
screw zones and melt .
time Speed Temp.

(wt.%/wt.%)

 

Top center Bottom Melt (Minutes) (rpm) (°C)

 

Neat PLA 185 185 185 178 3 100 50

 

PLA/HBP/PA
(9227552045) 185 185 185 179 10 100 50
OH to

anhydride 4:1

 

PLA/HBP/PA
(92:7.4:0.6) OH 185 185 185 178 10 100 50
to anhydride
3:]

 

PLA/HBP/PA
(92:7.1:0.9) OH 185 185 185 176 10 100 50

to anhydride
2: l

 

PLA/HBP/PA
(9226.42126) OH
to anhydride 1:1

185 185 185 178 10 100 50

 

PLA/HBP/PA
(9215,4226) OH
to anhydride 1:2

185 185 185 177 10 100 50

 

 

 

 

 

 

 

 

 

 

111

rotating twin—screw microcompounder having screw length of 150 mm, L/D of 18 and
barrel volume of 15 cm3. The test specimens were prepared with help of a mini-injection

molder. The mold temperature was 50°C.

4.1.3 Testing and Characterization
4.1.3.1 Dynamic Mechanical Analysis

The dynamic mechanical thermal analysis (DMTA) of the neat polymers and their
blends was performed with DMA Q800, TA Instruments, DE. The test was carried out at
a heating rate of 3°C/min from -90 to 150°C in the single cantilever mode. The drive

amplitude was 15 um, while the oscillating frequency was 1 Hz.

4.1.3.2 Ultracentrifugation and Fourier Transform Infrared Spectroscopy

Ultracentrifugation technique was used to determine the insoluble component or
the crosslinked HBP in PLA based blends. 0.5 gram of processed blends were dissolved
in 30 ml of methylene dichloride (CH2C12) (Jade Scientific) and stirred for 4 hrs at room
temperature. The solutions were observed for their physical appearance. The insoluble
fraction of the blends was extracted with repeated cycles of centrifugation (3000 rpm, 30
minutes) and solvent wash. The insoluble fraction was dried for 24 hrs at 40°C before
characterization. The chemical composition of insoluble fraction was evaluated with help
of Perkin Elmer system 2000 FTIR spectrometer having ATR assembly. The weight
fraction of the insoluble crosslinked HBP particles formed by the reaction of HBP and
PA was calculated by equation 4.1:

Wins

W = .................................... 4.1
CHBP W HBP + W PA

 

112

W cHBP = Weight fraction of crosslinked HBP particles.

Wins = Final weight of insoluble fraction after extraction and drying.

W HE P = Initial weight of HBP in PLA composition.

WP A: Initial weight of PA in PLA composition.

4.1.3.3 Tensile Testing

Tensile properties were measured using a United Calibration Corp SF M 20 testing
machine as per ASTM D638. The specimens having a gage length of 2.54 mm, were
tested at a cross-head speed of 15.4 mm/mm-min. At least five replications were done for

each sample.

4.1.3.4 Transmission Electron Microscopy

A transmission electron microscope (TEM) (JEOL 100 CX) was used to analyze
the morphology of PLA based blends at an accelerating voltage of 100 kV. Microtomed
ultra thin film specimens with thickness of less than 100 nm were used for TEM
observation. The PLA based blend samples were stained with ruthenium tetroxide
(RuO4) by dipping the bulk specimens in the solution for 3 hrs. The microtomy was
carried out at room temperature using a diamond blade on a RMC microtome station after

staining the samples.

4.1.3. 5 Contact Angle Measurements

Water contact angles of neat PLA, HBP, PA and crosslinked HBP were measured
with the help of N.R.L. C. A. Goniometer, ramé-hart instrument co., NJ, USA.

Compression molded film having uniform surfaces were used for neat PLA and PA. In

113

the case of HBP, a thin film of semi-solid HBP was spread on a glass slide and used for
measurements. In the case of crosslinked HBP, the extracted particles were annealed at
100°C and pressed into thin films. HPLC grade water was used for drop formation. The
drop volume was 50 ul. The contact angle of the drop with the substrate was measured

using an Olympus Microscope. At least 10 repetitions were conducted for each sample.

4.1.3.6 Gel Permeation Chromatography (GPC)

GPC was used to determine the molecular weight of neat PLA, processed PLA
and PLA extracted from PLA—crosslinked HBP blends. A 600 Multisolvent delivery
system equipped with 717 autosampler and 2410 R1 detector procured from Waters
(Milford, MA) was used to determine the molecular weight of samples. Initially 20 mg of
sample was dissolved in 30 ml of inhibitor free tetrahydrofuran (THF) solution (Sigma
Aldrich, Milwaukee, WI). 1 ml of the filtered solution (filtered with 0.2 mm pore size,
13mm disposable PTFE (polytetra-fluoroethylene) filter procured from Whatman
(Florham Park, NJ) was transferred to the 1 ml clear glass shell vials and capped with
polyethylene snap caps. The glass vials were placed on an autosampler stage. The
injection volume was 100 pl and flow rate of solvent was 1 ml/min. The three

replications for each sample were done.

4.2 Results and Discussion:

4.2.1 Dynamic Mechanical Thermal Analysis

Behavior of loss modulus with respect to temperature was evaluated to observe
the crosslinking of hyperbranched polymer in the presence of polyanhydride in the PLA

matrix. Hydroxy (-OH) to anhydride group molar ratio in the PLA/HBP/PA blends were

114

varied from 4:1, 3:1, 2:1, 1:1 and 1:2. Figure 4.1 represents the effect of molar ratio of
OH to anhydride on the glass transition temperature of crosslinked HBP phase in
PLA/cHBP blends. It is found that with increase in the anhydride molar ratios, there was
an increase in the glass transition temperature of the HBP phase in the PLA matrix
(Figure 4.1). The glass transition temperature of the HBP phase in the PLA/HBP (92/08)
blend was -40°C, while the glass transition temperature of polyanhydride was -27°C as
observed from loss modulus peak of dynamic mechanical analysis. It is found that there
was an increase in the glass transition temperature of cHBP phase with an increase in the

molar concentration of polyanhydride (PA). There was a marginal increase in the glass

 

O . L T g L L
A B C D E
Q . .
o -10 +Exper1mental
a +Fox Equation
g: -20
H .
E
g -30
[d #./—l/.
CD
:6 —40 9
6 a
1
-50 —-‘

Figure 4.1 Effect of OH to anhydride molar ratio on the glass transition temperature
of crosslinked HBP phase in PLA blends where, A: OH/anhydride (4:1); 8:
OH/anhydride (3: 1); C: OH/anhydride (2:1); D: OH/anhydride (1:1) and E:
OH/anhydride (1:2).

115

transition temperature when the OH to anhydride ratios were 4:1 and 3: 1, but increased to
larger extent when the OH to anhydride ratio was 2:1, 1:1. and 1:2. The maximum glass
transition temperature was -29°C when OH to anhydride ratio was 1:2. The experimental
values of glass transition temperature of crosslinked HBP were compared with the
theoretical values obtained from the Fox equation. The Fox equationm is used to estimate
the glass transition temperature (Tg) for miscible blends, random copolymers and
plasticized polymer systems in a bulk state. The Fox equation for the present system can

be represented by the equation 4.2.

 

l/T = WHBP + WPA ..................................... 4.2
chBP T T
gHBP gPA
Here W HBP and W p A are the mass fraction of HBP and PA while T g HB P and T g P A

are the glass transition temperature of HBP and PA in Kelvin. Figure 4.1 represents the
comparison of experimental and theoretical values of glass transition temperature of the
cross linked HBP as a function of OH to anhydride ratio. It is observed that the
crosslinked HBP tended to follow the concept of simple mixture when the OH to
anhydride ratio was more towards the hydroxyl group i.e. 4:1 and 3:1. There were
considerable deviations from Fox prediction in the values of glass transition temperature
for blends when the OH to anhydride ratios were 2:1, 1:1 and 1:2. This was due the
higher extent of crosslinking reaction between HBP and PA functionalities that led to the
formation of networks in the crosslinked HBP phase at these molar ratios. The extent of
deviation from theoretical values was more pronounced when the molar concentration of
polyanhydride was sufficiently high. This result suggested that crosslinking of HBP
occurred by reaction of its hydroxyl functionality with the anhydride functionality of

116

polyanhydride (PA). The behavior of the loss modulus with respect to temperature for
neat PLA and some selected blends is shown in Figure 4.2. It is interesting to note that
there was no significant increase in the glass transition temperature of crosslinked HBP.
It suggested that the crosslinking of HBP with polyanhydride resulted in a networked
structure, where a sufficient molecular mobility can occur at low temperature. The low

glass transition temperature of crosslinked HBP showed its rubbery nature at ambient

 

 

3.0-
,, 2.5-
O—c
21
E
3 2.0-4
O
5 1
E
1.5-
LL]
8"
._r 1
1.04
0.5-

 

 

! l ' l ' I ' I
-100 -50 0 50 100
Temperature, 0C

Figure 4. 2: Temperature dependency of loss modulus of neat PLA and its
blend with HBP and in- situ crosslinked HBP; (A) Neat PLA: (B) PLA/HBP
(92/08); (C) PLA/HBP/PA (92: 7.1 :0. 9) OH to anhydride 2:]; (D)
PLA/HBP/PA (92: 7.1.0. 9) OH to anhydride 1:1; (E): PLA/HBP/PA
(92: 7. 1:0. 9) OH to anhydride 1 :2

117

conditions. On the other hand, there was small decrease in the glass transition
temperature of PLA with addition of hyperbranched polymer. This result indicated the
presence of a small interaction of HBP with PLA. The in-situ crosslinking of HBP did not

bring any further change in the glass transition temperature of PLA.

The glass transition temperature of hyperbranched polymers is reported to depend
on the nature of the chain endsm. The lower polarity of the end group lowers the glass
transition temperature of a particular HBP. It is well established that glass transition
temperature of polymers is primarily due to the segmental mobility of the polymer
chains. But for HBP, it has been proposed that the glass transition occurs due to a
translational movement of the entire molecule instead of a segmental movementm. In
present case, the end hydroxy group of HBP reacted with anhydride groups of PA to form
crosslinks between different HBP molecules thus restricting their translation motion and
resulted in the increase in the glass transition temperature of HBP alter crosslinking. It is
well known that the crosslinking reduces the molecular mobility of networked systems
consisting of reactive polymersm. On the other hand, the extent of depression of glass
transition suggests that there was crosslinked HBP particles were in rubbery state in the

PLA matrix at room temperature for all hydroxy to anhydride ratios.

4.2.2 Ultra-Centrifugation and Fourier Transform Infrared Spectroscopy of

Crosslinked HBP

Table 4.2 represents estimation of reaction product of HBP and PA in the PLA
matrix. It was observed that the weight fraction of tightly crosslinked particles, which
were insoluble in methylene dichloride solution, increased until the OH to anhydride ratio
reached to 1:1 and then it leveled off. There was less formation of highly networked

118

particles, when the OH to anhydride ratio was more towards hydroxy group, i.e. 4:1 and
3:1. In these two compositions, there was partial crosslinking of HBP molecules with
polyanhydride, which were still soluble in methylene dichloride. It is well established that
a polymer system becomes insoluble in solvents after having sufficient crosslinking. This

result suggested that OH to anhydride ratio of 2:1, 1:1 and 1:2 were most effective in

Table 4.2: Estimation of amount of crosslinked hyperbranched polymer (using
equation 4.1) formed in various PLA blends after in-situ crosslinking of
hyperbranched polymer with different concentration of polyanhydride.

 

 

 

 

 

 

 

Initial weight
wt 0/ of Initial wt.% Fraction of In- Actual wt.% of
Material System HBP) in of PA in soluble Crosslinked
blend Blend Crosslinked HBP particles
HBP Particles
Neat PLA - - - -
PLA/HBP/PA
(92:7.55:0.45) 7.55 0.45 0.4li0.09 3.3
OH /anhydride
4:]
PLA/HBP/PA
(92:7.4:0.6) 7.4 0.6 0.55i0.07 4.4
OH/anhydride
3:1
PLA/HBP/PA
(92:7.1:0.9)OH/ 7.1 0.9 0.70:0.08 5.6
anhydride 2:1
PLA/HBP/PA
(92:6.4:1:6) 6.4 1.6 0.88:0.08 7.0
OH/anhydride
1:1
PLA/HBP/PA
(92:54:26) OH 5.4 2.6 0.90:0.05 7.2
/anhydride 1:2

 

 

 

 

 

 

 

119

generating sufficient crosslinking of hyperbranched polymers.

-F
E
-D

of .

01

$2

53

.:_C

E

U)

:2

:6

E B

c\°-'
I—A

 

 

3500 3000 2500 2000 1500 1000 500
Wavenumber (cm'l)

Figure 4.3: F T IR spectrum: A: Polyanhydride; B: HBP, C: Extracted
HBP-PA (2:1); D: Extracted HBP-PA (1:1); E: Extracted HBP-PA (1:2); F:
Neat PLA.

The FTIR spectrum of neat PLA, neat polyanhydride, neat HBP and extracted
crosslinked hyperbranched polymers from the PLA matrix having OH to anhydride ratios
i.e. 2:1. 1:1 and 1:2 are shown in Figure 4.3. The FTIR spectra suggested that the -OH
groups of HBP readily reacts with anhydride functionalities of polyanhydride as a result
there was a strong reduction in the absorption due to hydroxyl group in crosslinked HBP
to that of HBP only. In case of extracted HBP particles, where OH to anhydride ratio

was 2:1 and 1:1, there was no presence of unreacted anhydride group, while in case of

120

cHBP (OH to anhydride 1:2), there was absorption corresponding to anhydride peaks. So
it is found that in PLA/cHBP blend, the presence of unreacted anhydride functionality
can be minimized by using a lower amount of polyanhydride as a crosslinking agent or in
other words, the HBP can be easily crosslinked by keeping low molar ratio of
polyanhydride. It is expected that there might be a formation of a pseudo semi-
interpenetrating network by in-situ crosslinking of HBP in the PLA phase. A perfect
semi—interpenetrating network is usually formed by swelling a linear polymer with a
monomer, followed by its polymerization and subsequent crosslinking”). The formation
of a semi-interpenetrating network also depends on the solubility of monomer in the
linear polymer and the kinetics of the crosslinking reaction. In a previous research, brittle
polylactide (PLA) was toughened by a poly(l-caprolactone)(PCL) diol- and triol-based
polyurethane (PU) semi—interpenetrating networkHO]. In the present case, there were no
characteristic FTIR absorption peaks of PLA in the FTIR spectra of the extracted
crosslinked particles. This suggests that there was no true interpenetrating network
between PLA and crosslinked HBP particles. There is possibility of an entangled network

between crosslinked HBP and PLA, preferably at the interface of two phases.

4.2.3 Tensile Properties

The effect of varying HBP to PA weight contents, determined on the basis of
OH/anhydride molar ratios, on the tensile properties of PLA is shown in Table 4.3. It is
observed that a HBP to PA molar ratio as low as 2:1 was effective in improving the
elongation at break of PLA. The PLA/HBP/PA (92:7.1:0.9) blend, where the OH/
anhydride molar ratio was 2:], showed about 620% improvement in the elongation at

break as compared to neat PLA, while in the case of PLA/HBP/PA (92:6.4:1:6) blend

121

Table 4.3: Effects of HBP/ PA ratio on the tensile properties of PLA —cHBP blends.

 

 

 

 

 

 

 

 

Material Type Tensile Modulus, Tensile Strength at Elongation at
GPa Yield, MPa Break, %

Neat PLA 3.6i0.l 77 i1 .5 5 321.7

PLA/HBP/PA

(92:7.55:0.45) OH 3.3 i0.1 60 i1 .3 9 i1.2

to anhydride 4:1

PLA/HBP/PA

(92:7.4:0.6) OH to 3.2 i0.2 59 i0.4 18 i1.2

anhydride 3:]

PLA/HBP/PA

(92:7.1:0.9) OH to 3.2 i0.l 59 i1.5 37 i8.5

anhydride 2:1

PLA/HBP/PA

(92:6.4:1:6) OH to 3.2 $0.2 60 il.l 50 il.6

anhydride 1:1

PLA/HBP/PA

(92:54:26) OH to 2.8 i0.2 64 i1.7 48 i6.3

anhydride 1 :2

 

 

 

 

 

(OH/anhydride, 1:1) and PLA/HBP/PA (92:5.4:2.6) (OH/anhydride, 1:2), the percent
improvement was 877% and 847%, respectively. The tensile modulus and strength values
of these blends were not statistically different from each other. Figure 4.4 represents the
variation of elongation at break with respect to the actual amount of crosslinked HBP
particles in the blend. It was found that elongation at break increased with increase in the
weight % of crosslinked particles and reached a plateau for the PLA-cHBP Blends, where
OH to anhydride ratio was 1:1 and 1:2. The amount of polyanhydride cross linker tend to

affect the elongation at break value of PLA-cHBP blend. The crosslinked HBP particles

122

could be have sufficient crosslinked density and its concentration when the amount of
polyanhydride was above a certain value, which is 0.9 wt.% in the composition where
OH to anhydride ratio was 2: 1. Among all these compositions, the OH to anhydride ratio
of 1:1 in a PLA-cHBP blend is likely to be the best optimization of OH/anhydride molar
ratio. The reason for this selection is not the elongation at break only but there were
minimum unreacted hydroxyl groups as well as anhydride groups in crosslinked HBP

particles at this molar ratio.

60:

50» E

 

30 ' __

20 ,. I

Elongation at Break (%)

10‘ i
I

O ' "W" ' ’7" ‘ ' f""' i lV'Wi' ' ' i 1"“: I I i'TT’I 1 l {TUTTI I T iifivif" 1 i
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Weight % of Crosslinked HBP

Figure 4.4: Effect of weight% of crosslinked HBP particles on
the elongation at break value of PLA -cHBP blends

123

4.2.4 Transmission Electron Microscopy

Transmission electron microscopy (TEM) was used to study the effect of OH to
anhydride ratio on the size and dispersion of crosslinked HBP in the PLA blends. It is
observed that with an increase in the polyanhydride molar ratio, there was an increase in
the dispersion of crosslinked HBP particles in PLA-cHBP blends. TEM
photomicrographs (Figure 4.5) of PLA/HBP/PA (92:7.55:0.45) OH to anhydride, 4:1 and
PLA/HBP/PA (92:7.4:0.6) OH to anhydride, 3:1 blends showed low concentration and
sparse distribution of the crosslinked HBP particles, but there was an increase in the
concentration of particles when the OH to anhydride ratio were 2:1, 1:1 and 1:2 (Figure
4.6, 4.7). This trend suggests that the increase in the concentration of polyanhydride
provided more HBP molecules to crosslink with each other. There was evidence of better
particle distribution with an increase in the polyanhydride concentration in PLA-cHBP
blends. There was polydispersity in the domain size of crosslinked hyperbranched
polymer in all blends, which vary from 50 nm to 400 nm. In term of mechanical
properties, the PLA-cHBP blends having 2:1, 1:1 and 1:2, OH to anhydride ratio, showed
better ductility as compared to those of having 4:1 and 3:1 ration. So the presence of
significant number of particles in these blends helped in the energy dissipation of PLA-
cHBP blends. It is well established in other parallel toughening methodologies such as
rubber toughening that a critical concentration of rubber particles is required for the

brittle to ductile transitionlI 1’ ‘2].

124

 

Figure 4. 5: T EM photo-micrographs
A: PLA/HBP/PA (92: 7. 55 :0. 45) OH to anhydride, 4:]
B: PLA/HBP/PA (92: 7.4 :0. 6) OH to anhydride, 3:]

125

 

     

.q‘ r

Figure 4. 6: T EM photo-micrographs
A: PLA/HBP/PA (92: 7. 1:0. 9) OH to anhydride 2:1
B: PLA/HBP/PA (92:6. 4:1 :6) OH to anhydride 1 :1

126

 

Figure 4. 7: TEM photomicrograph of PLA/HBP/PA
(92/5. 4/2. 6). OH to anhydride ratio 1. 2.

4.2.5 Contact Angle Measurements

Water contact angle measurements of neat PLA, HBP, PA and crosslinked HBP
having variable amounts of PA, were conducted to correlate the extent of crosslinking of
hyperbranched polymer to its dispersion in the PLA matrix. Table 4.4 provides the values
of contact angle of various materials with water. PLA showed a contact angle 60° with
water; on other hand HBP had a contact angle of 13° only. This suggested that HBP had
more wettability with water, which reflected its hydrophilic nature. The chemical
structure of HBP has a good number of hydroxy groups on its periphery, which makes it
hydrophilic in nature. There was a large difference in the contact angle of HBP and PLA
reflecting the differences in the surface energy of two materials. When HBP was

crosslinked with polyanhydride, there was considerable increase in the value of water

127

contact angle. The water contact angle of polyanhydride film was 88°. The polyanhydride
has a polar anhydride group in its structure. So it is like that it should have a good affinity
with water. However, the chemical structure of PA has a long hydrocarbon (-C13H33)
stretch in its backbone. This hydrocarbon tail was responsible for the high water contact
angle of PA. The incorporation of PA changed the surface characteristic of HBP from
hydrophilic to considerably hydrophobic. This was attributed to the increase in the
crosslinked density and incorporation of more hydrophobic PA in the crosslinked HBP
network. The difference in the contact angle of crosslinked HBP and neat PLA was less
than that of neat PLA and pristine HBP. This result indicates that there was less
difference in the surface energies of crosslinked HBP and PLA. This property of

crosslinked HBP helped in improving its dispersion inside the PLA matrix.

Table 4. 4 .' Contact angle of various materials with water.

 

 

 

 

 

Material Type Contact angle, 0
Neat PLA 60i3
Hyperbranched Polymer l 3i2
(HBP)

Polyanhydride 88i3
Extracted crosslinked HBP 32i6

(2:1 OH/anhydride)

 

Extracted crosslinked HBP 78i5
(l :1 OH/anhydride)

 

Extracted crosslinked HBP 83i7
(1 :2 OH/anhydride)

 

 

 

 

128

4.2.6 Effect of Processing, Presence of HBP and Crosslinked HBP on the Molecular

Weight of PLA

Figure 4.8 represents the weight average molecular weight, number average
molecular weight and polydispersity index of neat PLA, processed PLA, and PLA
extracted from the blends of PLA-cHBP blends. The weight average molecular weight of
PLA decreased by 2 and 7% with the processing cycle of 3 and 10 minutes, respectively.
On the other hand, the number average molecular weight of PLA was unaffected at 3
minute of cycle time but it decreased by about 13% when PLA was processed for 10

minutes. The polydispersity index was unaffected at 3 minutes of processing cycle time

+ IVh + Pl + MN
200000 7 2

a M 5 E
4: ‘ a)
.99 ' 'o
g .5
E; 100000 - 1.5 *3
3 33‘
8 * %
T3 , ' 6
2 50000 , M 1.25 %-

j O4

0 1 - 1

A B C D E F G

Figure 4.8: Molecular weight and polydispersity of PLA in various
processing conditions, and in presence of HBP and crosslinked HBP: A: Neat
PLA; B: PLA (3 min.); C: PLA (10min.); D: PLA/HBP (92/08); E:
PLA/HBP/PA (92: 7.1 :0. 9) OH to anhydride 2:]; F: PLA/HBP/PA
(92: 7.1.0. 9) OH to anhydride 1:]; G: PLA/HBP/PA (92: 7.1 :0. 9) OH to
anhvdride 1:2

129

but increased from 1.21 to 1.29 when PLA was processed for 10 minutes. In previous
research, a decrease in the molecular weight of PLA was observed after melt
processing“3 1. The molecular weight of the PLA decreased to some extent in the presence
of HBP and crosslinked HBP but it was not drastic in nature. The blends of PLA with
HBP and cHBP were processed for 10 minutes. The melt processing of PLA in the
presence of pristine HBP caused 7% loss in weight average molecular weight and 13%
loss in number average molecular weight. The presence of hydroxyl functional
compounds is found to reduce the molecular weight of biopolyesters such as

polyhydroxybutyratel ‘4’ 15]

. So, it is likely that the presence of hydroxyl groups in
hyperbranched polymer contributed to the reduction in the molecular weight of PLA. It
was observed that there was a decrease in weight and number average molecular weight
of PLA when polyanhydride crosslinker was introduced in the PLA-HBP blend. There
was around 6-7 % loss in weight average molecular weight and 12-14% decrease in the
number average molecular weight of PLA in the presence of cHBP having 1:1 and 1:2
OH to anhydride ratio when compared to the molecular weight of the processed PLA (10
minutes). In the case of PLA—cHBP blend where HBP to OH ratio was 2:1, there was no
significant decrease in the molecular weight. The polydisperse index (PI) of these blends

was not significantly different from each other. The polydispersity index (P1) was

increased in the processed blend as compared to neat PLA.

130

4.3 References

10.

11.

12.

13.

Bucknall, C. B., Rubber Toughening. 2nd ed. Physics of glassy polymers, ed.
R.N. Harward, Young, R. J. 1997, London: Chapman & Hall.

Liang, J. Z., Li, R. K. Y, Rubber Toughening in Polypropylene: A Review.
Journal of Applied Polymer Science, 2000. 77: p. 409-417.

Jin, H. J., Chin, I. J., Kim, M. N., Kim, S. H., Yoon, J. S., Blending of poly(L-
1actic acid) with poly(cis-1,4-isoprene). European Polymer Journal, 2000. 36: p.
165-169.

Bhardwaj, R., Mohanty, A. K., Modification of Brittle Polylactide by Novel
Hyperbranched Polymer-Based Nanostructures. Biomacromolecules, 2007. 8: p.
2476-2484.

Wu, 8., A Generalized Criterion for Rubber Toughening: The Critical Matrix
Ligament Thickness. Journal of Applied Polymer Science, 1988. 35: p. 549-561.

Fox, T. G. , Bull. Am. Phys. Soc, 1956. 1: p. 1.23.

Kim, Y. H., Beckerbauer, R., Role of End-Groups on the Glass-Transition of
Hyperbranched Polyphenylene and Triphenylbenzene Derivatives.
Macromolecules, 1994. 27(7): p. 1968-1971.

Gan, 8., Seferis, J. C., Prime, R. B., A Viscoelastic Description of the Glass

Transition-Conversion Relationship for Reactive Polymers. Journal of Thermal
Analysis, 1991. 37(3): p. 463-478.

Sperling, L. H., Mishra, V., The current status of interpenetrating polymer
networks. Polymers for Advanced Technologies, 1996. 7(4): p. 197-208.

Yuan, Y., Ruckenstein, E., Polyurethane toughened polylactide. Polymer Bulletin
1998. 40: p. 485-490.

Bucknall, C. B., Cote, F. F. P., Partridge, I. K., Rubber Toughening of Plastics .9.
Effects of Rubber Particle-Volume Fraction on Deformation and Fracture in
HIPS. Journal of Materials Science, 1986. 21(1): p. 301-306.

Bucknall, C. B., Davies, P., Partridge, I. K., Rubber Toughening of Plastics .10.
Effects of Rubber Particle-Volume Fraction on the Kinetics of Yielding in HIPS.
Journal ofMaterials Science, 1986. 21(1): p. 307—313.

Gogolewski, S., Jovanovic, M., Perren, S. M.,Dillon, J. G., Hughes, M. K., The
Effect of Melt-Processing on the Degradation of Selected Polyhydroxyacids -

131

14.

15.

Polylactides, Polyhydroxybutyrate, and Polyhydroxybutyrate-Co-Valerates.
Polymer Degradation and Stability, 1993. 40(3): p. 313-322.

Femandes, E. G., Pietrini, M., Chiellini, E., Bio-based polymeric composites
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Chiellini, E.; Femandes, E. G.; Pietrini, M.; Solaro, R., Factorial design in
optimization of PHAs processing Macromol. Symp, 2003. 197 : p. 45-55.

132

Chapter 5: Phase Morphology and Brittle-to-Ductile Transition in PLA-
crosslinked HBP Blends

5.0 Introduction

Toughness is a very important property of polymeric materials for many
practical applications. Polymer modification techniques such as plasticization, co-
polymerization, rubber toughening, and polymer blends are commonly employed to
improve the toughness of brittle polymers. One of the desired aims of polymer
modification is to develop a polymeric system having high toughness and large
plastic deformation at break, while retaining the other desirable properties such as
stiffness and strengthm. Rubber toughening has been the most popular approach to
overcome the brittleness of amorphous and semi-crystalline polymers. In such
systems, rubber particles present as a dispersed phase in the continuous matrix of the
brittle polymer and initiate the toughening of polymer by mean of shear yielding or
multiple crazing of the matrix, and or cavitation in rubbery phase. The toughening
mechanism is usually a complex combination of these events in the multi-phase
polymer systems.

The semi-crystalline and amorphous polylactide fails under tensile loading by
the formation of crazesm. In conventional polymers, polystyrene (PS) deforms by
formation of crazes. Catastrophic failures of such polymers usually occur by
premature failure of crazes. Wum has correlated chain parameters such as
characteristic ratio and entanglement density with craze formation in a polymer. On
the other hand, various studies report that the toughness of glassy polymers can be

improved considerably by incorporating rubbery particles that initiate a large density

133

[4’ 5]. High-impact polystyrene (HIPS) is a well known system, where

of crazes
multiple crazing is a well established toughening mechanism. The rubber particles
can initiate crazes in the HIPS leading to reduction in the stress on a single craze and
thus can prevent localized deformation as compared to neat polystyrene. Bubeck et
al.[°] found that in high-impact polystyrene (HIPS), cavitation in rubbery particles and
deformation in the thin glassy ligament confined between rubber particles contribute
significantly to the deformation mechanism, while the crazing is responsible for
roughly half of the total plastic deformation. Wum put forward the concept of critical
matrix ligament as an essential criterion for brittle to ductile transitions in rubber
modified blends. Wu observed that for a brittle to ductile transition, the matrix
ligament, i.e. region of matrix between two nearest rubber particles, must be equal or
smaller than a critical matrix ligament thickness. The critical matrix ligament is found
to be the characteristic of the matrix and influenced by the mode, temperature and
rate of deformation. A percolation model for brittle —tough transition in the nylon-
rubber blend was also proposed“, which was based on multiple connectivity of thin
matrix ligaments and overlapping of stress field associated with rubber particles at
particular matrix ligament thickness. In polylactide based systems, a matrix ligament
thickness of 1pm was observed for PLA-LLDPE blendm.

In this chapter, different weight contents of crosslinked HBP particles having
OH to anhydride ratio (1:1) were in situ synthesized in PLA-crosslinked HBP (cHBP)

blends. The effects of the various weight contents of crosslinked HBP particles on the

tensile properties of the blend were studied. Particle size, particle size distribution,

134

and interparticle distance were studied and correlated with the brittle-to-ductile

transition in PLA-cHBP blends

5.1 Experimental Section
5.1.1 Reactive extrusion for fabrication of PLA-cHBP Blends

The PLA was dried for 4 hours at 40°C in a vacuum oven before processing.
The HBP was dried for 4 hours at 80°C in a convection oven. The detail of all
composition (wt.%/ wt.%) and their processing parameters are shown in Table 5.1.

The blends were prepared by melt mixing in a microcompounder/micro-extruder

Table 5.1: Details of various PLA based compositions and their processing
parameters

 

 

 

 

 

 

 

 

Processing temperature (°C) at
. three zones of mini-extruder Cycle Screw Mold
Materral Type .
o 0 time Speed Temp.
(W W“ A” (Minutes) ( m) (°C)
Top Center Bottom Melt rp
Neat PLA 185 185 185 178 3 100 50
PLA/HBP/PA
(98/1 .6/0.4) 185 185 185 179 10 100 50
PLA/HBP/PA
(96/3.2/0.8) 185 185 185 177 10 100 50
PLA/HBP/PA
(92/6.4/1.6) 185 185 185 178 10 100 50
PLA/HBP/PA
(88/9.7/2.3) 185 185 185 177 12 100 50
PLA/HBP/PA
(84/12.9/3.1) 185 185 185 178 15 100 50

 

 

 

 

 

 

 

 

 

 

135

(DSM Research, Netherlands). The instrument is a co-rotating twin-screw
microcompounder having screw length of 150 mm, L/D of 18 and barrel volume of
15cm3. The test specimens were prepared with help of a mini-injection molding
machine. Initially, neat PLA was fed in to the microcompounder followed by a
known amount of hyperbranched polymer (HBP) and polyanhydride (PA). The
reaction was observed by monitoring the behavior of the extrusion force with time,
and the appropriate cycle time was adopted depending the constancy or decrease in

the force with time.
5.1.2 Testing and Characterization

5.1.2.1 Transmission Electron Microscopy (TEM)

A transmission electron microscope (TEM) (JEOL 100 CX) was used to
analyze the morphology of PLA based blends at an accelerating voltage of 100 and
120 kV. Microtomed ultra thin film specimens with a thickness of less than 100 nm
were used for TEM observation. Before microtomy, the bulk specimens of all blends
were dipped in ruthenium tetroxide (RuO4) solution for 3 hrs. The microtomy was

carried out at room temperature using a diamond blade on a RMC microtome station.

5.1.2.2 Particle Size Analysis

The particle size analysis was performed with help of the Universal analysis
software. The particle diameter, d, and interparticle distance were manually
calculated using arbitrary distance measurements. This analysis was performed on at

[10-12]

least 150 particles. The following equations were used for calculations.

136

N
Zn; 1nd,-
_ i=1
1nd- ................................................... 5.1

 

Here, di is the diameter of ith particle.

 

 

 

N 2
Zniandi —lnd)
_ i=1
1110‘} N ................................. 5.2
Di
i=1

 

Here, 6 represents the particle size distribution parameter, 6:] for monodisperse

system and o>1 for polydisperse system.

5.1.2.3 Density and Volume Fraction of Crosslinked HBP Particles

The density of extracted crosslinked particles was calculated by dropping
them in different concentrations of aqueous sodium chloride (NaCl) solutions. The
particle were floating in salt solutions that have density more than 1.089 g/cc, while
were sinking in solutions having density less than 1.059 g/cc. The density of solution,
where HBP particles suspend in the solution was considered as density of particle. A
table of density of various salt solutions is given in the appendix of the dissertation.
The average density of the particles was found to be 1.073 gm/cc. The volume

fraction of crosslinked HBP particles, (icmgp, was calculated from the equation 5.3.

¢cHBP = pm WcHBP /{(,0m - PcHBP)WcHBP + ,oCHBp} ........................ 5.3

137

Here, pm, PcHBP are the density of polylactide, 1.25 gm/cc and the crosslinked
HBP, 1.073 gm/cc. The weight fraction, WcHBP, of actual crosslinked particles in

each composition was calculated by equation 5.4 and the value was used in equation
5.3.

Wins
W HBP + W PA

........................................................... 5.4

 

W cHBP =

WcHBP= Weight fraction of crosslinked HBP particles.
Wins = Final weight of insoluble fraction after extraction and drying.
W HB p = Initial weight of HBP in PLA composition.

W p A = Initial weight of PA in PLA composition.

5.1.2.4 Tensile Properties

Tensile properties were measured using a United Calibration Corp SFM 20
testing machine as per ASTM D638. The specimens having a gage length of 2.54

mm, were tested at a strain rate of 15.4 mm/mm-min.

5.1.2.5 Scanning Electron Microscopy (SEM)

A scanning electron microscope, JEOL (model ISM-6400) was used to
evaluate the morphology of tensile—fractured surfaces of the neat PLA and its blends.

An accelerating voltage of 13 kV was used to produce the SEM photomicrographs.

5.2 Results and Discussions

5.2.1 Phase morphology of PLA-crosslinked HBP Blends

138

 

Figure 5.1: Transmission Electron Photomicrograph (Scale bar: 500
nm); A: (PLA/HBP/PA (98/1. 6/0. 4)
B: PLA/HBP/PA (96/3. 2/0. 8)

139

 

’Iv': in

Figure 5.2: Transmission Electron Photomicrograph (Scale Bar: 500
nm); A: PLA/HBP/PA (92/6. 4/1. 6)
B: PLA/HBP/PA (88/9. 7/2. 3)

140

 

Figure 5.3: TEMphotomicrograph (Scale bar: 500 nm) of
PLA/HBP/PA (84/12.9/3.1)

The phase morphology of PLA-crosslinked blends were investigated with the help of
transmission electron microscopy (TEM). TEM photomicrographs (Figure 5.1, 5.2
and 5.3) of all blends showed the two-phase morphology of the PLA-cHBP blends,
where the cross linked HBP particles (dark phase) were present as a dispersed phase.
Figure 5.1 A and B shows the phase morphology of PLA/HBP/PA (98/1.6/0.4), and
PLA/HBP/PA (96/3.2/0.8) blends. The crosslinked HBP particles were well separated
and there was considerable interparticle distance. The average particle size was 60
and 51 nm respectively, in these compositions. In the case of other blends i.e.
PLA/HBP/PA (92/6.4/1.6) and PLA/HBP/PA (88/9.7/2.3), there was considerable
increase in the particle concentration without having any significant aggregation. The

interparticle distance was reduced quite significantly in these blends. The interparticle

141

distance, also known as matrix ligament thickness, has proven to play a significant

role in the toughening mechanism of heterogeneous polymer systems. The critical

criteria/condition for brittle/tough transition is “3]
13%

Where 1: is the surface- to- surface interparticle distance and Tc is its critical value.

The Tc value of a polymer system is dependent on characteristic ratio of the matrix,

“4’ 15]. If the matrix ligament

rate, temperature, loading mode and internal stress
thickness is below a critical value, then a local plane strain to plain stress transition
can occur in these thin matrix ligaments leading to the reduction in the excessive
hydrostatic stressn’ 8’ '3’ 16’ ‘7].

In another theory by Michlerllg], Tc is related to the thickness of craze/bulk
boundary layer. i.e.

rc ~ 2t

Here t is the thickness of craze/bulk boundary layer, which is defined by the region of
plastically deformed unvoided material, which consists of entanglement strands that
are stretched from the undeforrned state in the bulk to the deformed state in the craze
interior (fibrils and voids). The thickness of this layer is found to be a characteristic of
a materialm]. It was suggested that if the matrix ligament is thinner the 2t, shear
yielding of the matrix occurs instead of crazing.

There was a polydispersity in the size (from spherical to elliptical) of
crosslinked HBP particles in PLA/HBP/PA (92/6.4/ 1.6) and PLA/l-IBP/PA

(88/9.7/2.3) blends. It is reportedm that uniform sized particles are more effective in

toughening than polydisperse ones, while, asymmetric particles such as ribbons and

142

networks are more effective than spheroidal ones. The crosslinked particles were
elliptical in shape due their orientation in the flow direction during melt processing as
well as due to sample beam interaction during TEM observation, where PLA tends to
expand due to high electron beam energy. The domain size of the particles varied
from 50 nm to 200 nm (measuring axial length), a large population of particles were
below 100 nm. The phase boundaries of crosslinked particles were highly indistinct
suggesting the possibilities of a networked interface between PLA and crosslinked
HBP particles. In the TEM photomicrograph of PLA/HBP/PA (84/12.9/3.1), where
HBP and PA had a maximum concentration, the phase morphology was consisted of
considerably larger particles (~200nm). This particular blend lacked the good
distribution of particles as it was observed in the blends of (92/6.4/1.6) and
PLA/HBP/PA (88/9.7/2.3). The potential reason for this difference could be the
extensive phase separation of HBP from the PLA phase before its crosslinking,

primarily due to the high concentration of HBP in this composition.

5.2.2 Mechanical Properties and Deformation Mechanism of PLA-crosslinked

HBP Blends

In chapter 3 and 4, it was concluded that crosslinking of HBP in PLA matrix
can be effectively used to improve its plastic deformation and toughness. The
crosslinked HBP particles were effective, when there was a critical concentration of
crosslinker i.e. 0.9 wt.% as well as of tightly crosslinked hyperbranched polymer, i.e.
~5.4 wt.%. Here, different combined weight (2, 4, 8, 12 and 16%) contents of HBP
and PA at a OH to anhydride molar ratio of 1:1, were incorporated in the PLA matrix

and their effect on tensile properties is studied. The detail of tensile properties of neat

143

PLA and PLA-cHBP blends is shown in Table 5.2, which also shows the amount of
actual crosslinked particles in PLA—cHBP system, which was calculated by equation

5.4.

It was observed that the increasing weight content of crosslinked HBP
improved the elongation at break of the blend. Elongation at break of PLA improve
from 5.1 to 9.9 and 15.6%, respectively, when the weight content of crosslinked
particles were 1.76 and 3.52% in the PLA/HBP/PA(98/1.6/0.4) and PLA/HBP/PA

Table 5.2: Effects of amount of crosslinked hyperbranched polymer of 0H /anhydride
ratio (1:1) on the tensile properties of PLA-cHBP blends.

 

 

 

 

 

 

 

Material TYPE Wt.% of Tensile Tensile Strength Elongation at
(wt.%/wt%) crosslinked Modulus, at Yield, MPa Break, %
HBP( eq. 5.4) GPa
Neat PLA 0 3.6i0.l 76.5 21.-1.5 5.1 :17
PLA/HBP/PA
(98/1 .6/0.4) 1.76 3.5 :01 66.0 i1.3 9.9 :03
PLA/HBP/PA
(96/3.2/0.8) 3.52 3.5 i0.2 62.4 i1.5 15.6 i3.5
PLA/HBP/PA
(92/6.4/l .6) 7.04 3.2 i0.2 59.5 i1.1 49.9i1 .6
PLA/HBP/PA
(88/9.7/2,3) 10.56 3.1 i0.2 55.5 i1.8 84.3 i120
PLA/HBP/PA
14.08 2.9 i0.l 49.0 $1.7 33.7 i5.1

(84/12.9/3.1)

 

 

 

 

 

 

 

144

(96/3.2/0.8) blends. The critical concentration of crosslinked HBP particles for strong
brittle-to-ductile transition in PLA-cHBP blends, was found to be 7.04 wt.% in the
PLA/HBP/PA (92/6.4/1.6) blend, where the elongation at break of reached to 49.9%.
The elongation at break reached a maximum value i.e. 84.3% for PLA/HBP/PA
(88/9.7/2.3) blend but decreased for PLA/HBP/PA (84/12.9/3.l) to 33.7%. On the
other hand, properties like tensile modulus and tensile strength at yield decreased
with increasing amount of crosslinked hyperbranched polymer in the blend. For the
highly ductile blend i.e. PLA/HBP/PA (88/9.7/2.3), the decrease in tensile modulus

and tensile strength at yield was about 14% and 27% respectively, as compared with

 
  
  

 

 

 

80-

60- B
a: C
9.1
E
3;" 40-
8 E
F13 “W.

20 ,\ 1:

OI I I I I f I I

Strain, %

Figure 5. 4: Stress — strain curves of neat PLA and various PLA -cHBP blends
obtained at a strain rate of 15. 4 mm/mm-min. A: Neat PLA; B: PLA/HBP/PA
(98/1.6/0. 4); C: PLA/HBP/PA (96/3. 2/0. 8); D: PLA/HBP/PA (92/6.4/1.6); E:
PLA/HBP/PA (88/9. 7/2. 3); F: PLA/HBP/PA (84/12.9/3.1).

145

neat PLA. Figure 5.4 shows the stress- strain curves of the neat PLA and its blends
with variable amount of crosslinked hyperbranched polymer having a similar OH to
anhydride ratio of 1:1. The curves were obtained at room temperature, i.e. 23°C, at a
strain rate of 15.4 mm/mm-min. It was observed that neat PLA failed in the brittle
fashion after showing some strain softening behind its yield point. PLA used in this
study, has a glass transition temperature of 55°C, it will be in a glassy state at room
temperature. Its crystalline structure would be frozen-in at room temperature and its
deformation would be primarily dictated by the amorphous region. There is an
established conformity that crazes are the main precursor of failure of glassy
polymersm’ 2”. In case of PLA, high chain stiffness and low entanglement density of

can be the reasons for formation of crazes, leading to its catastrophic failure. A

L»
O

      

'-‘ N N
LII 0 UI
_ __ _;__ 1

Tensile Toughness (MJ/nf)
'ES

0 2.5 5 7.5 10 12.5 15
Wt. % of Crosslinked HBP

Figure 5.5: Effect of concentration of crosslinked hyperbranched polymer
on the tensile toughness of neat PLA and PLA-cHBP blends

146

localized nature of crazes put a limitation on energy dissipation and leads to

premature craze failure, causing the brittleness of a polymer.

In the present system, in—situ synthesis of crosslinked hyperbranched polymer
particles in the PLA matrix was found to delay its premature craze/brittle failure
(Figure 5.4). A considerable plastic deformation of the PLA/cHBP blend was
observed when the weight contents of crosslinked particles were 7.04 and 10.56%. A
minimum concentration of crosslinked HBP particles i.e. 7.04 wt% was found to be
effective for brittle to ductile transition in the investigated PLA-cHBP blends. The
ductile PLA-cHBP blends showed significant post yield deformation by the
mechanism of strain softening and cold drawing. Figure 5.5 represents the variation
of tensile toughness (area under the stress-strain curve) as a firnction of weight
contents of cHBP particles. The tensile toughness showed an increasing trend from
2.3 MJ/m3 for neat PLA to a maximum of 23.7 MJ/m3 when 10.56 wt.% of cHBP
particles i.e. for PLA/HBP/PA(88%/9.7%/2.3%) blend, was present and declined to
11.3 MJ/m3 for 14.08 wt.% of cHBP particles in PLA/HBP/PA (84%/12.9%/3.l%)
blend. The flocculation of particles and loss of finely distributed particle morphology
were the potential reasons for the decline in the tensile toughness at this concentration.

The SEM photomicrograph (Figure 5.6 and 5.7) revealed that the tensile
fractured surface of neat PLA and various PLA-cHBP blends. The fracture surface of
neat PLA was flat and showed its brittle nature (Figure 5.6 A). The surface
characteristics of the tensile fractured surfaces changed progressively from flat to
rougher, when the weight contents of cHBP increased in the blends. A marked change

in the morphology of fractured surface was observed when the weight contents of

147

cHBP concentration were 7.04 and 10.56%. In these blends, the matrix was highly
drawn and formed fibrils, which strongly indicated shear yielding of the polylactide
matrix. This suggest that the onset of the brittle to ductile transition started at 7.04
wt.% of cHBP. The surface characteristics of PLA-cHBP blend having 14.08 wt.% of

cHBP were not significantly different from the blends having 7.04 and 10.56 wt.% of

cHBP.

148

Figure 5. 6: SEMphotomicrograph of tensile
fractured samples. A: Neat PLA

B: (PLA/HBP/PA (98/1. 6/0. 4)

C: PLA/HBP/PA (96/3. 2/0. 8)

149

 

;__ ........

    

Figure 5. 7: SEMphotomicrograph of tensile
fractured samples.

A: PLA/HBP/PA (92/6. 4/1. 6)

B: PLA/HBP/PA (88/9. 7/2. 3)

C: PLA/HBP/PA (84/12.9/3.1)

150

 

5.2.3 Particle Size Analysis

In the present section, the discussion of the PLA/HBP/PA (84/12.9/3.1) blend
is omitted due to its anomalous effect on the tensile toughness of polylactide and an
absence of well defined particle morphology of cHBP in the blend. Table 5.3 presents
the particle size analysis of crosslinked HBP particles in various PLA-cHBP blends.
There was a variation in the cHBP particle diameter with increase in the amount of
crosslinked HBP in the PLA matrix. The maximum average particle diameter was 97
nm in the blend having 10.56 wt.% of cHBP particles i.e. for PLA/HBP/PA
(8/9.7/2.3) blend . There was no clear trend obtained from the particle size
distribution parameter value but it indicated towards polydisperse size of crosslinked
HBP in PLA matrix. The minimum polydispersity occurred in the PLA/HBP/PA
(88/9.7/2.3) blend. The particle size distribution parameter c was found to 1.56 in the

PLA/HBP/PA (92/6.4/l .6), while it was 1.31 in the PLA/HBP/PA (88/9.7/2.3) blend.

Table 5.3: Particle size characteristics of crosslinked HBP particles in PLA-
cHBP blends.

 

Diameter (nm) of crosslinked

 

 

 

 

 

 

HBP P31110165 Particle size distribution
Materials Parameter, 0
Mean Median Mode

PLA/HBP/PA

(98 /l . 6 /0. 4) 60 40 54 1.45
PLA/HBP/PA

(96/3.2/0.8) 51 49 49 1.39
PLA/HBP/PA

(92/6.4/1.6) 86 72 5° 1'56
PLA/HBP/PA 97 90 68 1.31

(88/9.7/2.3)

 

 

 

 

 

 

151

Number of Particles

90
80 -.
70 .~
60 1
50 ~
40 1
30 j
20 l
10 -1

O k, , I - I - l . l l lle- i -'T’-’T 1._,.,‘,, . f". ,-,.

513’ e9 :\9 9:Q :{1‘9 e9 «9.99.,r1§°.ga9¢\999&g€’.,e9g.g\9,99 ,11‘?

Q, 63’ Q, I\ IN I I I I I I
‘L ‘3 ‘3 ‘3 Q ‘3 00 ‘3 Q ‘3‘; (5‘3’ ‘3’

Particle Diameter (nm)

(A)

70
60
50
40
30

20
(B)

Number of Particles.

1

O

I . - - -_. _ 1,,”

O

Q ‘3 Q ‘3 Q ‘3 ‘3 Q ‘3 Q ‘3 Q ‘3 Q ‘3
99’ be 9° e909 (L529! e5"0 9' (1,6359 95° $0959ng 95° @999
3 9300 semeeeoeee

Particle Diameter (nm)

Figure 5.8: Particle size distribution of crosslinked HBP particles.
(A) PLA/HBP/PA (92/6.4/1.6); (B) PLA/HBP/PA (88/9. 7/2. 3)

152

The histograms of cHBP particles in ductile blends, i.e. PLA/HBP/PA
(92/6.4/l.6) and PLA/HBP/PA (88/9.7/2.3), are presented in Figure 5.8. The
histograms showed the polydispersity of the crosslinked particles in these blends. In
case of PLA/HBP/PA (92/6.4/1.6) blend, a major population of crosslinked HBP
resided in the diameter interval of 50-75 nm, while in case of the PLA/HBP/PA
(88/9.7/2.3) blend, the major population of particles had diameters between 75-100
nm. The particle size distribution spread was low in case of PLA/HBP/PA
(88/9.7/2.3), compared to that of PLA/HBP/PA (92/6.4/ 1.6) blend. This suggested
that the crosslinking reaction was more controlled in PLA/HBP/PA (88/9.7/2.3)blend

and resulted in the particles having more uniform sizes.

5.2.4 Brittle-to-Ductile Transition in PLA-crosslinked HBP Blend

[7, l3]

Wu et al. suggested that a sharp brittle to ductile transition occurs in

rubber modified plastics, when the inter-particle distance, also known as matrix

ligament thickness, is smaller than a critical value. They observed this phenomenon

['31

in nylon-rubber blends . Figure 5.9 shows schematics of the matrix ligament

 

A L
‘ r

 

iHTHi

Figure 5. 9: Schematic of Matrix Ligament Thickness.
Redrawn after ref 7.

153

thickness, where r is the matrix ligament thickness, d is the diameter of particles and
L, is the distance between the centers of the particles.

The condition of toughening in case the of brittle matrices is

1: S 17c

Where I is the surface- to- surface interparticle distance and Tc is its critical
value.

The matrix ligament thickness is correlated with the chain stiffness/or
characteristic ratio of the polymer, where the critical matrix ligament thickness
decreases with an increase in the characteristic ratio of the matrix13 1.

The theoretical calculation of matrix ligament thickness can be obtained from the
following equations.
The matrix ligament thickness (T) of the monodisperse system can be related to

1131

particle diameter (d) and volume fraction ((1)). Wu obtained an equation assuming

that the particle occupies a cubic lattice in a system. The equation is given as

T(d,¢) = d[(7r/6¢)1/3 — 1] ...................................................... 5.5

Wum later incorporated the parameter of particle size distribution parameter, c, in

equation 5.5 and obtained equation 5.6.

T = d[(7r/6¢)1/3 —1]exp(ln2 0') ................................................ 5.6

Equation 5.6 was further modified by Liu et al.“21 for the calculation of matrix
ligament thickness of a polymer blend having well dispersed polydisperse particle

morphology. The equation 5.7 is as follows

T = d[(n/6¢)1/3 exp(1.5 1112 0) — exp(0.51n2 0)] ............................ 5.7

154

In PLA-crosslinked HBP blends, with increase in the volume fraction of
crosslinked hyperbranched polymer, matrix ligament thickness or interparticle
distance decreased (Figure 5.10). This was due to the increase in number density of
crosslinked HBP particles with increase in the volume fraction of crosslinked HBP
particles. The average matrix ligament thickness decreased from 461 nm to 72 nm,
when the volume fraction of crosslinked HBP particles was increased from 0.02 i.e. in
PLA/HBP/PA (98/1.6/0.4) blend to 0.12 i.e. in PLA/HBP/PA (88/9.7/2.3) blend. The
decrease in the matrix ligament with increase in the volume fraction was evident from
the TEM photomicrograph of various PLA-cHBP blends ((Figure 5.1, 5.2 and 5.3).
The increase in the number of thinner matrix ligaments with increase in the volume
fraction of crosslinked HBP particles was also observed from these TEM photo

micrographs.

500 .
450 *-
400
350 l
300 -
250 1
200 ~
150 1
100
50 -
O l

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

Avg. Interparticle Distance.
(nm)

Volume Fraction of Crosslinked HBP particles

Figure 5.10: Effect of volume fraction on the inter-particle distance of
crosslinked HBP particles in PLA -cHBP Blends

155

On the other hand, the tensile toughness was found to increase with decrease
in the matrix ligament thickness (inter-particle distance) for various PLA—crosslinked
HBP blends and the onset of brittle to ductile transition was observed for the
PLA/HBP/PA (92/6.4/ 1.6) blend, where the matrix ligament thickness was 114 nm
(Figure 5.11). The tensile toughness of this blend was 15.3 MJ/m3 as compared to 6.6
and 4.7 MJ/m3 for the blends of PLA/HBP/PA (96/3.2/0.8) and PLA/HBP/PA
(98/l.6/0.4), where the average matrix ligament thickness was 219 and 461 nm,
respectively. The tensile toughness further increased to 23.7 MJ/m3 when the average
matrix ligament thickness decreased to 72 nm for PLA/HBP/PA (88/9.7/2.3) blends.
This suggested that there was an increase in the number of thinner ligaments with
increase in the volume fraction of crosslinked HBP particles. The experimental value
of matrix ligament thickness in toughened blends, were compared with theoretical

values obtained from equations 5.5, 5.6 and 5.7 and given in Table 5.4

25.0
20.0 '
15.0 '

10.0

5"
o

Tensile Toughness (MJ/rri).

.0
<3

0 100 200 300 400 500

Average Interparticle Distance (nm)

Figure 5.11: Effect of inter-particle distance on the tensile toughness of
PLA-cHBP blends.

156

Table 5.4: Comparison of experimental matrix ligament thickness with
theoretical values for toughened nanoblends; here type A: PLA/HBP/PA
(88/9. 7/2. 3. type B: PLA/HBP/PA, (92/6.4/1.6).

 

 

 

 

 

 

 

 

 

 

Matrix Ligament Thickness MParticle Type Type
orphology
A B
Experimental Matrix Ligament Thickness, nm 72 114
T(d,¢) = d[(7r/6¢)1/3 -1], nm Monodisperse 42 97
T = d[(7; /6¢)1/3 —1]exp(ln2 0), nm Polydisperse 45 116
T = d[(7r /6¢)1/3 exp(1.51n2 0) — exp(0.51n2 0)] Polydisperse
,nm with well 54 145
dispersion

 

Wun’ ‘31 suggested that shear yielding can take place in matrix ligaments,
when their thickness is below a critical matrix ligament thickness. An essential
condition for this to happen is that there should be a reduction in triaxial dilative
stress in these thin ligaments. This is usually happen by the cavitation in rubber
particles. It is also suggested that the cavitation is indeed not necessary to relieve the
triaxial stress state; a dispersed phase having modulus lower than that of the matrix
modulus can relieve this stress state in these thin ligamentsn’ 8]. This can facilitate a
plane strain-to-plane stress transition in these ligaments leading to their shear yielding.
In present PLA-crosslinked HBP blends, the dispersed phase i.e. crosslinked HBP has
a low glass transition temperature, and expectedly low modulus than the PLA matrix.
When these particles were present above a certain volume fraction, the extremely thin

matrix ligaments formed in the PLA-cHBP blend. As shown in Chapter 3, the

157

crosslinked HBP particles tend to extend under tensile loading. By this mechanism,
the triaxial stress state in thin matrix ligaments could be relieved and helped in their
shear yielding. So, the inter-particle distance or matrix ligament thickness was found

to be a critical parameter in the toughening of PLA-cHBP blends.

158

5.3 References

l.

10.

ll.

12.

Michler, G. H., Adhikari, R., Henning, S., Toughness Enhancement of
Nanostructured Amorphous and Semicrystalline Polymers. Macromol. Symp.,
2004. 214: p. 47-71.

Kulinski, Z., Piorkowska, E., Crystallization, structure and properties of
plasticized poly(L-lactide). Polymer, 2005. 46(23): p. 10290-10300.

Wu, S, Chain structure, phase morphology, and toughness relationships in
polymers and blends. Polymer Engineering and Science, 1990. 30(13): p. 753-
761.

Michler, G. H., The Role of Interparticle Distance in Maximizing the
Toughness of High-Irnpact Therrnoplastics. Acta Polymerica, 1993. 44(3): p.
113-124.

Magalhaes, A. M. L., Borggreve, R. J. M., Contribution of the Crazing
Process to the Toughness of Rubber-Modified Polystyrene. Macromolecules,
1995. 28(17): p. 5841-5851.

Bubeck, R. A., Buckley, D. J., Kramer, E. J., Brown, H. R., Modes of
Deformation in Rubber-Modified Therrnoplastics During Tensile Impact.
Journal of Materials Science, 1991. 26(23): p. 6249-6259.

Wu, S. H., A Generalized Criterion for Rubber Toughening - the Critical
Matrix Ligament Thickness. Journal of Applied Polymer Science, 1988. 35(2):
p. 549-561.

Margolina, A., Wu, S. H., Percolation Model for Brittle-Tough Transition in
Nylon Rubber Blends. Polymer, 1988. 29(12): p. 2170-2173.

Anderson, K. S., Lim, S. H., Hillmyer, M. A., Toughening of polylactide by
melt blending with linear low-density polyethylene. Journal of Applied
Polymer Science, 2003. 89(14): p. 3757-3768.

Premphet, K., Paecharoenchai, W., Quantitative Characterization of Dispersed
Particle Size, Size Distribution, and Matrix Ligament Thickness in
Polypropylene Blended with Metallocene Ethylene-Octene Copolymers.
Journal of Applied Polymer Science, 2001. 82: p. 2140-2149.

Irani, R. R., Callis, F. C., Particle Size: Measurement, Interpretation and
Application. 1963, New York: Wiley.

Liu, Z. H., Zhang, X. D., Zhu, X. G., Qi, Z. N., Wang, F. 8., Effect of
morphology on the brittle ductile transition of polymer blends: l. A new

159

13.

14.

15.

16.

17.

18.

19.

20.

21.

equation for correlating morphological parameters. Polymer, 1997. 38: p.
5267 5273.

Wu, S. H., Phase-Structure and Adhesion in Polymer Blends - a Criterion for
Rubber Toughening. Polymer, 1985. 26(12): p. 1855-1863.

Borggreve, R. J. M., Gaymans, R. J ., Luttmer, A. R., Influence of Structure on
the hnpact Behavior of Nylon-Rubber Blends. Makromolekulare C hemie-
Macromolecular Symposia, 1988. 16: p. 195-207.

Flexman, E. A., New High-Irnpact Acetal Elastomer Has Broader Use
Potentials. Modern Plastics, 1985. 62(2): p. 72-76.

Wu, S. H., Margolina, A., Percolation Model for Brittle Tough Transition in
Nylon Rubber Blends - Reply. Polymer, 1990. 31(5): p. 972-974.

Wu, S. H., Impact Fracture Mechanisms in Polymer Blends - Rubber-
Toughened Nylon. Journal of Polymer Science Part B-Polymer Physics, 1983.
21(5): p. 699-716.

Michler, G. H., Private Communication, in 32nd Microsymposium on Polymer
Blends. 1989: Prague, Czechoslovakia.

Michler, G. H., Crazes in Amorphous Polymers .1. Variety of the Structure of
Crazes and Classification of Different Types of Crazes. Colloid and Polymer
Science, 1989. 267(5): p. 377-388.

Kramer, E. J., Microscopic and Molecular Fundamentals of Crazing.
Advances in Polymer Science, 1983. 52-3: p. 1-56.

Kambour, R. P., Structure and Properties of Crazes in Polycarbonate and
Other Glassy Polymers. Polymer, 1964. 5(3): p. 143-155.

160

Chapter 6: Conclusions and Recommendations for Future Work

6.1 Conclusions

A nanostructure-controlled polylactide (PLA) was created by in-situ cross—linking
of a hyperbranched polymer (HBP) in the PLA matrix through reactive melt blending.
The generation of new HBP-based nanostructures in the PLA matrix improved the tensile
toughness and elongation at break more than 500% and 800%, respectively, as compared
to unmodified PLA. The modified PLA remarkably maintained high strength and
modulus values. The cross-linking reaction of HBP was confirmed by the FTIR analysis
and DMTA analysis. The transmission electron microscopy and atomic force microscopy
observations of modified PLA confirmed the nanoscopic dimensions of cross-linked HBP
particles in the PLA matrix. The scanning electron microscopy analysis clearly exhibited
the toughening of PLA. The toughening mechanism of PLA was comprised of shear
yielding and crazing. The TEM photomicrograph of deformed regions of neat PLA
showed micron size crazes in neat PLA. On other hand, there was extension of craze
fibrils in the modified PLA, which was correlated with the stabilization of these crazes.
The crosslinked HBP particles were found to extend under tensile loading and helped in
the energy dissipation mechanism. There was no cavitation observed in the crosslinked
HBP particles. Rheological studies revealed that there was a network formation in PLA
matrix after in-situ crosslinking of HBP. The HBP was found to reduce the melt viscosity
of PLA dramatically and this effect was maintained even after its in-situ crosslinking in

PLA matrix.

161

The effect of crosslinker concentration on the mechanical properties of PLA-
crosslinked HBP blend was studied. It was found that the amount of crosslinked HBP
synthesized inside the PLA matrix increased with the increase in the concentration of
polyanhydride crosslinker. The increased weight fraction of crosslinked HBP in PLA
matrix was found to have a promising effect on the elongation at break value of
polylactide. The onset of the brittle to ductile transition for polylactide was observed
when hydroxyl (OH) to anhydride ratio was 2:1 for a blend having 92 wt.% PLA/7.1
wt.%HBP/0.9 wt.% PA. The toughening effect originated from the amount of crosslinked
HBP formed inside the PLA matrix. The crosslinking of HBP was observed with the
help of dynamic mechanical thermal analysis by monitoring the glass transition
temperature obtained from the relaxation peak associated with loss modulus. The glass
transition temperature values of the crosslinked HBP phase were correlated with the Fox
equation and found to deviate from it when OH to anhydride ratios were 2:1, 1:1 and 1:2
for crosslinked HBP. The hydrophilic nature of the hyperbranched polymer was altered to
hydrophobic by incorporation of polyanhydride crosslinker, as demonstrated by the
increase in the contact angle with water. Gel permeation chromatography (GPC) revealed
that there was no dramatic decrease in the molecular weight of PLA in the presence of
pristine as well as crosslinked HBP.

The concentration of crosslinked HBP having OH to anhydride ratio (1:1) was
varied from 2 to 16 wt.% by in-situ synthesizing them in PLA matrix, for understanding
the brittle-to-ductile transition in modified PLA. It was observed that toughness was
dependent on the volume fraction of crosslinked HBP and matrix ligament thickness

(inter-particle distance) in the modified blends. The tensile toughness was found to

162

increase with increase in volume fraction and decrease in the matrix ligament thickness.
The maximum critical matrix ligament thickness was found to be 114 nm for
PLA/HBP/PA (92wt.%/7.4wt.%/1.6wt.%), which further reduced to 72 nm for
PLA/HBP/PA (88wt.%/9.7wt.%/2.3 wt.%) blend. The experimental matrix ligament
thickness values were correlated with the values obtained from theoretical equations.
Shear yielding in these thin ligaments was likely the most effective toughening
mechanism in the toughened polylactide nanoblends.

Finally, hyperbranched polymer-based modification of polylactide resulted in a
high performance polylactide based material having good mechanical and rheological
properties. The current research unwraps new opportunities available to us from the
unique physical and chemical properties of the highly functional hyperbranched polymers

in generating new nanostructured multiphase polymer systems with enhanced properties.

6.2 Recommendations for Future Work

Polylactide has shown much promise towards building of a bioplastic regime,
where it will play a pivotal role due to its affordable cost and useful property attributes.
Unconventional research methodology must be encouraged to improve the PLA
properties. The present work revealed that hyperbranched polymers can provide a
gateway to such kind of research in the field of PLA based materials. There is still vast
opportunity available to us for tailoring the architecture of HBP and study its influence on
the PLA properties. HBP modified PLA can generate new materials, which can have
applications ranging from packaging to automotives.

In the present system, structural modification of a second generation of hydroxyl

functional aliphatic hyperbranched polymer is carried by in—situ crosslinking it in the

163

PLA matrix. This work can be extended to higher generation of hyperbranched polymers,
which will have an effect on the size and molecular weight between crosslinks of
crosslinked hyperbranched polymer. In this way, we can tailor the glass transition
temperature and modulus of the crosslinked HBP particles. In present system, a
polyanhydride is used for crosslinking of hyperbranched polymer. A crosslinker having
better compatibility with polylactide base resin can be opted for the improved
compatibilization effect in the PLA-cHBP blends. Interfacial adhesion of PLA-
crosslinked HBP blend can be investigated to understand its local deformation
mechanism.

Beside crosslinking, end capping of hyperbranched polymer with compounds
compatible with polylactide can be attempted. This can generate new nano-size core-shell
particles. Such particles can be studied for their toughening effect on polylactide. The
hyperbranched polymers have been used for reducing interfacial tension in conventional
polymer blends. Such approach can be used in bioplastics based blends.

The modified PLA can be used as a base resin for filler reinforcement such as talc
and nano-clay. The functional nature of crosslinked HBP can provide compatibility
between the filler and the PLA matrix. In this way, properties such as heat deflection
temperature and gas barrier of the modified resin can be improved, along with preserving
its toughness. Effects of rigid filler enhanced toughening in modified PLA can also be
investigated. The lowered melt viscosity of PLA in. presence of crosslinked HBP can
provide ideal conditions for the filler dispersion. Such concepts can be extended to

polymer-polymer blends and fiber reinforced composites.

164

7: Appendix: Density of NaCl Salt Solutions of Different Concentrations

Density Table of NaCl Salt Solutions of Different Concentrations

 

 

 

 

 

 

 

 

 

Wt of Nacl Salt, Volume of Weight of Densrty Of
S'NO Solution ml Solution in solutron,

gm ’ ’ g gm/CC
1 5.3148 100 102.9609 1.0296
2 8.7154 100 105.0367 1.0503
3 9.3146 100 105.4965 1.0549
4 9.8930 100 105.9073 1.0590
5 11.5530 100 106.9443 1.0694
6 12.0067 100 107.2864 1.0728
7 14.6050 100 108.9893 1.0898
8 20.3524 100 113.7321 1.1373

 

 

 

 

 

 

 

165

      

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