FUNCTIONAL PROPERTIES AND STABILITY OF PLLA-METAL ORGANIC
FRAMEWORK BASED MIXED MATRIX MEMBRANES
By
Ajay Kathuria

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
Packaging-Doctor of Philosophy
2013

i

ABSTRACT
FUNCTIONAL PROPERTIES AND STABILITY OF PLLA-METAL ORGANIC
FRAMEWORK BASED MIXED MATRIX MEMBRANES
By
Ajay Kathuria
Poly(lactic acid) (PLA) is a commercially available bio-based, biodegradable and
compostable polymer with many new applications in the packaging industry. However,
PLA has certain limitations such as poor barrier, low impact resistance, poor tear resistance
and low toughness, which hinder its functionality as a packaging material. The purpose of
this study was to explore and understand new avenues to improve the mechanical properties
and functionality of the PLA for various industrial applications including packaging. PLAmetal organic framework (MOFs) mixed matrix membranes (MMMs) were fabricated with
these objectives in mind. MOFs are a new class of crystalline coordinate polymers with
applications in gas storage, catalysis and gas separation, and they can be used to produce
PLA composites to improve its mechanical properties and functionality.
Copper benzene tricarboxylate Cu3(BTC)2 MOF was successfully synthesized by
microwave synthesis and characterized using X ray diffraction (XRD), scanning electron
microscopy (SEM) and surface area studies. During the fabrication of MMMs, it is
important to maintain the integrity of MOF crystals to achieve the desired properties and
functionality. Activated Cu3(BTC)2 MOF crystals were incorporated into PLA by melt
extrusion process using twin screw micro-compounder without any damage to the
Cu3(BTC)2 MOF crystal structure, which helped in preserving the functional properties of
MOF in the polymer matrix. The effect of residual water in the MOF structures was

ii

evaluated. It was found that residual water could be detrimental to the morphology of the
crystals during extrusion processing and can compromise the final properties of MMM.
The presence of MOF particles in the polymer improves the toughness of the PLA
matrix. We observed that under the uniaxial tensile stress the triaxial stress generated at the
interface of PLA MOF crystals led to cavitation induced by debonding. Cavitation
mechanism generated local plastic deformation followed by strain softening leading to the
improved toughness. Parallel plate rheological studies were performed to understand the
interaction of MOF particle with the PLA and the effect of these particles on the processing
of MMM. Rheological and differential scanning calorimetric studies provided evidence of
strong PLLA-MOF interactions.
CO2 permeability of PLLA-20% wt. Cu3(BTC)2 MOF mixed matrix membranes
increased by around 38% as compared to neat PLLA. The perm-selectivity (αCO2/O2) of
PLLA-20% MOF increased from 7.6 to 10.3, respectively. The permeability coefficient
for trans-2-hexenal increased by 60% with the addition of 20% MOF. Permeability
coefficients of various gases and organic molecules are strongly influenced by the kind of
interactions between the microporous materials, matrix and permeants. This study will
help in the advancement of our understanding of mixed matrix membranes prepared from
bio-based polymeric matrix for various industrial and commercial applications including
food and pharmaceutical packaging.

iii

To Grace, and Sumiti

iv

ACKNOWLEDGEMENTS

I would like to thank my advisor, Dr. Rafael Auras for his unabated motivation, support
and enthusiasm. I have been truly blessed that he allowed me pursue research work under
his guidance. During this dissertation work he provided a lot of freedom to conduct
research which helped me to develop as a young researcher.
I would also like to thank Dr. Mohamad G. Abiad for his guidance as a friend and
mentor. His sheer presence brings a lot of positive energy. To solve a problem, he brings
lot of new ideas to the table along with different approach. It has been a pleasure to work
with him. Some of the research work in this dissertation would not have been possible
without his guidance.
Dr. Maria Rubino has been very helpful and instrumental to help me understand
some of the basic fundamentals concepts. I have worked as a TA for her for Instrumental
analysis course (PKG 817). She helped me a lot in absorbing the concepts during my
graduate studies.
I would like to express my sincere thanks to Dr. Susan Selke for her guidance,
support and motivation. She has been kind and caring as a teacher and as a mentor. I also
enjoyed working with her on a project funded by the packaging industry during the first
year of my doctorate studies.
I would like to thank Dr. Ramani Narayan for providing the guidance and
allowing me to use his processing lab facilities. He has been inspirational. I am thankful
to Dr. K. Jayaraman for providing me access to his rheological laboratory to complete a
part of my studies. I would also like to thank Dr. Rahul Bhardwaj, Edward Drown, Ren

v

Weije and Shawn Shi for their useful discussions and comments during the course of this
work.
I would also like to express my gratitude to faculty and staff of the School of
Packaging, MSU. I am grateful to Dr. Joseph Hotchkiss, Dr. Bruce Harte, Dr. Diana
Twede, Dr. Gary Burgess, Dr. Robert Clarke, Dr. Laura Bix and Dr. Laurent Matuana
and Dennis Young for their wonderful teaching and support. I am thankful to Kelby
Thayer, Sherrie Lenneman, Cimbery Weir, Ronald Iwaszkiewicz and Sean Harwood for
providing help and support during my graduate studies.
I am thankful to my friends and colleagues from Dr. Auras research group: Dong
Ho Kang, Hayati Samsudin, Sukeewan, Turk for their inputs and suggestions during
various presentations in the group.
I would also like to acknowledge financial support from my parents, Bimbo
bakery project and graduate assistantship funding from the School of Packaging, MSU.
Without their financial support this work would not have been possible.

vi

TABLE OF CONTENTS

LIST OF TABLES ...............................................................................................................x
LIST OF FIGURES ........................................................................................................... xi
KEY TO SYMBOLS OR ABBREVIATIONS ............................................................... xiv
Chapter 1. Introduction ........................................................................................................1
1.1 Introduction ................................................................................................................1
1.2 Goals and objectives ...................................................................................................5
1.3 Document outline .......................................................................................................5
1.4 REFERENCES ...........................................................................................................7
Chapter 2. Literature review ..............................................................................................11
2.1 Structure, synthesis and properties of PLA ..............................................................12
2.2 Toughening of polymer composites .........................................................................14
2.2.1 Cavitation ..........................................................................................................14
2.2.2 Crazing ..............................................................................................................14
2.2.3 Shear yielding ....................................................................................................14
2.3 Adsorption ................................................................................................................15
2.3.1 Difference between physisorption and chemisorption ......................................15
2.3.2 Methods to evaluate the sorption isotherms ......................................................16
2.4 Metal organic Framework ........................................................................................19
2.5 Mixed matrix membranes .........................................................................................24
2.6 REFERENCES .........................................................................................................28
Chapter 3. Synthesis and characterization of copper-benzene 1,3,5 tricarboxylate metal
organic framework .............................................................................................................34
3.1 Introduction ..............................................................................................................35
3.2 Methodology ............................................................................................................40
3.2.1 Microwave synthesis .........................................................................................40
3.3 Characterization of Cu3(BTC)2 MOF ......................................................................42
3.3.1 Surface area and porosity measurement ............................................................42
3.3.2 Scanning electron microscope ...........................................................................44
3.3.3 X ray powder diffraction ...................................................................................45
3.4 Results and discussions ............................................................................................45
3.5 REFERENCES .........................................................................................................50
Chapter 4. Deterioration of metal organic framework crystal structure during fabrication
of poly(L-lactic acid) mixed matrix membranes ...............................................................53
4.1 Introduction ..............................................................................................................55
4.2 Methodology ............................................................................................................57
4.2.1 Materials ............................................................................................................57

vii

4.2.2 Sample preparation ............................................................................................59
4.2.3 Characterization .................................................................................................60
4.2.3.1 Thermo-gravimetric analysis .....................................................................60
4.2.3.2 X-ray diffraction ........................................................................................60
4.2.3.3 Differential scanning calorimetry ..............................................................60
4.2.3.4 Tensile test .................................................................................................61
4.2.3.5 Weight average molecular weight .............................................................62
4.2.3.6 Scanning electron microscopy ...................................................................62
4.2.3.7 CO2 and O2 permeability coefficients .......................................................63
4.2.3.8 Data analysis ..............................................................................................63
4.3 Results and discussion ..............................................................................................64
4.3.1 Thermo-gravimetric analysis ........................................................................64
4.3.2 X-ray diffraction ...........................................................................................65
4.3.3 Differential scanning calorimetry .................................................................72
4.3.4 Tensile test ....................................................................................................75
4.3.5 Gel permeation chromatography...................................................................79
4.3.6 Carbon dioxide (CO2) and oxygen (O2) permeability ..................................80
4.4 Conclusions ..............................................................................................................83
4.5 REFERENCES .........................................................................................................84
Chapter 5. Toughening of poly(L-lactic acid) with metal organic framework crystals
5.1 Introduction ..............................................................................................................91
5.2 Methodology ............................................................................................................93
5.2.1 Materials ............................................................................................................93
5.2.2 Sample preparation ............................................................................................93
5.2.3 Characterization .................................................................................................94
5.2.3.1 Tensile test .................................................................................................94
5.2.3.2 Rheology ....................................................................................................94
5.2.3.3 X-ray diffraction ........................................................................................95
5.2.3.4 Differential scanning calorimetry ..............................................................95
5.2.3.5 Thermogravimetric analysis.......................................................................96
5.2.3.6 Weight average molecular weight .............................................................96
5.2.3.7 Scanning electron microscopy ...................................................................97
5.2.3.8 Data analysis ..............................................................................................97
5.3 Results and discussion ..............................................................................................97
5.3.1 Tensile test/toughness ...................................................................................97
5.3.2 Rheological studies .....................................................................................104
5.3.3 X-ray diffraction .........................................................................................113
5.3.4 Differential scanning calorimetry ...............................................................115
5.3.5 Thermogravimetric analysis........................................................................115
5.3.6 Gel permeation chromatography.................................................................120
5.4 Conclusions ............................................................................................................120
5.5 REFERENCES .......................................................................................................121
Chapter 6. Mass transfer properties .................................................................................126
6.1 Introduction ............................................................................................................127
viii

6.2 Methodology ..........................................................................................................131
6.2.1 Materials ..........................................................................................................131
6.2.2 Fabrication of membranes ...............................................................................131
6.2.3 Mass transfer properties ..................................................................................132
6.2.3.1 Carbon dioxide transmission rate.............................................................132
6.2.3.2 Water vapor transmission rate .................................................................133
6.2.3.3 Oxygen transmission rate .........................................................................133
6.2.3.4 Aroma transmission rate ..........................................................................133
6.2.4 Data analysis ....................................................................................................134
6.3 Results and discussions ..........................................................................................134
6.3.1 Carbon dioxide permeability coefficient .........................................................134
6.3.2 Oxygen permeability coefficient .....................................................................135
6.3.3 Water vapor permeability coefficient ..............................................................136
6.3.3 Trans-2-hexenal permeability coefficient........................................................137
6.4 Conclusions ............................................................................................................138
6.5 REFERENCES .......................................................................................................139
Chapter 7. Conclusions and future work..........................................................................144
7.1 Conclusions ............................................................................................................145
7.1.1 Synthesis and characterization of Cu3BTC2 MOF .........................................145
7.1.2 Stability of Cu3BTC2 MOF crystals during fabrication of MMMs ...............145
7.1.3 Effect of MOF crystals on the toughness of PLLA ........................................146
7.1.4 Mass transfer properties of PLLA ..................................................................147
7.1.5 Overall conclusions ........................................................................................147
7.2 Recommendations for future work .........................................................................148
7.2.1 Chemical modification of metal organic framework .......................................148
7.2.2 Modeling mass transfer properties .................................................................148
7.2.3 Environmental impact ....................................................................................148
7.2.4 Shelf life studies .............................................................................................149

ix

LIST OF TABLES

Table 4.1: X-ray diffraction intensities of activated and saturated MOF ..........................69
Table 4.2: Intensity ratios, and area under the curve of PLLA-A-MOF and
PLLA-S-MOF ...................................................................................................70
Table 4.3: Detailed DSC analysis data of PLLA and its composites.................................73
Table 4.4: Tensile properties of PLLA, PLLA-A-MOF and PLLA-S-MOF .....................78
Table 4.5: Tensile strength and elongation at break of PLLA and PLLA-TMA
composites........................................................................................................78
Table 4.6: Molecular weight of PLLA, PLLA-A-MOF and PLLA-S-MOF .....................79
Table 4.7: CO2 and O2 permeability coefficients of PLLA, PLLA-A-MOF and
PLLA-S-MOF ..................................................................................................81
Table 5.1: Tensile properties of PLLA, PLLA-5% MOF, PLLA-10% MOF and
PLLA-20% MOF ...........................................................................................100
Table 5.2: Detailed DSC analysis information of PLLA and its composites ..................118
Table 5.3: Molecular weight of PLLA, PLLA-5% MOF, PLLA-10% MOF and
PLLA-20% MOF ............................................................................................119
Table 6.1: CO2 and O2 permeability coefficients of PLLA, PLLA-5% MOF, PLLA-10%
MOF and PLLA-20% MOF ............................................................................135
Table 6.2: Water vapor permeability coefficients of PLLA, PLLA-5% MOF, PLLA-10%
MOF and PLLA-20% MOF ............................................................................137
Table 6.3: Trans-2-hexenal permeability coefficients of PLLA, PLLA-5% MOF,
PLLA-10% MOF and PLLA-20% MOF .......................................................138

x

LIST OF FIGURES

Figure 1.1: Schematic representation of mixed matrix membranes ....................................3
Figure 2.1: Chemical structure of (a). L-lactic acid (b). D-lactic acid...............................12
Figure 2.2: Chemical structure of (a). L-lactide (b). D-lactide c. meso-lactide .................13
Figure 2.3: IUPAC classifications of sorption isotherms ..................................................18
Figure 2.4: (a). Benzene .....................................................................................................21
Figure 2.4: (b). Hexaflourobenzene ...................................................................................21
Figure 2.5: Some carboxylate based ligands......................................................................23
Figure 3.1: Dicopper(II) tetracarboxylate building block or repeating unit of Cu3(BTC)2
MOF ................................................................................................................39
Figure 3.2: Ethos microwave .............................................................................................40
Figure 3.3: Images of activated and saturated Cu3(BTC)2 MOF .......................................41
Figure 3.4: Micromeritics ASAP

TM

2020 .........................................................................44

Figure 3.5: N2 isotherm of Cu3(BTC)2 MOF at 77 K .......................................................46
Figure 3.6: SEM images of Cu3(BTC)2 MOF acquired at 12kV and 39 mm working
distance ..........................................................................................................47
Figure 3.7: X-ray diffraction pattern of Cu3(BTC)2 MOF ................................................48
Figure 4.1: Chemical structure of building block of Cu3(BTC)2 MOF ............................57
TM

Figure 4.2: SEM image of Basolite
C300 acquired at 12kV and 39 mm working
distance ...........................................................................................................58
Figure 4.3: TGA thermograms of PLLA, PLLA-A-MOF, PLLA-S-MOF, saturated and
activated MOF before extrusion .....................................................................65

xi

Figure 4.4: XRD patterns of (a). activated Cu3BTC2 MOF and (b). saturated Cu3BTC2
MOF ................................................................................................................67
Figure 4.5: XRD patterns of (a). PLLA-A-MOF disc, (b). PLLA-S-MOF disc, and (c)
injection molded PLLA disc ...........................................................................68
Figure 4.6: DSC thermograms of PLLA, PLLA-A-MOF and PLLA-S-MOF: (a) first
heating cycle; (b) second heating cycle ..........................................................74
Figure 4.7: Stress-strain curves of PLLA, PLLA-A-MOF and PLLA-S-MOF .................77
Figure 4.8: Permeability coefficients of PLLA, PLLA-A-MOF and PLLA-S-MOF ........81
Figure 4.9: SEM image of PLLA-A-MOF fractured surface at 12kV and 39 mm working
distance ...........................................................................................................82
Figure 4.10: SEM images: (a). PLLA-S-MOF fractured surface acquired at 12kV and 47
mm working distance; (b). PLLA-S-MOF XRD disc acquired at 12kV and
39mm working distance showing collapsed MOF particle ..........................82
Figure 5.1: Toughening mechanism of PLLA-MOF composites adapted from references
[28,30]. Step I: polymer MOF composite, step II: debonding at the interface
of the polymer-MOF composite under tensile stress, step III: plastic
deformation .................................................................................................99
Figure 5.2: Stress-strain graph of PLLA and PLLA-MOF composites ...........................102
Figure 5.3: Tensile fractured cross section of (a). PLLA, (b). PLLA-5% MOF, (c). PLLA10% MOF, and (d) PLLA-20% MOF ...........................................................104
Figure 5.4: (a). Storage modulus (G’) of PLLA and PLLA-MOF composites as a function
of oscillatory frequency ...............................................................................106
Figure 5.4: (b). Loss modulus (G”) of PLLA and PLLA-MOF composites as a function of
oscillatory frequency ...................................................................................107
Figure 5.4: (c). Tan delta curves of PLLA and PLLA-MOF composites ........................108
Figure 5.4: (d). Complex viscosity of PLLA and PLLA-MOF composites ....................109
Figure 5.4: (e). Cole-Cole plot of PLLA and PLLA-MOF composites ...........................110
Figure 5.4: (f). van Gurp-Palmen plot of PLLA and PLLA-MOF composites ..............111
Figure 5.5: Schematic representation of the configuration of polylactide MOF composites
(a). at rest (b). low shear rates (c). at high shear rates adapted from reference

xii

[40] ...............................................................................................................112
Figure 5.6: X-ray diffraction pattern of PLLA-MOF composites ...................................114
Figure 5.7: Thermogravimetric analysis of PLLA, Cu3(BTC)2 MOF and PLLA-MOF
composites.....................................................................................................117

xiii

KEY TO SYMBOLS OR ABBREVIATIONS

Symbols

Key

Tg

Glass Transition Temperature

Tm

Melt Temperature

TM

Trade Mark

K

Degrees Kelvin

PLLA-A-MOF

PLLA-5% activated MOF

PLLA-S-MOF

PLLA-5% saturated MOF

MOF

Metal Organic Framework

MMMs

Mixed matrix membranes

Cu3(BTC)2

Copper benzene 1,3,5 tricarboxylate metal organic framework

η*

Complex viscosity

xiv

Chapter 1. Introduction

1

Chapter 1. Introduction
1.1 Introduction
Conventional petroleum derived polymers such as polyethylene

(PE),

polypropylene (PP), polyethylene terephthalate (PET) and polystyrene (PS) are widely
utilized in packaging and other industrial applications [1]. However, their non-renewable
origin, solid waste management issues, depleted landfill space and carbon footprints have
spurred the development of bio-based, biodegradable and compostable polymers. Biobased polymers can be directly extracted from biomass, synthesized from bio-derived
monomers or can be produced by microorganism [2]. Polymers synthesized from bio
derived monomers have achieved great commercial momentum in the last decade.
Polylactic acid (PLA), one such polymer in this category, is synthesized from lactic acid,
which can be produced from corn, sugar beets, rice starch and cellulosic waste [3-7]. In
the United States, PLA is produced from corn. It is one of the leading biopolymers with
commercial applications in the packaging industry and is marketed under the trademark
IngeoTM by NatureWorks LLC, a subsidiary of Cargill Incorporated. NatureWorks has an
annual capacity of 300 million pounds [8]. PLA has high mechanical strength and
modulus. The poor thermal stability, high permeability, poor tear strength, low heat
deflection temperature (HDT), low melt strength and poor functionality of PLA hinders
its large scale application in the packaging industry [9-11].
Separation of gases using polymeric membranes has increased with significant
advancement in membrane technology [12-13]. Polymer based membranes are cheap,
durable, easy to handle and have low operating cost [13, 14]. However, low selectivity,
poor stability at elevated temperature and solubility in organic compounds limits

2

applications of polymeric membranes [13, 15-16]. Porous inorganic, organic and hybrid
materials such as zeolites, carbon molecular sieves and metal organic framework (MOF)
have higher selectivity than polymeric membranes, but they are expensive, difficult to
process and have poor mechanical strength [16-19]. In order to extract the benefits of
both porous and polymeric membranes the focus has shifted towards the development of
mixed matrix membranes (MMMs), which are composed of porous materials dispersed in
a polymeric matrix. Figure 1 represents a schematic of MMM.

CO2
CH4
CH4

CO2

CO2

CO2

CH4
CH4
CH4

CO2
CO2

CO2 2
CO

CO2

CO2

CH4

CO2
CH4

CO2

CO2
CH4

CO2

CO2

CO2
CO2

CH4

Figure 1.1: Schematic representation of mixed matrix membranes. For interpretation of
the references to color in this and all other figures the reader is referred to the
electronic version of this dissertation

.
Although MMMs prepared from inorganic materials like zeolites have been
widely researched in the last few decades, the poor compatibility between an inorganic

3

filler and an organic polymeric matrix can lead to sub-micron size holes in the MMMs.
MOFs are a new class of organic-inorganic hybrid porous crystalline materials which
consist of metal ions coordinated with organic molecules into multidimensional structure.
They are popular for properties such a gas separation, gas storage and catalysis. The most
attractive property of MOFs is their ultrahigh surface areas, which is greater than porous
2 -1

carbons or zeolites and may exceed at times 5900 m .g

[20]. The utilization of MOFs

with polymeric materials can allow the creation of new functional MMMs. MOF particles
with organic linkers can offer better compatibility than zeolites [21-23].
Various researchers have reported the preparation of MMMs from MOF [14, 2425]. The presence of MOFs in the polymers can aid and enhance the perm-selectivity of
the membranes by influencing the sorption and diffusion factors [24]. Polymers such as
polydimethyl siloxane (PDMS) and polysulfone (PSf) have been extensively studied for
the membrane applications. These polymers have high carbon footprints. Therefore,
utilization of a more carbon neutral polymeric matrix such as PLA can help in mitigating
some environmental concerns. PLA matrix also offers high permselectivity for gases such
as CO2/CH4 [2, 26]. Therefore, this can be exploited for applications such as getting high
purity methane from natural gas streams, which contain CO2 impurities, or it can be
utilized to obtain high calorific value methane fuel from a biogas plant.
Thus, the synthesis of novel PLA Cu3BTC2-MOF-MMMs can be used to improve
functionality of PLA for various applications such as packaging, petrochemical, energy,
etc. [27]. The synthesis of Cu3(BTC)2-MOF and production of PLA-Cu3(BTC)2-MOFMMMs are the main focus of this dissertation.

4

1.2 Goals and objectives
The main objectives of this dissertation are:
1. To synthesize Cu3(BTC)2 MOF using a microwave assisted rapid synthesis
technique and to characterize it using X-ray diffraction, BET surface area
analysis and scanning electron microscopy.
2. To evaluate the effect of residual water on the stability of the MOF crystals during
melt extrusion processing. The integrity of these crystals can significantly affect
the final properties of the membranes.
3. To understand the toughness mechanism and PLA-MOF interactions using
electron microscopy and rheological properties of the PLA-MOF MMMs.
4. To determine the mass transfer properties of the developed PLA-MOF
composites.

1.3 Document outline
This dissertation is divided into seven chapters. A brief summary of the following
chapters is as follows:
Chapter 2 provides a summary of the literature review of PLA, MOF, the
difference between physical adsorption and chemical adsorption, adsorption mechanism
followed by MOF, polymeric membranes and MMMs.
Chapter 3 describes the various synthesis methodologies that can be utilized for
the synthesis of MOF. Benefits of microwave assisted synthesis of MOFs over other

5

synthesis procedures as well as characterization of copper benzene 1,3,5 tricarboxylate
metal organic frameworks (MOFs) using XRD, SEM and BET surface area analysis
techniques.
Chapter 4 focuses on PLA–MOF based MMMs and evaluation of the morphology
and stability of the MOF particles during fabrication of the MMMs. This chapter
elaborates the effect of the residual water on the deterioration of the Cu3BTC2 MOF
crystal structure.
Chapter 5 describes the effect of the MOF particles based on the thermal,
mechanical, rheological and electron microscopy studies. This chapter describes the
toughness mechanism, which helps in understanding the improved toughness of the PLAMOF composites as compared to the neat PLA matrix. Rheological and thermal studies
helped in understanding the interfacial interactions between PLA and MOF.
Chapter 6 presents the mass transfer properties of PLLA and the effect of the
presence of metal organic framework on the mass transfer properties of mixed matrix
membranes for CO2, O2, H2O and trans-2-hexenal.
Chapter 7 is conclusions and recommendation for future work.

6

REFERENCES

7

REFERENCES

1. Swain SN, Biswal SM, Nanda PK, Nayak PL. Biodegradable soy based plastics:
opportunities and challenges. Journal of polymers and the environment 2004; 12:
35-42.
2. Weber CJ, Haugaard V, Festerson R and Bertelsen G. Biobased packaging
materials for the food industry. Food additives and contaminants 2002; 19: 172177.
3. Auras R, Harte B and Selke S. An overview of polylactides as packaging
materials. Macromolecular Bioscience 2004; 4: 835-864.
4. Drumright RE, Gruber PR and Henton DE. Polylactic acid technology. Advanced
materials 2000; 12: 1841-1846.
5. Fukushima K, Sogo K, Miura S and Kimura Y. Production of D-lactic acid by
bacterial fermentation of rice strach. Macromolecular biosciences 2004; 4: 10211027.
6.

Shen XL and Xia LM Lactic acid production from cellulosic waste by
immobilized cells of lactobacillus delbrueckii. World journal of microbiology

7. Groot W, Krieken JV, Sliekersl O and Vos SD. Production and purification of
lactic acid and lactide in Poly(lactic acid) synthesis, structures, properties,
processing, and applications. Edited by Auras R, Lim LT, Selke SEM and Tsuji
H. Wiley series 2010.
8. Nampoothiri KM, Nair NR and John RP. An overview of the recent developments
in polylactide (PLA) research. Bioresource technology 2010; 22: 8493-8501.
9. Lim LT, Auras R and Rubino M. Processing technologies for poly(lactic acid).
Progress in polymer science 2008; 33; 820-852.
10. Rasal RM, Janorkar AV and Hirt DE. Poly(lactic acid) modifications. Progress in
polymer science 2010; 35; 338-356.
11. Bhardwaj R and Mohanty A. Advances in the properties of polylactides based
materials: A review. Journal of biobased materials and bioenergy 2007; 19: 191209.
12. Robeson LM. Polymer membranes for gas separation. Current Opinion in Solid
State and Material Science 1999; 4: 549-552.
13. Freemantle M. Membranes for gas separations. Chemical and Engineering News
2005; 83: 49-57.

8

14. Chung TS, Jiang LY, Li Y and Kulprathipanja S. Mixed matrix membranes
comprising organic polymers with dispersed inorganic fillers for gas separation.
Progress in Polymer Science 2007; 32: 483-507.
15. Ghosal K and Freeman BD. Gas separation using polymer membranes: an
overview. Polymers for Advanced Technologies 1993; 5: 673-697.
16. Ismail AF and David LIB. A review on the latest development of carbon
membranes for gas separation. Journal of Membrane Science 2001; 193: 1-18.
17. Mahajan R and Koros WJ. Factors controlling successful formation of mixed
matrix gas separation materials. Industrial and Engineering Chemistry Research
2000; 39: 2692-2696.
18. Mahajan R and Koros WJ. Challenges in forming successful mixed matrix
membranes with rigid polymeric materials. Journal of Applied Polymer Science
2002; 86: 881-890.
19. Li JR, Kuppler RJ and Zhou HC. Selective gas adsorption and separation in
metal–organic frameworks. Chemical Society Reviews 2009; 38: 1477-1504.
20. Ferey G, Mellot-Draznieks C, Serre C, Millange F, Dutour J, Surble S and
Margiolaki I. A chromium terephthalate based solid with unusually large pore
volumes and surface area. Science 2005; 309: 2040–2042.
21. Mahajan R, Burns R, Schaeffer M and Koros WJ. Challenges in forming
successful mixed matrix membranes with rigid polymeric materials, Journal of
Applied Polymer Science 2002; 86: 881–890.
22. Kathuria A, Abiad M and Auras R. Deterioration of metal organic framework
crystal structure during fabrication of poly(L-lactic) acid mixed matrix
membranes. Polymer International 2013.
23. Elangovan D, Yuzay I, Selke S and Auras R. Poly(lactic acid) metal organic
framework composites: optical, thermal and mechanical properties. Polymer
International 2012; 61: 30-37.
24. Car A, Stropnik C and Peinemann KV, Hybrid membrane materials with different
metal organic frameworks for gas separation. Desalination 2006; 200: 424-426.
25. Keskin S, Liu J, Rankin RB, Johnson JK and Sholl DS, Industrial and engineering
chemistry research 2009; 48: 2355-2371.
26. Almenar E, and Auras R. Permeability, Sorption and diffusion of pure PLA in
Poly(lactic acid) synthesis, structures, properties, processing, and applications.
Edited by Auras R, Lim LT, Selke SEM, and Tsuji H. Wiley series 2010.

9

27. Kusuma VA, Freeman BD and Yacaman MJ. Structure/property characteristics of
rubbery membranes for carbon dioxide removal in Advanced membrane
technology and applications. Edited by Li NN, Fane AG, Ho WSW and Matsuura.
Wiley Series 2008.

10

Chapter 2. Literature Review

11

Chapter 2. Literature Review

2.1

Structure, synthesis and properties of PLA
Polylactic acid can be synthesized from lactic acid by polycondensation reaction,

azeotropic

dehydration

condensation

polymerization

and/or

by

ring

opening

polymerization using lactide molecules [1]. Lactic acid, which is produced by the
fermentation of dextrose, exists in two enantiomeric forms, D-lactic acid and L-lactic
acid. Figure 1 shows the chemical structure of D and L-lactic acid.

O
HO

O
HO

OH
CH3

OH
CH3

L-(+)-Lactic acid

D-(-)-lactic acid

Figure 2.1: Chemical structure of a. L-lactic acid b. D-lactic acid

Mass production of PLA is pursued using ring opening polymerization (ROP) of
lactide dimer to achieve high molecular weight PLA. Lactide dimer exists in three
enantiomeric forms called D lactide, L lactide and meso lactide, represented in Figure 2.2
[2]. The stereochemical configuration of the lactide molecule affects the crystal structure,
degree of crystallinity, melting point, mechanical and barrier properties of the final PLA
[3-6]. High molecular weight PLA, a semicrystalline polymer when the L-lactide amount
is more than 92%wt., can crystallize in three-crystal form known as α, β, γ forms [6]. The
processing parameters and drawing conditions strongly affect the extent of crystallinity

12

and type of crystal formation in the final product [8]. The α crystal form is the most
common form present in PLA.

H
H3C

O

O

O

O

CH3

D-lactide

H

H3C
O

H

O
O

L-lactide

O
CH
H 3

H
H3C

O

O

O

O

H

CH3

meso-lactide

Figure 2.2: Chemical structure of a. L-lactic acid b. D-lactic acid c. meso-lactide

PLA has found application in the biomedical, packaging and textile industries.
However, some limitations such as poor gas barrier properties, poor impact strength and
toughness, poor tear resistance, low heat deflection temperature and poor functionality
hinder applications in the packaging industry [9-11]. The crazing phenomenon is
primarily responsible for the brittle failure of PLA [12]. Various methodologies can be
employed to improve the toughness of PLA, such as plasticization, copolymerization,
melt blending, reactive extrusion and addition of fillers [13-18]. Fillers are primary added
to the polymeric matrix to improve the stiffness. However, favorable interfacial
interactions, distance between the particles suspended in the matrix or the right
concentration of the filler can contribute to improved toughness. Toughening of polymer
composites can be explained by various mechanisms such as shear yielding, cavitation,
crazing, etc.

13

2.2 Toughnening of polymer composites
2.2.1 Cavitation
When uniaxial stress is applied on polymers filled with toughening agents or modified
inorganic particles, the uniaxial stress can generate triaxial stress at the interface of the
polymer and the filler, which is relieved by formation of microvoids, leading to improved
toughness. This mechanism is known as the cavitation mechanism. The cavitation
mechanism can progress in two fashions: (a). Interfacial debonding leading to the
formation of micro size cavities [19] and (b). cavitation within the fillers such as rubber
particles [20].
2.2.2 Crazing
Glassy polymers such as polystyrene and PLA undergo brittle deformation by craze
formation due to the localized nature of the crazes. These Crazes are defined as
microvoids separated with fibrils of 5-20 nm diameter [21]. In some cases, these crazes
act as a source of energy dissipation leading to a plastic deformation. These fibrils break
under stress, leading to the absorption of energy.
2.2.3 Shear yielding
Shear yielding is known as a localized plastic deformation phenomenon. During the
crazing and cavitation mechanism, a change in the volume of the matrix is observed,
however, there is no volume change in the case of shear yielding. Increased entanglement
or cross-linking density supports shear yielding over crazing as it hinders the creation of
the voids.
2.3 Adsorption

14

Adsorption is a surface phenomenon. It is a process where molecules condense at the
surface. The adsorption of various molecules on the surface is dependent on factors such
as the partial vapor pressure of the adsorbate, interaction between the adsorbate and the
adsorbent, temperature, etc. Adsorption is an exothermic process and adsorption
isotherms are utilized to accurately determine the performance characteristics of porous
materials. The state of the adsorbate (atomic or molecular) can be determined by
spectroscopic analysis. An adsorption isotherm can be generated by using gravimetric,
constant volume and dynamic adsorption methods. The adsorbate can form a monolayer
or multilayer over the adsorbent. There are various models such as the Langmuir and
Brunauer-Emmett-Teller (BET) model which describe monolayer and multilayer
adsorption.
2.3.1 Difference between physisorption and chemisorption
Based on the strength of the bond between the adsorbate molecule and adsorbant,
adsorption can be divided into two classes: (a). chemical adsorption (chemisorption) and
(b). physical adsorption (physisorption). Chemisorption occurs due to ionic, covalent or
metallic bonds/coordinates. It may or may not be reversible. Chemisorption may or may
not be dissociative in nature. Non-dissociative adsorption is also referred to as molecular
adsorption. The enthalpy of adsorption is generally greater than 40 kJ/mol. On the other
hand, physisorption is a reversible and non-dissociative process. Physisorption primarily
occurs due to van der Waals interactions between the adsorbate and adsorbent, such as
dipole-dipole, dipole quadrupole, and quadrupole-quadrupole interactions. The enthalpy
of adsorption is generally less than 40 kJ/mol.
2.3.2 Methods to study adsorption isotherms
15

Physical adsorption or physisorption can be studied by static methods or dynamic
methods. Static methods are preferred over the dynamic method because they are simple
and provide more accuracy [22].
1. Volumetric technique: Sample preparation is performed using degassing where the
adsorbed molecules are removed from the surface or pores of the sample at a higher
temperature. The samples are typically analyzed by gases such as N2 or Ar at their
liquefaction temperature, for example 77K for N2. The sample is exposed to incremental
(or decremental) doses of pressure to obtain an adsorption (or desorption) isotherm. The
probability of gas molecules attaching to the surface increases with an increase in the
relative pressure. The adsorbed volume of gas as a function of relative pressure is
recorded by the instrument. This technique is popular for measuring specific surface
area, pore volume and pore size. For measuring the BET surface area, a relative pressure
between 0.05 and 0.3 is utilized. The disadvantages of this technique are cost and
complexity, as it involves the use of vacuum pumps.
2. Gravimetric technique
This technique utilizes a precision microbalance to obtain weight gain/loss as a
function of change in the pressure at a fixed temperature. For example, the VTI
instrument measures the weight gain. The gravimetric technique is utilized to determine
the isosteric heat of sorption and molecular interactions between the sorbate and sorbent
[23]. It is also used to study the morphological stability of pharmaceutical products at
elevated humidity conditions.

16

Porous materials are carefully engineered to perform specific functions. Their
physical characteristics such as specific surface area and pore size influence the quality
and utility of the material. Adsorption isotherms are utilized to accurately determine the
performance characteristics of the porous materials by using gases such as N2, Ar.
Sample preparation is performed using degassing where the adsorbed molecules are
removed from the surface or pores of the sample at higher temperature. The samples are
typically analyzed at the liquefaction temperature of the gas for example 77K for N2. A
gas molecule is attracted to the surface by intrinsic surface energy of the sample. The
physisorption of the molecules tend to increase with the increasing partial pressure over
the sample. The shape of the pore, pore size and pore size distribution strongly affects the
adsorption behavior of the porous materials. Therefore, the texture of a porous material
can be analyzed using the shape of the isotherm.
The majority of the isotherms can be classified into six types; type I to type VI; as
per the IUPAC classification depicted in the figure 2.5 [24].

17

Type I

Type II

Type III

Type IV

Type V

Type VI

Figure 2.3 IUPAC classifications of sorption isotherms

Type I isotherm also known as the Langmuir isotherm does not generally exhibits
hysteresis. This type of isotherm is associated with microporous materials (pores<2 nm).
A plateau is observed at a low relative pressure because the pores are so small that they
are filled at low pressure. Once the pores are filled the remaining adsorption occurs on
the exterior of the porous material. The Type II isotherm is generally observed in nonporous or macroporous materials. The knee observed in the isotherm indicates the
formation of a monolayer. At higher relative pressures, multilayer adsorption is observed
in such type of isotherms. Type III isotherms are also related to macroporous materials;
however, there is no knee formation in these types of curves. These isotherms indicate
weak adsorbate and adsorbant interactions. Type IV isotherms are typically observed in

18

mesoporous materials (2 to 50 nm). A hysteresis effect is commonly observed in such the
porous materials as the shape and size of the pores causes the adsorption or desorption
portions of the isotherm to have different path. In the type V isotherm there is no plateau
indicating the completion of a monolayer. This effect is due to poor interaction between
adsorbate and adsorbent, for example adsorption of water on carbon. The hydrophobic
carbon does not absorb on the carbon and the observed adsorption can be attributed to the
condensation of water rather than the adsorption. Type VI isotherms shows step-by-step
formation of multilayers on a nonporous material.
2.4 Metal organic frameworks
Porous materials can be categorized into three categories based on the ingredients:
inorganic porous material, hybrid organic-inorganic solid (coordination polymers) and
carbon based high surface area materials. Some examples of high surface area porous
materials are zeolites, silica gel, metal organic frameworks, carbon nanotubes, fullerene,
activated carbon, and carbon fiber composite molecular sieves. Charcoal and activated
carbon have surface area of 500-1000 m2/g. Silica gel has a surface area of around 800
m2/g. Zeolites have surface area ranging from 500 to 800 m2/g. Metal organic
frameworks (MOFs), a relatively new class of porous materials, have been discovered in
late 1990’s. MOFs are popular for various properties such as catalysis, gas separation,
sorption and high internal surface area, etc. [25-26]. They are coordinate polymers
formed by the co-ordinate bond between the organic linkers and metal ion. These
coordinate bonds are weak in comparison to the Si/Al–O bonds present in the zeolite
structure. Therefore, these coordination polymers have poor thermal stability compared to
zeolites. However, they have sufficient strength for many applications. Some MOFs have

19

surface areas in excess of 5900 m2/g [26]. High selective interaction of MOF with various
gases and molecules opens various avenues for deploying these materials in various
applied disciplines such as the petrochemical, energy, environmental and packaging
industries [27]. These materials are easy to design and can also be tailored to required
applications. MOF based membranes can be used to selectively purge molecules
considered undesirable in the package over a period of time, which can affect the quality
of the product, aroma or otherwise influence the customer perception.
The adsorption mechanism of various molecules is governed by factors such as
electrostatic interactions, dipole moment, magnetic dipole moment, ionization potential,
vapor pressure of adsorbent, pore size in the MOF, etc. Permanent magnetic dipole
moment in atoms, molecules or ions originate from the orbital motion of the electrons
and from the spin of the electrons. Adsorption of various molecules by MOF can occur
through various ways:
a. Adsorption of the molecules by unsaturated metal sites present in the MOF.
b. Interaction between the ligand and the molecules.
c. Interaction between the polar groups attached to the metal and the molecules.
Better chemical specificity for the guest molecule can be achieved by engineering
customized metal sites and organic linkers. The metal or metal oxide sites present in the
MOF get saturated very quickly. Molecules can bind to metal/metal oxide more strongly
as compared to ligands. This can be ascribed to electrostatic and orbital donation
interactions between the transition metal and the adsorbed molecule. The structure of the
d electron shell plays a significant role in interactions. A strong magnetic effect is
observed when d and f electrons are involved. Such interactions can lead to dissociate

20

chemisorption of the molecules on the metal [28]. However, chemisorption is also
dependent on the strength of the bond of the molecule to be dissociated. Van der Waal
forces dominate the interactions between the ligand and the adsorbed molecule [29]. The
heat of adsorption defines the bond strength between the metal and sorbates. Initial heats
of adsorption in metals follow this trend [30]:

Ta > W > Cr > Fe > Ni > Rh > Cu > Au

Torrisi et al [31, 32] studied the van der Waal interactions between CO2 and the
aromatic structures present in the ligands using ab initio PW 91/DNP Approximation
(GGA) density functional theory (DFT). Two classes of functional group were selected
for substitution on the benzene rings: (i) electrophilic halogen groups (ii) nucleophilic
methyl groups. It was observed that the halogen substitution of the benzene rings
destabilizes the π-quadrupole interactions. This was ascribed to the charge withdrawal
effect of hydrogen-substituted groups. Methyl and polymethyl substitution on the
benzene ring increased the CO2 affinity.

δδ

δ+

δ

δ+
δ
H

H

δ+
δ
H

δδ

δ+
δ
H
δ

F

F

F

δ-

δ+
H

δ δ-

δ

δ

F

δ-

F
δ
δ- F

δ
δ+ H

Fig 2.4 a. Benzene

Fig 2.4 b. Hexaflourobenzene

21

Rowsell et al [33] studied H2 adsorption by Zn4O (BDC)3, Zn4O (NDC)3, Zn4O
(HPDC)3, Zn4O (TMBDC)3 and Zn4O (BTB)3 MOF up to pressure of 750 Torr. They
observed that the interaction of the guest molecules varies with the chemical structure of
the ligand. It was observed that the uptake of H2 increases with the number of organic
rings per unit formula. Initial hydrogen uptakes revealed that Zn4O (NDC)3 and Zn4O
(HPDC)3 have more affinity for the H2 molecules than to the other MOF structures
utilized in the study.
The ubiquitous presence of aromatic linkers in the various varieties of MOF can
be related to the thermal stability of the benzene ring. The presence of the delocalized π
electrons in the benzene ring in the MOF structure also helps in the secondary forces
between the ligand structure and the sorbent [31, 32]. The interaction between these
aromatic ligands or secondary building units attached to the metal ions also defines the
geometry and pore size in these porous molecules [34, 35]. The stacking interactions of
these aromatic molecules are also affected by the substitute groups attached to the
aromatic ring. Electron donating groups help in ramping up the electron density of the
delocalized π electrons of the aromatic group whereas electron withdrawing groups like
NO2 can reduce the electron density of the delocalized π electrons. All these groups in
turn will effect on the geometry of the MOF.

22

OH

O

C

HO

C
O

HO

OH

O

C

O

C

C

OH

O

Benzene tricarboxylic acid
Benzene tricarboxylic acid

H2BDC

OH

O
C

OH

O
C

C
HO
C
HO

O

O

H2BPDC
H2BPDC

H2H2BPDC
H2TPDC
TPDC

H2BPDC

Figure 2.5: Some carboxylate based ligands

23

The good stability, and good water and CO2 sorption properties of Cu3(BTC)2
MOF inclined us to further explore its synthesis and characterization.
commercially manufactured by BASF under the trade name Basolite

TM

It is also
C 300. By

embedding Cu3(BTC)2 MOF we can enhance the permeability and permselectivity of
polymeric membranes.
2.5 Mixed Matrix Membranes
Gas separation using polymeric membranes is popular because of their adequate
mechanical strength, inherent transport properties, ease of processing and upscaling.
Conventional non-porous polymeric membranes follow a solution-diffusion model for the
gas separation [36]. The slow rate of diffusion through polymers reduces their
commercial attractiveness. Molecular interactions between the permeants and polymer
play important roles in determining the perm-selectivity. Solubility of polymers to
organic compounds and their segmental mobility limits the gas selectivity. The use of
polymeric membranes at elevated temperature can reduce the life of polymeric
membranes.
Polyimide, polydimethylsiloxane (PDMS), polysulfone (PSf) and polyethylene
oxide (PEO) are some of the widely used polymeric membranes. Polyimide is
commercially available under the trade name Matrimid

TM

. It has high thermal stability,

good chemical resistance and high mechanical strength [37]. PEO is popular for its high
CO2 selectivity due to strong interactions between CO2 and the polar ether oxygens
present in PEO [38]. However, crystallization due to aging can reduce the permeability of

24

such membranes. The crystallization process can be reduced through copolymerization,
controlling the molecular weight or by branching the main chain [38, 39]. PSF is popular
for membrane applications for water purification, ultra filtration, reverse osmosis and gas
separation [39]. It is a glassy polymer (Tg ~ 186oC) with good mechanical strength,
excellent chemical stability and ease of processing [39, 40].
Low cost and easy processibility of the polymers and high selectivity of porous
membranes motivates the fabrication of polymer-porous material composite membranes
also known as MMMs [41]. These MMMs can provide attractive properties for gas
separation. Appropriate selection of the porous material and polymeric matrix is
necessary for producing efficient MMMs. The choice of polymeric matrix and porous
material should be synchronized to achieve the desired selectivity and mass transfer
properties [42]. The polymer should be compatible with porous material and the polymerporous material interface should be free of voids [42-45].
Traditionally, MMMs have been fabricated by dispersing zeolite particles in
polymeric membranes. Different types of Zeolites have been widely researched with a
variety of polymeric materials [42-50] to obtain increased permeability and superior
selectivity as compared to polymeric membranes alone. Most of the studies indicated the
surface modification need to achieve the good compatibility. Mahajan et al [46] studied
®

MMMs prepared from zeolite 4A and Matrimid . They reported voids at the interface of
zeolite 4A and Matrimid

®

polymer which reduced the perm-selectivity of the MMMs.

Modification of the zeolite 4A with silane linkers before fabrication of MMMs can help
in eliminating the voids at the interface of the seives and polymer matrix [47].

25

However, Yuzay et al [48] fabricated PLA - zeolite type 4A composites using
melt extrusion processing. The authors observed void free interface of PLA and zeolite
type 4A in the microscopic images, indicating good interfacial interactions between PLA
and zeolite type 4A.
In recent years MOFs have attracted great attention for fabrication of MMMs
because of their high surface area, ease of synthesis and availability of thousands of
different structures [49-57]. In addition MOFs are composed of metal ions connected
together by organic linkers, the presence of these organic linkers helps to improve the
interfacial interactions [52, 58-59]. Elongavon et al [59] studied PLLA and Cu3(BTC)2
MOF using contact angle studies using geometric and harmonic means and obtained low
interfacial tension and high work of adhesion, indicating good compatibility between the
MOF and the PLLA. Adams et al [49] prepared MMMs from poly(vinyl acetate) (PVAc)
and 15% copper-terephthalic acid based (CuTPA) MOF by polymer solution processing.
SEM images of the MMM cross section showed well dispersed CuTPA MOF particles.
The synthesized MMMs had increased permeability for He, O2, N2 and CH4 gases as
compared to neat polymer. The authors also reported improved selectivity for various
combinations of gases such as CO2/CH4 and O2/N2.
Car et al [50] studied copper benzene 1,3,5-tricarboxylate (Cu3(BTC)2) MOF and
Mn(HCOO)2 (manganese (II) formate) MOFs with rubbery dimethylsiloxane (PDMS)
and glassy polysulfone (PSf). The authors reported slight improvement of CO2/N2
selectivity for PDMS-10% Cu3(BTC)2. Increased H2, N2, O2, CH4 and CO2 permeability

26

was observed for PSf-5% Cu3(BTC)2 MOF and PSf-10% Cu3(BTC)2 MOF. In other
research [51] on Matrimid

®

with 10, 20 and 30% MOF-5 w/w, strong polymer-MOF

interactions were observed by SEM microscopy. Plastic deformation was observed in the
Matrimid-MOF-5 composites due to the formation of cavities. The permeability of
various gases increased up to 120%, however there was no improvement in the selectivity
of the membranes.
Bae et al [52] fabricated MMMs containing zeolitic imidazolate framework-90
(ZIF-90) porous crystals. Good adhesion was observed between ZIF-90 and the
polyimide polymer matrix. The authors reported increased CO2 permeability without any
loss of CO2/CH4 selectivity.
Various computational analyses have been published to estimate and validate the
performance

of

MMMs

[54-56].

Keskin

et

al

[54]

theoretically

studied

®

Cu(hfipbb)(H2hfipbb)0.5 particles in a Matrimid film and their calculation predicted that
MMMs provided high selectivity for CO2/CH4 separations but mediocre selectivity for
H2/CO2. They also concluded that MMMs prepared from a particular polymer and
porous microporous crystalline material may act as a good membrane for a particular gas
combination but it may not work for other gas combination.
Apart from the separation of gases, such composite systems can be used to
selectively purge or scavenge gases from the headspace of the food or pharmaceutical
package. These porous materials can also be encapsulated in canisters and caps, opening
windows for various packaging applications.

27

REFERENCES

28

REFERENCES

1. Kricheldorf HR. Synthesis and application of polylactides. Chemosphere 2001;
43: 49-54.
2. Garlotta D. A literature review of poly(lactic acid). Journal of Polymers and the
Environment, 2002; 9: 63-84.
3. Södergard A and Stolt M. Properties of lactic acid based polymers and their
correlation with composition. Progress in Polymer Science 2002; 27: 1123-1163.
4. Cocca M, Lorenzo MLD, Malinconico M and Frezza V. Influence of crystal
polymorphism on mechanical and barrier properties of poly(L-lactic acid).
European Polymer Journal 2011; 47: 1073-1080.
5. Tsuji H. Poly(lactide) stereocomplexes: Formation, structure, properties,
degradation, and applications. Macromolecular Bioscience 2005; 5: 569-597.
6. Jiang X, Luo Y, Tian X, Huang D, Reddy N and Yang Y. Chemical structure of
poly(lactic acid) in Poly(lactic acid) synthesis, structures, properties, processing,
and applications. Edited by Auras R, Lim LT, Selke SEM, and Tsuji H. Wiley
series 2010.
7. Hoogsteen W, Postema AR, Pennings AJ and Brinke GT. Crystal structure,
conformation and morphology of solution-spun poly(L-lactide) fibers,
Macromolecules 1990; 23: 634-642.
8. Auras R, Harte B and Selke S. An overview of polylactides as packaging
materials. Macromolecular Bioscience 2004; 4: 835-864.
9. Ahmed J and Varshney SK. Polylactides-chemistry, properties and green
packaging technology: A review. International Journal of Food Properties 2011;
14: 37-58.
10. Dorgan RJ, Lehermeier H and Mang M. Thermal and rheological properties of
commercial grade poly(lactic acid)s. Journal of Polymers and the Environment
2000; 8: 1-9.
11. Rasal RM, Janorkar AV and Hirt DE. Poly(lactic acid) modifications. Progress in
polymer science 2010; 35; 338-356.
12. Berger LL. Relationship between craze microstructure and molecular
entanglements in glassy polymers 1. Macromolecules 1989; 22: 3162-3167.
13. Liu H and Zhang J. Research progress in toughening modification of poly(lactic
acid). Journal of Polymer Science Part B: Polymer Physics 2011; 49: 1051-1083.

29

14. Martin O and Averous L. Poly(lactic acid): plasticization and properties of
biodegradable multiphase systems. Polymer 2001; 42: 6209-6219.
15. Grijpma DW and Penning AJ. (Co) polymers of L-lactide of L-lactide, 1.
Synthesis, thermal properties and hydrolytic degradation. Macromolecular
Chemistry and Physics 1994; 195: 1633-1647.
16. Zhang L, Xiong C, Deng X. Miscibility, crystallization and morphology of
poly(β-hydroxybutrate)/poly (d,l-lactide) blends. Polymer 1996 ; 37: 235-241.
17. Bhardwaj R, and Mohanty A K. Modification of Brittle Polylactide by Novel
Hyperbranched Polymer-Based Nanostructures. Biomacromolecules 2007; 8:
2476-2484.
18. Ray SS and Okamoto M. Polymer/layered silicate nanocomposites: a review from
preparation to processing. Progress in Polymer Science 2003; 28 1539–1641.
19. Zuiderduin WCJ, Westzaan C, J. Hue´tink J, Gaymans RJ. Toughening of
polypropylene with calcium carbonate particles. Polymer 2003; 44: 261-275.
20. Kim GM, and Michler GH. Micromechanical deformation processes in toughened
and particle filled semicrystalline polymers. Part 1: Model representation for
micromechanical deformation processes. Polymer 1998; 39:5689-5679.
21. Processing and finishing of polymeric materials Vol 1 Wiley Series 2011.
22. Keller J and Staudt R. Gas adsorption equilibria: experimental methods and
adsorption isotherms. New York: Springer 2005.
23. Sorption isotherms and isosteric heat of sorption for grapes, apricots, apples and
potatoes. Food Science and Technology 2004; 37: 429-438.
24. Adamson AW and Gast AP. Physical Chemistry of Surfaces; Sixth Edition; A
Wiley-Interscience publication, 1997.
25. Mueller U, Schubert M, Teich F, Puetter H, Schierle-Arndt K and Pastre J. Metal–
organic frameworks—prospective industrial applications. Journal of Materials
Chemistry 2006; 16: 626–636.
26. Ferey, G, Mellot-Draznieks C, Serre C, Millange F, Dutour J, Surble S,
Margiolaki I, A Chromium Terephthalate-Based Solid with Unusually Large Pore
Volumes and Surface Area. Science 2005; 309: 2040–2042.
27. Auras R, Selke S, Elangovan D and Yuzay IE. Polymers and metal organic
frameworks composites and methods of preparation thereof. US patent application
US 2011/0297 610/A1 (2011).

30

28. Deuss H and Vander Avoird A. Model for dissociative H2 chemisorption on
transition metals, Physical Review B; 1973; 8: 2441-2444.
29. Lochan RC and Head-Gordon M. Computational studies of molecular hydrogen
binding affinities: The role of dispersion forces, electrostatics, and orbital
interactions, Physical Chemistry Chemical Physics 2006; 8: 1357–1370.
30. Dwyler FP and Mellor DP. Chelating agents and metal chelates. New York
Academic Press 1964.
31. Torrisi A, Mellot-Draznieks C and Bell RG. Impact of ligands on CO2 adsorption
in metal-organic frameworks: First principles study of the interaction of CO2 with
functionalized benzenes. I. Inductive effects on the aromatic ring, The Journal of
Chemical Physics 2009; 130, 194703: 1-13.
32. Torrisi A, Mellot-Draznieks C and Robert G. Bell. Impact of ligands on CO2
adsorption in metal-organic frameworks: First principles study of the interaction
of CO2 with functionalized benzenes. II. Effect of polar and acidic substituents,
Inductive effects on the aromatic ring. The Journal of Chemical Physics 2010;
132, 044705:1-13.
33. Rowsell JLC, Millward AR, Park KS and Yaghi OM. Hydrogen Sorption in
Functionalized Metal-Organic Frameworks. Journal of American Chemical
Society 2004; 126: 5666-5667.
34. Hunter CA, Lawson KR, Perkins J and Urch CJ. Aromatic interactions, Journal of
Chemical Society, Perkin Transactions 2001; 2: 651–669.
35. Hunter CA and Sanders JKM. The Nature of π- π interactions. Journal of
American Chemical Society 1990; 112: 5525-5534.
36. Inorganic-polymeric and composite membranes: structure, function and others.
Edited by Oyama ST and Stagg-Williams SM.
37. Du N, Park BH, Dal-Cin MM and Guiver MD.Advances in high permeability
polymeric membrane materials for CO2 separations. Energy and Environmental
Science 2012; 5: 7306-7322.
38. Lin H, Wagner EV, Freeman BD, Toy LG and Gupta RP. Plasticization enhanced
hydrogen purification using polymeric membranes. Science, 2006, 311, 639–642.
39. Kim HW and Park HB. Gas diffusivity, solubility and permeability in
polysulfone–poly(ethylene oxide) random copolymer membranes 2011; 372: 116124.
40. Ghosal K, Chern RT, Freeman BD, Daly WH, Negulescu II. Effect of basic
substituents on gas sorption and permeation in polysulfone. Macromolecules 29
(1996) 4360–4369.

31

41. Moore TT, Mahajan R, Vu DQ and Koros WJ. Hybrid membrane materials
comprising organic polymers with rigid dispersed phases. American Institute of
Chemical Engineers 2004; 50: 311-321.
42. Mahajan R and Koros WJ. Factors controlling successful formation of mixed
matrix gas separation materials. Industrial and Engineering Chemistry Research
2000; 39: 2692-2696.
43. Suer MG, Bacü N and Yilmaz L. Gas permeation characteristics of polymerzeolite mixed matrix membranes. Journal of Membrane Science 1994; 91: 77-86.
44. Chung TS, Jiang LY, Li Y, Kulprathipanja S. Mixed matrix membranes (MMMs)
comprising organic polymers with dispersed inorganic fillers for gas separation.
Progress in Polymer Science 2007; 32: 483−507.
45. Duval JM, Folkers B, Mulders MVH, Desgrandchamps G, Smolders CA.
Adsorbent filled membranes for gas separation. Part 1. Improvement of the gas
separation properties of polymeric membranes by incorporation of microporous
adsorbents. Journal of Membrane Science 1993; 80: 189-198.
46. Mahajan R and Koros WJ. Mixed Matrix Membrane Materials with glassy
polymers. Part 1. Polymer Engineering and Science 2002; 42: 1420-1431.
47. Mahajan R and Koros WJ. Mixed Matrix Membrane Materials with glassy
polymers. Part 2. Polymer Engineering and Science 2002; 42: 1432-1440.
48. Adams R, Carson C, Ward J and tannenbaum. Metal organic framework mixed
matrix membranes for gas separations. Microporous and Mesoporous Materials
2010; 131: 13-20.
49. Yuzay I, Auras R and Selke S. Poly(lactic acid) and zeolite composites prepared
by melt processing: morphological and physical-mechanical properties. Journal of
applied polymer science 2010; 115: 2262-2270.
50. Hybrid membrane materials with different metal–organic frameworks (MOFs) for
gas separation. Desalination 2006; 200:424–426.
51. Perez EV, Balkus KJ, Ferraris JP and Musselman IH. Mixed matrix membranes
containing MOF-5 for gas separations. Journal of Membrane Science 2009; 328:
165-173.
52. Bae TH, Lee JS, Qiu W, Koros WJ, Jones CW and Nair S. A High performance
gas separation membrane containing submicrometer sized metal organic
framework crystals. Angewandte Chemie International Edition 2010; 49: 98639866.

32

53. Zhang Y, Musselman IH, Ferraris JP, Balkus KJ. Gas permeability properties of
Matrimid® membranes containing the metal organic framework Cu–BPY–HFS.
Journal of Membrane Science 313 (2008) 170–181.
54. Keskin S and Sholl DS. Selecting metal organic frameworks as enabling materials
in mixed matrix membranes for high efficiency natural gas purification. Energy
and Environmental Science 2010; 3: 343-351.
55. Keskin S and Sholl DS. Assessment of a metal organic framework membrane for
gas separations using atomically detailed calculations: CO2, CH4, N2, H2 Mixtures
in MOF-5. Industrial and Engineering Chemistry Research 2009; 48: 914-922.
Energy and Environmental Science 2010; 3: 343-351.
56. Keskin S. Molecular simulation study of CH4/H2 mixture separations using metal
organic framework membranes and composites. The Journal of Physical
Chemistry 2010; 114, 13047–13054.
57. Ren H, Jin J, Hu J and Liu H. Affinity between metal organic frameworks and
polyimides in asymmetric mixed matrix membranes for gas separations. Industrial
and Engineering Chemistry Research 2012; 51: 10156-10164.
58. Kathuria A, Abiad MG and Auras R. Deterioration of metal–organic framework
crystal structure during fabrication of poly( L-lactic acid) mixed-matrix
membranes 2013.
59. Elangovan D, Yuzay IE, Selke SEM and Auras R. Poly(L-lactic acid) metal
organic framework composites: optical, thermal and mechanical properties 2012;
61: 30-37.

33

Chapter 3. Synthesis and characterization of Cu3(BTC)2 MOF

34

Chapter 3. Synthesis and characterization of Cu3(BTC)2 MOF

3.1 Introduction

Coordination polymers or supramolecules first came into attention in the 1960s.
However, the interest in porous coordination polymers and MOFs was invigorated in the
late 1990s [1]. The synthesis of MOFs has attracted increasingly attention in the last two
decades [2]. Thousands of MOF structures can be synthesized by varying the metal and
organic components, two primary building blocks required for the synthesis of MOF. The
synthesis of MOF can be pursued using various routes as summarized below:
1. Solovothermal or hydrothermal synthesis
a. Synthesis using autoclave
b. Microwave assisted synthesis
c. Ultrasonic synthesis
2. Slow diffusion process
3. Synthesis under ambient pressure using refluxing
4. Mechano-chemical synthesis
5. Electrochemical synthesis

As the name suggests, the solvothermal or hydrothermal technique uses heat with
a solvent or aqueous solution for the synthesis of crystals. In this technique, the synthesis
has traditionally been carried out in an autoclave. The conventional heating process is
inconsistent; there is variation in the temperature across the solution. It may take days to
a week to synthesize MOF crystals using an autoclave, and it is not viable for commercial

35

production of MOF. Solvothermal and hydrothermal reactions can be carried out by
microwave assisted rapid synthesis or sonochemical synthesis. Uniform morphology,
small distribution of crystal size, even nucleation and fast reaction rate are some of the
popular benefits of microwave synthesis as compared to autoclave [3-5]. Sonochemical
synthesis, also known as ultrasonic synthesis, is another energy efficient synthesis
technique, which decreases the reaction time significantly.
Ni et al [4] reported that microwaves create uniform seeding conditions which
generates the narrow size distribution. The fast reaction rate also leads to shorter reaction
time and higher energy efficiency for microwave synthesis. Solvothermal synthesis by
autoclave generates crystals, that can be analyzed using single crystal X ray diffraction
analysis [5] whereas microwaves synthesis cannot generate single crystals, which are big
enough for single crystal X-ray analysis [4] and high temperature and pressure are still
required for the synthesis of these crystals.
Li et al [6] utilized sonochemical synthesis technique for the synthesis of
Cu3(BTC)2 MOF at atmospheric pressure and room temperature for 1 hour.
Sonochemical synthesis induces homogeneous nucleation and rapid crystal synthesis. It is
environmental friendly, efficient, easy and cheap process [6,7].
Li et al [8] fabricated [Zn(BDC)(H2O)]n (BDC = 1,4 benzenedicarboxylate)
MOF using synthesis times of 10, 20, 30, 60 and 90 minutes. Nanocrystals were obtained
at 10 to 60 minutes. However, at 90 minutes of reaction time ultrasound radiations
damaged the crystals.

36

Hartmann et al [9] synthesized Cu3(BTC)2 MOF by solvothermal synthesis,
electrochemical synthesis, and synthesis under ambient pressure. The authors reported
2

surface areas between 1153 to 1624 m /g. The selection of the synthesis process affected
the surface area and specific pore volume. The highest surface area was achieved at
ambient temperature by a rigorous stirring and refluxing process followed by
solvothermal process using an autoclave. The electrochemical synthesis provided the
lowest surface area. The authors compared ethanol and 50% ethanol plus 50% water as
solvents for solvothermal synthesis. Ethanol as a solvent yielded higher pore volume and
higher specific surface area as compared to ethanol-water solution. For example,
2

autoclave synthesis in ethanol yielded a surface area of 1510 m /g whereas autoclave
2

synthesis in ethanol water solution provided 1253 m /g.
Pichon et al [10] utilized a mechanochemical synthesis technique for the synthesis
of Cu(INA)2 MOF (INA = isonicotinic acid). The authors ground copper acetate with
isonicotinic acid using a ball mill at a 25 Hz oscillation rate for 10 minutes without any
heat or solvent to synthesize the MOF. The grinding process generated acetic acid as a
byproduct. The authors suggested that the grinding process was required to uniformly
mix the reactants, and the reaction can be initiated after 1 minute of grinding, and it can
be completed in 6 hours unaided. However, continuation of grinding accelerated the rate
of reaction.
Biemmi et al [11] studied the influence of reaction temperature and time on the
synthesis of HKUST-1 [Cu3(BTC)2] MOF and MOF-5. They synthesized Cu3(BTC)2 at
348, 393, 423 and 453 K. The formation of Cu2O (2θ= 36.43) in a significant amount
37

was observed at 423 and 453 K in the XRD patterns. The authors obtained a max yield
of 80% at a synthesis time of 320 h. The choice of salt also affected the purity of the
MOF.
Krawiec et al [12] synthesized Cu3(BTC)2 by pressure free synthesis using 50
mmol copper (II) acetate monohydrate and 50 mmol trimesic acid (TMA) in dimethyl
formamide. The mixture was refluxed and stirred for 12 h. After cooling and filtering the
solution, the powder was heated in 150 mL DMF for 1 h at 423 K to remove the residual
TMA and washed with water for 1 h at 403 K. The product was activated at 433 K for 48
hr and analyzed for BET surface area. The Cu3(BTC)2 powder had a single point BET
2 -1

3 -1

surface area of 1239 m g with pore volume of 0.62 cm g , as measured by N2 physical
adsorption isotherm.
Cu3(BTC)2 has also been synthesized by electrochemical synthesis route [13].
The authors utilized copper as anode and cathode in the electrochemical cell with TMA
dissolved in methanol as the electrolyte. The electrochemical process was carried out at
12-19 V for 150 min using 1.3 A current level and generated Cu3BTC2 MOF with a
2 -1

surface area of 1650 m g .

38

The crystal structure of various biomolecules is influenced by the Pai-Pai
interactions between the aromatic molecules. The benzene ring has quadrupole moment
of -29.0 x 10

-40

2

cm . Hunter et al [14,15] described quadrupole-quadrupole interactions

benzene ring for stack. Similar forces might be responsible for the creation of the rigid
framework with micropore structure in the MOF. Synthesis at elevated temperature and
pressure increases the adhesion between the particles.

Figure 3.1: Dicopper (II) tetracarboxylate building block or repeating unit for HKUST-1

Cu3(BTC)2 is a good water and carbon dioxide adsorbent. The chemical structure
of the buiding block of Cu3(BTC)2 is represented in Figure 3.1. To take advantage of
faster reaction rates, we decided to use a microwave assisted solovothermal synthesis
technique for the synthesis of Cu3(BTC)2 MOF also known as HKUST-1.
39

3.2 Methodology
3.2.1 Microwave Synthesis
An Ethos EZ microwave digester system from Milestone Inc., CT was utilized for
increasing the rate of chemical reaction. The Microwave digester was equipped with an
SK-10 segmented rotor. The rotor has a closed PTFE vessel, and it is designed to
withstand 100 bar pressure and 300oC. The PTFE vessel has an inbuilt thermo well to
insert thermocouple, which controls the temperature of reaction. Figure 3.2 shows the
picture of Ethos Ez Microwave digester.

Figure 3.2: Ethos Microwave, adapted from http://www.speciation.net/

Hydrothermal synthesis of the MOF utilized in this study was conducted according to the
following steps:
Synthesis of MOF
Twenty-four mL of water was mixed with 24 mL of ethanol. After that, 0.84 g of
benzene 1,3,5 tricarboxylic acid and 1.75 g of copper nitrate trihydrate were
added into the reaction vessel. The solution was allowed to react in the closed

40

vessel in the Ethos microwave for 4 h at 110oC under autogeneous conditions and
cooled. The blue powder recovered by filtering the solution was washed once
with water followed by one wash with ethanol. After evaporating the residual
alcohol from the powder, the powder was heated overnight at 120oC using
vacuum oven.
The presence of ethyl alcohol in the reaction reduces the hydrophobic
repulsions between water and benzene 1,3,5 tricarboxylic acid since smaller
carboxylic acids are soluble in water. Nitrate ions exist as counter anions in the
copper (II) nitrate trihydrate. Copper (II) nitrate trihydrate can form hexa aqua
metal ions with water. These hexa aqua metal ions are stable in the acid solution,
but they tend to be less stable in alkaline solutions.

Figure 3.3: Images of activated (left) and partially saturated Cu3(BTC)2
Figure 3.3 shows the activated MOF Cu3(BTC)2 which is dark violet and partially
saturated MOF – Cu3(BTC)2(H2O)3xH2O. After complete saturation the MOF acquires a
turquoise color. The water molecules axially coordinated to the two exchangeable copper

41

sites can be removed by heat activation under vacuum, yielding a lewis acid site with
catalytic properties. The effect of the adsorbed water on the X-ray diffraction pattern is
discussed in chapter 4 in more detail.

3.3 Characterization of Cu3(BTC)2 MOF
3.3.1 Surface Area and Porosity Measurement
A micromeritics accelerated surface area and porosity system (ASAP

TM

) 2020

was used to determine surface area and porosity of the micro porous material. Degassing
o

was carried out at 200 C for 48 h. The MOF was allowed to cool for half an hour and
then mounted on the analysis port. BET surface area analysis was performed using N2 gas
®

at 77 K using pressure ratios between 0 and 0.3. The ASAP 2020 software was utilized
to calculate the BET (Brunauer, Emmer and Teller) surface area. Generally, relative
pressures between 0.05 and 0.3 are utilized for BET surface area calculation using a static
volumetric technique [16]. From the isotherm data BET surface area can be calculated
using equation 1.

P
1 c 1 P


v  Po  P  cvm cvm Po

Where,
Po = Saturation Vapor Pressure
P = adsorption pressure

42

(1)

v = quantity of the gas adsorbed
C = constant, which constant c can be mathematically expressed as:

c  exp(

E1  EL
)
RT

R= Gas constant = 8.314 J/K/mol
E1 is the heat of adsorption for the first layer.
EL= Heat of adsorption for the subsequent layers.
T = Absolute temperature

The volume of the monolayer calculated from the equation 1 is then utilized to estimate
the BET surface area using equation 2.

SBET

(2)

vmNs

V

S = area occupied by a small gaseous molecule = 0.16 nm2 for N2.
N = Avogadro number = 6.023 x 10

23

Vm = Volume of the monolayer
V = Molar volume of the adsorbate gas

43

Programmable two
station degas system
Isothermal jacket

Vacuum cold trap
Long-duration
dewar

Figure 3.4: Micromeritics ASAP

TM

2020 adapted from www.micromeritics.com

3.3.2 Scanning electron microscope (SEM)
The MOF crystals were sputter coated with gold using an Emscope SC500
(Emscope Laboratories Ltd, Ashford, UK) sputter coater. The SEM analysis was carried
out using a Joel JSM 6400 SEM. The microscope was equipped with a LaB6 filament.
Images were acquired with a 3 nm resolution. The images were collected at 12keV
accelerating voltage and 39 mm working distance.

44

3.3.3 X-Ray Powder Diffraction
X-ray diffraction studies on the MOF crystals were performed using a Bruker D8
advance X-ray diffractometer (Bruker AXS G mgH, Karlsruhe, Germany) at 40kV,
40mA using Cu Kα radiation (λ = 1.5418 Å). A 1.2 mm primary beam slit and 2.0 mm
detector slit were used. The X-ray scans were carried out using 0.02 degrees per second.
Data was collected in triplicate.

3.4 Results and discussions

Figure 3.5 shows the N2 isotherm of the Cu3BTC2 MOF. The isotherm is a Type 1
isotherm (IUPAC classification) also known as a Langmuir isotherm.

Such isotherms

are associated with microporous materials (pores<2 nm) like MOFs and activated carbon
[17]. A Plateau was observed at low relative pressure because the pores are so small that
they get filled at low pressure. Once the pores are filled the remaining adsorption occurs
on the exterior of the porous material. We obtained an average BET surface area of 1236
2 -1

± 103 m g .

The SEM images of the Cu3BTC2 MOF crystal acquired using the JEOL 6600 are
presented in figure 3.6. The crystal size ranges from 5-30 µm with average crystal size
~15 µm.

45

Figure 3.5: N2 isotherm of Cu3(BTC)2 MOF

46

50 µm

5 µm

Figure 3.6: SEM images of Cu3(BTC)2 MOF acquired at 12kV and 39 mm working
distance

47

10 µm

20 µm

The powder X-ray diffraction pattern (PXRD) of the Cu3(BTC)2 MOF is represented in
Fig. 3.7. The crystals are in agreement with already published PXRD patterns of
Cu3(BTC)2 MOF [18, 19]. Hartmann et al [19] reported that peak at 2θ = 36.43 and 2θ =
35.5 and 38.7 relate to Cu2O and CuO, respectively. These impurities are generated at
reaction temperature above 110oC. Our samples were free of Cu2O and CuO impurities.

Figure 3.7: X ray diffraction pattern of Cu3(BTC)2 MOF

48

The Cu3(BTC)2 MOF was successfully synthesized using microwave assisted synthesis
process. However, for the simplicity, ease and reproducibility we used Basolite

TM

C 300,

supplied by sigma Aldrich, for the fabrication of membranes discussed in detail in
chapter 4, 5 and 6.

49

REFERENCES

50

REFERENCES

1.

James SL. Metal organic frameworks. Chemical Society Reviews 2003; 32:
276-288.

2.

Stock N and Biswas S. Synthesis of metal organic frameworks (MOFs):
Routes to various MOF topologies, morphologies, and composites. Chemical
Reviews 2012; 112: 933-969.

3.

Khan NA, Haque E and Jhung SH. Rapid syntheses of a metal–organic
framework material Cu3(BTC)2(H2O)3 under microwave: a quantitative
analysis of accelerated syntheses. Physical Chemistry and Chemical Physics
2010; 12: 2625–2631.

4.

Ni Z, and Masel RI. Rapid production of metal-organic frameworks via
microwave-assisted solvothermal synthesis. Journal of American Chemical
Society 2006; 128: 12394-12395.

5.

Tranchemontagne DJ, Hunt JR, Yaghi OM. Room temperature synthesis of
metal-organic frameworks: MOF-5, MOF-74, MOF-177, MOF-199, and
IRMOF-0. Tetrahedron 2008; 64: 8553–8557.

6.

Li ZQ, Qiu LG, Xu T, Wu Y, Wang W, Wu ZY, and Jiang X. Ultrasonic
synthesis of the microporous metal–organic framework Cu3(BTC)2 at ambient
temperature and pressure: An efficient and environmentally friendly method.
Materials Letters 2009;63: 78–80.

7.

Kim SH, Yang ST, Kim J, and Ahn WS. Sonochemical synthesis of
Cu3(BTC)2 in a deep eutectic mixture of choline chloride/dimethylurea.
Bulletin of Korean Chemical Society 2011; 32 (8): 2783-2786.

8.

Li Z-Q, Qiu L-G, Wang W, Wu Y, and Xia J. Fabrication of nanosheets of a
fluorescent metal-organic framework [Zn(BDC)(H2O)]n (BDC=1,4benzenedicarboxylate): ultrasonic synthesis and sensing of ethylamine.
Inorganic Chemistry Communications 2008; 11 1375-1377.

9.

Hartmann M, Kunz S, Himsl D, and Tangermann O. Adsorptive separation of
isobutene and isobutane on Cu3(BTC)2. Langmuir 2008; 24: 8634-8642.

10.

Pichon A, Lazuen-Garay A and James SL. Solvent-free synthesis of a
microporous metal–organic framework. Crystal Engineering Communications
2006; 8: 211–214.

11.

Biemmi E, Christian S, Stock N, and Bein T. High-throughput screening of
synthesis parameters in the formation of the metal-organic frameworks MOF5 and HKUST-1. Microporous and Mesoporous Materials 2009;117: 111–117.
51

12.

Krawiec P, Kramer M, Sabo M, Kunschke R, Fröde H, and Kaskel S.
Improved hydrogen storage in the metal organic framework Cu3(BTC)2.
Advanced Engineering Materials 2006; 8 (4): 293-296.

13.

Mueller U, Schubert M, Teich F, Puetter H, Schierle-Arndt K and Pastre J.
Metal–organic frameworks—prospective industrial applications. Journal of
Materials Chemistry 2006; 16: 626–636.

14.

Christopher A. Hunter and Jeremy K. M. Sanders, The Nature of π- π
Interactions, Journal of American Chemical Society 1990, 112, 5525-5534.

15.

Christopher A. Hunter, Kevin R. Lawson, Julie Perkins a and Christopher J.
Urch, Aromatic interactions, J. Chem. Soc., Perkin Trans. 2, 2001, 651–669

16.

Paul A. Webb and Clyde Orr, Analytical methods in fine particle technology,
Page 145, Chapter 3.

17.

Kitagawa, S.; Kitaura, R.; Noro, S.-I. Functional porous coordination
polymers. Angew. Chem. Int. Ed. 2004; 43: 2334-2375.

18.

Schlichte K, Kratzke T, and Kaskel S. Improved synthesis, thermal stability
and catalytic properties of the metal-organic framework compound
Cu3(BTC)2. Microporous and Mesoporous Materials 73 (2004) 81-88.

19.

Hartmann M, Kunz S, Himsl D, and Tangermann O. Adsorptive separation of
isobutene and isobutane on Cu3(BTC)2. Langmuir 2008; 24: 8634-8642.

52

Chapter 4. Deterioration of metal organic framework crystal structure during
fabrication of poly(L-lactic) acid mixed matrix membranes*
*version of the article
Kathuria A, Abiad MG and Auras R. Polymer International 2013; 62: 1144-1151.

53

Chapter 4. Deterioration of metal organic framework crystal structure during
fabrication of poly(L-lactic) acid mixed matrix membranes

Poly(L-lactic acid) (PLLA) and metal organic framework (MOF) mixed matrix
membranes (MMM) were prepared by melt extrusion of PLLA with 5% w/w of either
activated or water saturated Cu3(BTC)2 (Cu3(C9H3O6)2(H2O)3·xH2O or HKUST-1).
The morphology and the stability of injection-molded samples were evaluated using
thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), gel
permeation chromatography (GPC), X-ray diffraction (XRD), and scanning electron
microscopy (SEM). The presence of activated and saturated MOF crystals increased the
cold crystallization onset temperature as compared to neat PLLA. It can be attributed to
the MOF crystals incorporated in the PLLA matrix, which decreased the mobility of
PLLA and thus impeded the crystallization process. According to the XRD results, the
activated MOF crystals were successfully incorporated into the PLA matrix without
altering the crystal structure of the MOF. Moreover, the findings from the permeability
and tensile tests as well as SEM imaging indicated good interfacial interactions between
PLLA and activated MOF. However, during melt extrusion of PLLA with saturated
MOF, water molecules from the saturated MOF altered the MOF crystal structure and
contributed to the degradation of the PLLA polymer by reducing its molecular weight
(Mw) by around 21%.

54

4.1. Introduction
During the last couple of decades, growing environmental concerns along with
limited petroleum resources and technological improvements provided impetus to the
development of new bio-based, biodegradable and compostable polymers such as
poly(lactic acid) (PLA). Due to technological advancements, PLA has become one of the
most remarkable commercial polymers in this category particularly for medical and
packaging applications [1, 2].
PLA is generally synthesized by ring opening polymerization of lactide
monomers which are linked together in one of the three isomeric forms – (D,D) lactide,
(L,L) lactide and (D,L) lactide. The final properties of the PLA such as thermal,
mechanical, and barrier properties are significantly impacted by the isomeric composition
of the resin. Although PLA has many appealing properties such as transparency, low
processing temperature, and compostability, it is brittle and has low heat deflection
temperature (HDT) which limits the scope of its application. Consequently, researchers
attempted to use different methodologies including plasticization, chemical modification,
composite fabrication as well as blending to reduce PLA’s inherent brittle nature and
improve its HDT, gas and water vapor barrier properties among others [3-6].
Metal organic frameworks (MOFs) is a class of crystalline materials with high
+

2+

+

2+

2+

+

porosity, which are composed of metal ions (Cu , Cu , Ag , Zn , Co , K , etc.)
linked together by organic bridging ligands forming multi-dimensional structures [7].
They exhibit high thermal and mechanical stability [8]. Perhaps the most attractve
property of MOFs is their ultrahigh surface areas, which is greater than porous carbons or
2 -1

zeolites and may exceed at times 5900 m .g

55

[9], large pore sizes and high micro-pore

volume. The porosity and large surface area of these materials along with their varied
chemical compositions render them candidates for high capacity adsorption of various
gases and molecules [10]. The structure of MOFs allows guest molecules to diffuse into
the bulk structure while the size, shape and affinity of these pores provide selectivity
among the guests, which can be incorporated into them. There is a plethora of literature
investigating various applications of MOFs in the fields of catalysis [11], gas purification
and separation [12], hydrogen storage [13] and as an option for efficient controlled drug
delivery [14].
One of the commercially available MOFs is Cu3(BTC)2 [Cu3(C9H3O6)2(H2O)3],
also known as HKUST-1. Cu3(BTC)2 is characterized by a face centered cubic (FCC)
cell as the repeating unit [15]. The Cu

2+

dimer coordinated with four oxygen atoms of

benzene tricarboxylate makes a paddle wheel which constitutes the building block of
HKUST-1 whose structure is represented in Figure 4.1 [15]. The water molecules axially
coordinated to the two exchangeable copper sites can be removed by heat activation
under vacuum yielding a Lewis acid site with catalytic properties [16].

56

Figure 4.1: Chemical structure of building block of Cu3(BTC)2 MOF
Interest in improving the functionality of polymer based membranes have led
researchers to develop new mixed matrix membranes (MMM) by integrating inorganic
particles such as zeolites, MOFs, carbon, and carbon nanotubes, among other sieving
materials, within the organic matrices [18-23]. The thermal and mechanical stabilities are
key factors in the development of such systems. For this reason, the aim of the present
study was to understand the effect of the adsorbed water present in Cu3BTC2 MOF on the
morphology and stability of poly(L-lactic acid)–Cu3BTC2 based MMMs prepared by melt
extrusion followed by injection molding.

4.2. Methodology
4.2.1 Materials
Poly(L-lactic acid) (PLLA) resin grade 4043 D, 98% L-lactide, with weight
average molecular weight (Mw), number average molecular weight (Mn) and
polydispersity index (Mw/Mn) of 111 Kda, 84 Kda and 1.3 respectively, was procured

57

from NatureWorks LLC (Minnetonka, MN, USA). Trimesic acid (TMA) - Benzene 1,3,5
tricarboxylic acid- (95 wt.% pure) and BasoliteTM C300 Metal Organic Framework
[Cu3(BTC)2] were purchased from Sigma Aldrich (St Louis, MO, USA). BET surface
area of Cu3(BTC)2 MOF as reported by the manufacturer ranges between 1500 and 2100
2 -1

m2.g-1, and was previously measured as 1566 m .g

[24]. The particle size of

Cu3(BTC)2 averages to 15 μm measured using a JEOL JSM 6400 SEM (Japan Electron
Optics Laboratories Ltd., Tokyo, Japan) with an accelerating voltage of 12 kV and 39
mm working distance (Figure 4.2). The detailed methodology of SEM is provided below.

10 µm
Figure 4.2: SEM image of Basolite™ C300 acquired at 12 kV and 39 mm working distance

58

4.2.2 Sample preparation
Prior to processing and extrusion, the PLLA resin was dried at 80oC for 4 h while
activated Basolite

TM

C 300 MOF [Cu3(BTC)2) - to be referred to as ‘activated MOF’
o

here after - was activated for 24 h at 200 C using vacuum oven. On the other hand,
saturated Basolite

TM

C 300 MOF – to be referred to as ‘saturated MOF’ here after – was

obtained by conditioning at 23oC and 50% RH for 4 days to equilibrate and reach water
storage capacity. This time was predetermined to be sufficient for the MOF to saturate
and stabilize [25].
MMMs (PLLA + 5% wt. MOF) were extruded using a vertical co-rotating twinscrew micro-compounder (DSM Research, Geleen, The Netherlands) followed by
injection molding. The extruder was equipped with a 150 mm screw and L/D ratio of 18.
3

The volume of the barrel was approximately 15 cm . The extruded compositions were
transferred into a transferring cylinder and injected to the mini-injection molder (DSM
Research, Geleen, The Netherlands). The temperature profile for the extrusion process
o

from the top to bottom zone was set at 190 C with an extrusion cycle time of 5 min. The
pressure for injection molding was ~1 MPa (140 psi) while the transfer cylinder and mold
o

temperatures were set at 195 and 65 C; respectively. Dogbone tensile bars and XRD
discs of PLLA, PLLA-5% activated MOF and PLLA-5% saturated MOF composites
®

were prepared and stored in a desiccator over desiccant (Drierite ) at room temperature
(~23

o

C). The PLLA-5% activated MOF and PLLA-5% saturated MOF will be

59

represented as (PLLA-A-MOF) and (PLLA-S-MOF), respectively. MOF 5 wt.% was
previously determined as enough MOF to produce a meagninful change in the MMMs
final thermal, mechanical and barrier properties [24, 25].
4.2.3 Characterization
4.2.3.1 Thermogravimetric analysis (TGA)
The thermal stability of Cu3(BTC)2 MOF, PLLA and PLLA-MOF composites
was examined using a TGA model 2950 from TA-Instruments (New Castle, DE, USA).
TGA was also used to determine the initial water content in the Cu3BTC2 MOF.
o

-1

Approximately 5 mg of sample were heated from 23 to 700 C at a rate of 10°C.min .
The TGA data was analyzed using the Universal Analysis software version 2000 (TA
Instruments, New Castle, DE, USA). Samples were run in triplicate.
4.2.3.2 X-Ray diffraction (XRD)
X-ray diffraction data was collected for the injection molded XRD discs of PLLA,
o

PLLA-A-MOF, and PLLA-S-MOF stored at 23 C in a desiccator over desiccant
®

(Drierite ) as well as for activated and saturated Cu3BTC2 MOF powder using a Bruker
D8 advance X-ray diffractometer (Bruker AXS GmgH, Karlsruhe, Germany) at 40kV,
40mA (1,600W) using Cu Kα radiation ( = 1.5418 Å), a 1.2 mm primary beam slit and
2.0 mm detector slit. The X-ray scans were carried out using 0.02 degrees per second.
Data was collected in triplicate.
4.2.3.3 Differential scanning calorimetry (DSC)

60

DSC analysis was performed using a DSC Q100 from TA instruments. The
o

o

samples were equilibrated to 0 C followed by heat/cool/heat cycles from 0 to 180 C at a
o

-1

rate of 10 C.min . The melting temperature (Tm) and associated heat enthalpy (ΔHm)
were computed from the first heating cycle. On the other hand, the glass transition (Tg)
temperature and cold crystallization onset (Tco), cold crystallization peak (Tcc) and
enthalpy of cold crystallization (Δ Hc) were calculated from the second heat cycle to
eliminate any thermal history caused by processing or storage. DSC data were obtained in
triplicates and were analyzed using the Universal Analysis software version 2000.
The percent crystallinity of the samples was determined using equation (1):

X c  % 

H m  H c
100
c
H m 1  x 

(1)

c

where ΔHm is enthalpy of fusion; ΔHc is the enthalpy of cold crystallization; ΔH m is
c

enthalpy of fusion of pure crystalline PLA; ΔH m = 93.1 J/g [26]; and x is the mass
fraction of the MOF in the MMM.
4.2.3.4 Tensile test
Tensile properties were evaluated according to ASTM D638-10 using a Universal
Tensile Machine model UTS SFM 20 from United Calibration Corporation, (Huntington
Beach, CA, USA). The machine was equipped with a laser extensometer. Injection
molded dogbone samples with a gauge length of 25.4 mm were stored at room

61

®

temperature in a dessicator over desiccant (Drierite ) for 40 h prior to testing. The
samples were then tested at room temperature (~23oC) using a 453 kg load cell at 0.023
kg of preload. PLLA and PLLA-MOF composites were tested at the crosshead speed of
50.8 mm.min

-1

(2 in/min), whereas PLLA and PLLA-Trimesic acid (TMA) composites

were tested at the crosshead speed of 2.54 mm.min

-1

(0.1 in/min) (later elaborated in

section 3.4). At least five replicates were tested for each type of sample.
4.2.3.5 Weight average molecular weight
In order to examine the effect of processing conditions and water content of the
Cu3(BTC)2 MOF on the molecular weight of the PLLA matrix, Mw, and Mn of the PLLA
resin, extruded PLLA resin and PLLA-A-MOF and PLLA-S-MOF were determined
using a GPC (Waters Inc., Milford, MA, USA). A flow rate of 1 mL.min-1 and runtime of
45 min at 35°C were used. The instrument was calibrated with polystyrene standard
3

6

materials with a molecular weight ranging from 2.9 x 10 to 3.64 x 10 Da using a third
order polynomial equation. The Mark-Houwink corrected constant K = 0.000174 (mL·g

-

1

) and a = 0.736 for dilute PLLA solution in tetrahydrofuran (THF) were used [27]. The

instrument was equipped with a Waters 1515 isocratic pump, Waters 717 autosampler, a
series of Waters Styragel Columns (HR4, HR3 and HR2) and Waters 2414 refractive
index detector. Approximately 20 mg of specimen were dissolved in 10 mL of HPLC
grade Tetrahydrofuran (THF) with 99.99 % purity (Pharmco-Aaper, Brookfield, CT,

62

USA) in order to determine the Mw, Mn, and the polydispersity index (PI). The solution
was then filtered with a 0.45 μm filter.
4.2.3.6 Scanning electron microscopy (SEM)
A JEOL JSM 6400 scanning electron microscope equipped with a Lanthanum
Hexaboride (LaB6) electron gun was utilized to study the surface topography of the
PLLA, BasoliteTM C 300, PLLA-A-MOF and PLLA-S-MOF. The samples were sputtercoated with 10nm gold using an Emscope SC 500 (Emscope Laboratories Ltd., Ashford,
UK). SEM photomicrographs were acquired at accelerating voltage of 12 kV.

4.2.3.7 CO2 and O2 permeability coefficients
The films used for the permeability studies were prepared using a PHI 30 ton
compression molding machine (City of Industry, CA, USA) with 12” x 12” plate size.
The injection molded samples prepared using the twin screw DSM micro-compounding
o

instrument were compressed at 170 C and ~1.0 MPa for 5 min. The CO2 and O2
permeation rates were measured using a Permatran

TM

C 4/41 and an Oxtran

TM

2/21

o

(MOCON Inc, Minneapolis, MN, USA); respectively, at 23 C, 0% RH, and 100%
2

permeant gas between aluminum masks with areas of 3.14 and 5 cm . The permeability
coefficients of CO2 and O2 were calculated using equation (2):

Permeability  Gas Transmission Rate 

63

1
t
P

(2)

where ΔP is the difference in permeant partial pressure across the films expressed in Pa,
and t is the film thickness.
4.2.3.8 Data analysis
Statistical analyses were performed using SAS 9.0 Software (SAS Institute Inc.,
Cary, NC, USA). Tukey's HSD (Honestly Significant Differences) tests were used to
determine significance at the 95% confidence (α = 0.05).

4.3. Results and discussion
4.3.1 Thermo-gravimetric analysis
The TGA thermographs for activated MOF, saturated MOF, PLLA, PLLA-AMOF and PLLA-S-MOF are shown in Figure 4.3. The saturated MOF which was stored
at 23oC and 50% RH for 4 days adsorbed around 36% w/w moisture while the activated
MOF exhibited 5% w/w moisture, mostly attributed to water adsorbed during sample
handling. Comparably, Schlichte et al [28] obtained similar results for HKUST-1 dried in
a vacuum oven at 100 oC, rehydrated in air, and rehydrated in moist argon stream and
they reported 2, 27 and 36.4% w/w moisture; respectively.
Figure 4.3 also shows that the organic component of the framework, benzene
tricarboxylate, starts degrading at around 325oC. The thermal degradation onset of the
polymer matrix (PLLA) and the MMM (PLLA-A-MOF and PLLA-S-MOF) starts at
o

around 322 C. Difference can be observed in the final decomposition temperature (Td)
for PLLA, PLLA-S-MOF and PLLA-A-MOF – reported as 385, 410 and 500oC;
respectively. The final plateau for the PLLA-A-MOF and PLLA-S-MOF samples can be
mainly attributed to the copper present in the MOF.

64

PLLA
PLLA-A-MOF
PLLA-S-MOF
Saturated MOF
Activated MOF

100

80

60

40

20
0
0

100

200

300

400

500

600

700

Figure 4.3: TGA thermograms of PLLA, PLLA-A-MOF, PLLA-S-MOF, saturated and
activated MOF before extrusion

65

Huang et al [29] studied the thermal stability of Cu3BTC2 MOF using ReaxFF
reactive molecular dynamics (RMD). They reported good thermal stability of the MOF
structure for temperatures up to 300oC while observing structural collapse at temperatures
above 327oC. The simulation from 327 to 427oC indicated that MOF retained the
molecular formula but sacrificed the microporous nature of the material as further heating
released CO2 and CO.

4.3.2 X-Ray Diffraction (XRD)
XRD diffraction patterns of activated MOF and saturated MOF are represented in
Figures 4.4a and 4.4b; respectively, matching previously reported XRD patterns [28, 30,
31]. The percentage intensity of various planes is denoted in Table 4.1. The data
presented in this study confirms the observations previously reported by Schlichte et al
[28] regarding the intensity ratios I200/I220 and I331/I420 which are significantly higher in
activated MOF than saturated MOF as observed. The crystal retained all the planes of
FCC crystal structure, the changes in the intensity ratio of plane (200), (400), and (441)
planes are due to the adsorption of water by the Cu3BTC2 MOF crystal [28]. Figures
4.5a, 4.5b and 4.5c represent the diffractograms of PLLA-A-MOF, PLLA-S-MOF, and
PLLA XRD discs; respectively. The crystalline peak (Figure 4.5c) observed for PLLA
o

composites at 2θ = 16.4 corresponds to α-PLLA crystals with a broader background
hump, suggesting a primarily amorphous nature of the polymer [32, 33, 34]. The high
amorphous content of the injection molded PLLA, PLLA-A-MOF, and PLLA-S-MOF
composites can be ascribed to the slow crystallization kinetics of the PLLA [35]. This

66

observation was further endorsed by the DSC analysis discussed in the next section. The
X-ray diffractogram of injection molded PLLA-A-MOF discs is represented in Figure
4.5a. PLLA-A-MOF have similar peaks with minor changes in the intensity ratios
calculated from the area under the curve at the same planes as observed in the Cu3(BTC)2
MOF as shown in Table 4.2. The extensive change in the intensity ratio of plane (200) in
the case of activated MOF and PLLA-A-MOF may be related to the possible hydration of
MOF during sample handling after MMMs fabrication.

67

a

Cu3(BTC)2
Activated MOF

6

(333)
8
(440)

(422)

(331)

(400)
8

(200)
8
(111)
8
4

Cu3(BTC)2(H2O)3.xH2O
Saturated MOF

(220)
8

(222)
8

b

8

Figure 4.4: XRD patterns of (a) activated MOF and (b) saturated MOF

68

(333)

(400)

(220)

(111)

(200)

(422)

(222)

a

b

c

5

10

15

20

25

30

35

Figure 4.5: XRD patterns of (a) PLLA-A-MOF disc, (b) PLLA-S-MOF disc, and (c)
injection molded PLLA disc

69

40

Table 4.1 XRD intensities of activated and saturated MOF
Plane

2θ

Intensity Ratio*
d spacing (Å)
Activated MOF

Saturated MOF

(111)

5.7

15.5

1.1

0.9

(200)

6.6

13.5

97.8

35.2

(220)

9.5

9.3

52.8

46.0

(222)

11.6

7.6

100

100

(400)

13.5

6.6

12

32.4

(331)

14.6

6.1

9.5

5.5

(420)

15.0

5.9

1.2

5.8

(422)

16.4

5.4

4.3

7.1

(333)

17.4

5.1

12.4

13.8

(440)

19.0

4.7

10.3

20.8

*Background subtracted intensity

70

Table 4.2 Intensity ratio, and area under the curve of PLLA-A-MOF and PLLA-S-MOF
Plane

2θ

d-spacing
(Å)

PLLA-A-MOF

PLLA-S-MOF

Area under the curve

Intensity Ratio*

Area under the curve

Intensity Ratio*

(111)

5.7

15.5

195

19.8

-

-

(200)

6.6

13.5

661

59.9

-

-

(220)

9.4

9.4

348

47.0

68

51.5

(222)

11.5

7.7

1172

100

132

100

(400)

13.4

6.6

136

14.5

25

47

(331)

14.6

6.1

79.5

5.3

-

-

(420)

15.0

5.9

-

-

-

-

(422)

16.4

5.4

53

4.7

17.95

14

(333)

17.4

5.1

128

9.0

-

-

(440)

19.0

4.7

167.5

15.8

*Background subtracted intensity

71

The d-spacing, peak intensity and area under the peak for various planes of
PLLA-S-MOF are presented in table 4.2. It can be observed that peaks (111) and (200)
which were present in the saturated MOF disappeared in the PLLA-S-MOF and the peak
intensity of other planes such as (220) and (222) were also drastically reduced. The
absence of these peaks in the diffraction pattern indicates that saturated Cu3(BTC)2
MOF lost the face centered cubic (FCC) crystal structure after extrusion. The loss of FCC
crystal structure of the MOF during compounding of the composites can be attributed to
the exposure to both moisture and temperature during extrusion. Prestipino et al [36]
utilized a combination of extended X-ray absorption fine structure (EXAFS), X-ray
absorption near edge structure (XANES), UV-Vis and Infrared (IR) spectroscopy
techniques and reported that the water sorption properties of HKUST-1 are provided by
“an open framework with an intersecting 3D-channel system – composed by dimeric
cupric tetracarboxylate.” The tetracarboxylate and the benzene rings in the MOF (see
Figure 1b) are responsible for the sorption properties. Prestipino et al [35] also illustrated
that the adsorption properties of HKUST-1 are not only given by unspecific van der
Waals interactions, but also by specific Coulomb-type electrostatic interactions. They
demonstrated that upon dehydration at 453 K under vacuum the first coordination sphere
of Cu2+ sites was significantly altered. The shortening of the Cu–Cu distance and the
distortion of the Cu–O bonds produce coordinately unsaturated Cu2+ dimer sites
overlooking the cages. Küsgens et al [37] also studied the water stability of Cu3(BTC)2
(HUKST-1) MOF at elevated temperatures by dipping the MOF particles into de-ionized
o

water at 50 C for 24 h. After filtering and drying the MOF at room temperature they

72

performed XRD studies and evaluated the surface area demonstrating the presence of
irreversible changes in the MOF structure along with reduction in BET surface area from
2 -1

1340 to 647 m .g

[37]. Mustafa et al [38] studied the stability of the Cu3(BTC)2 MOF

under steam conditions. They observed changes in the MOF structure starting at 70oC,
with the formation of unidentified low crystalline material at 120oC. The Cu3(BTC)2
MOF transformed completely to an irreversible form [Cu2OH(BTC)(H20)]n.2nH2O at
150oC. The alterations in the crystal structure during processing attributed to the presence
of adsorbed water in the saturated Cu3(BTC)2 MOF embedded in the PLLA may have
led to permanent changes in the pore size and sorption capacities of the final MMMs.
4.3.3 Differential Scanning Calorimetry (DSC)
Figures 4.6a and 4.6b represent the thermograms of the injection molded PLLA,
PLLA-A-MOF and PLLA-S-MOF obtained during the first and second heating cycles;
respectively. Table 4.3 summarizes the glass transition, crystallization and melting
enthalpies and temperatures. The average percentage crystallinity of PLLA, PLLA-AMOF and PLLA-S-MOF obtained from the second heating cycle were 2.0, 1.0 and 0.6%;
respectively, suggesting the amorphous nature of the PLLA, PLLA-A-MOF and PLLAS-MOF composites and complementing the observations from the XRD spectra. The
mass fraction percentage crystallinity of PLLA, and PLLA-A-MOF as calculated from
the XRD are 2.8 and 0.6 %; respectively. The presence of activated MOF particles
slightly reduced the crystallinity of the MMMs compared to neat PLLA.

73

Table 4.3 Detailed DSC analysis information of PLA and its composites
o

Cold Crystallization

Tg ( C)
Sample

o

o

Tco ( C)
A

PLLA

60.8 ± 0.1

PLLA-A-MOF

60.9 ± 0.0

PLLA-S-MOF

59.8 ± 0.4

A
B

103.8 ± 0.2

113.8 ± 0.2
109.8 ± 0.6

Δ Hc (J/g)

Tcc ( C)
A
B

C

119.1 ± 0.1

Melting

A

129.8 ± 0.4
123.7 ± 1.5

19.6 ± 0.9

B

3.6 ± 0.1

C

A

B

25.0 ± 0.9

C

o

o

Tmo ( C)
143.3 ± 0.1

Δ Hm (J/g)

Tm ( C)
A

145.5 ± 0.1
145.0 ± 0.3

B
C

147.9 ± 0.2

A

150.8 ± 0.1
149.2 ± 0.4

21.9 ± 0.5

B
C

6.9 ± 0.3

B

27.2 ± 2.1

Note: Values in the same column with same capital superscript letters are not statistically significantly different at =0.05

74

A

C

a
PLLA
PLLA-S-MOF
PLLA-A-MOF

First heating cycle

0

20

40

80

60

100

120

140

160

180

b
PLLA
PLLA-S-MOF
PLLA-A-MOF

Second heating cycle

0

20

40

60

80

100

120

140

160

180

Figure 4.6: DSC thermograms of PLLA, PLA-A-MOF and PLA-S-MOF: (a) first heating
cycle; (b) second heating cycle
75

The addition of 5 wt.% activated and saturated MOF increased the cold
crystallization onset (Tco) and the cold crystallization (Tcc) temperatures. For PLLA-Ao

o

MOF Tco increased from 103.8 to 113.8 C and Tcc from 119.1 to 129.8 C, whereas for
o

o

PLLA-S-MOF Tco increased from 103.8 to 109.8 C and Tcc from 119.1 to 123.7 C. It
was also observed that the enthalpy of crystallization (ΔHc) of PLLA-A-MOF decreased
-1

from 19.6 to 3.6 J.g , designating a decrease in PLLA mobility and thus a decline in the
ability for the PLA matrix to crystallize in the presence of 5 wt.% activated MOF. No
o

crystallization peak was observed in the cooling cycle at the cooling rate of 10 C.min

-1

for any of the materials (Figure 4.6b).
4.3.4 Tensile Test
The tensile strength of MMM depends on various factors such as interfacial
interactions, aspect ratio, dispersion of the fillers, and thermal stability of the filler [39].
Fillers can also influence the crystallinity of the matrix, leading to change in various
properties including strength, elongation, permeability, etc. PLLA is known for high
tensile strength and poor flexibility [40]. Elangovan et al [25] studied the surface energy,
interfacial tension and spreading coefficient of PLLA and HUKST-1 MOF using the
contact angle measurement. The contact angles of PLLA and HKUST-1 measured using
o

ethylene glycol were 57.1 and 53.3 respectively, whereas the contact angles of PLLA
and HUKST-1 obtained from methylene iodide were 37.6

o

o

and 45.4 . The authors

utilized the geometric and harmonic means and obtained low interfacial tension and high

76

work adhesion indicating good compatability between the MOF and the PLLA. They
also observed improved flexibility of PLLA added with more than 1 wt.% MOF.
The stress strain graph of PLLA, PLLA-A-MOF and PLLA-S-MOF is presented
in Figure 4.7. The corresponding tensile strength, elongation at break, and modulus of
elasticity values are presented in Table 4.4. The average tensile strength and percentage
(%) elongation of injection molded neat PLA samples were 76.9 MPa and 3.6%
respectively. Bhardwaj et al [6] studied the modification of PLA with hyberbranched
polymers. They reported that PLA has tensile strength 76.5 MPa and 5.1 % elongation.
The elongation of PLLA-A-MOF and PLLA-S-MOF were 16.9 and 9.14 %, respectively.
Improvement in the elongation of PLLA-A-MOF as compared to neat PLLA can be
attributed to good interfacial interaction between the activated MOF particles and PLLA
matrix, as observed in the SEM images (Figure 4.8). PLLA-S-MOF has 300%
improvement in elongation over neat PLLA. Elongation of PLLA-S-MOF was less than
the PLLA-A-MOF. The presence of water in the MOF might have affected the interfacial
interactions between MOF and PLLA. The tensile strength of PLLA-A-MOF decreased
by around 23% as compared to neat PLLA. We observed 13% reduction in the tensile
strength of PLLA-S-MOF as compared to neat PLLA. The exact mechanism of the
improved toughness of the PLLA-A & S-MOF composites is not fully understood.
Benzene based carboxylic acid and other small molecule polyhydric carboxylic acid ester
based compounds have been used as plasticizers [41, 42]. Wang et al [43] investigated
the co-polycondensation of lactic acid with TMA and reported a decrease in the glass
transition (Tg) temperature of the copolymer. In order to further explore this behavior,
PLLA samples added with 1, 2.5 and 5% wt TMA, the basic constitutional unit of MOF,

77

were produced to isolate the possible plasticizing effect of TMA due to any possible
release of benzene 1,3,5 tricarboxylate (organic linker) to PLLA after detachment from
MOF during processing. Table 4.5 shows that melt extrusion of PLLA with TMA did not
substantially increase the elongation at break for PLLA samples added with 1, 2.5 and 5
wt % TMA. Therefore, the extension at break improvement can be mainly attributed to
the MOF particles.

PLLA
PLLA-A-MOF
PLLA-S-MOF

Figure 4.7: Stress–strain curves of PLLA, PLLA-A-MOF and PLLA-S-MOF

78

Table 4.4 Tensile properties of PLLA, PLLA-A-MOF and PLLA-S-MOF
Material

Tensile Strength
(MPa)

Elongation at break
(%)

A

PLLA

76.9 ± 1.5

PLLA-A-MOF

58.9 ± 0.8

PLLA-S-MOF

66.7 ± 1.1

Modulus of Elasticity
(GPa)

3.6 ± 1.5

B

A

A

3.0 ± 0.3

B

A

16.9 ± 3.2

C

9.1 ± 2.5

3.0 ± 0.2

C

A

3.1 ± 0.1

Note: Values in the same column with same capital superscript letters are not statistically
significantly different at α = 0.05

Table 4.5 Tensile strength and elongation at break of PLLA, PLLA-TMA composites
Material

Tensile Strength (MPa)
A

PLLA

66.3 ± 0.4

PLLA-1 % TMA

59.8 ± 0.5

PLLA-2.5% TMA

55.9 ± 0.4

PLLA-5% TMA

52.2 ± 0.9

Elongation at break (%)
3.8 ± 0.6

B

A

5.8 ± 1.1

C

5.5 ± 0.8

D

6.5 ± 1.0

B
B
B

Note: Values in the same column with same capital superscript letters are not statistically
significantly different at α = 0.05

79

4.3.5 Gel Permeation Chromatography (GPC)
The Mw, Mn, and polydispersity index (PDI=Mw/Mn) of the PLLA resin, extruded
PLLA, PLLA-A-MOF and PLLA-S-MOF are shown in Table 6. The Mw of PLLA resin,
extruded PLLA, PLLA-A-MOF, and PLLA-S-MOF were 110, 105, 98.6, and 83.5 kDa,
respectively. The 6 % drop in Mw of PLLA-A-MOF as compared to extruded PLLA can
be associated with oxidative degradation. However, this Mw reduction cannot contribute
to 470% improvement in the elongation. [44]. Hydrolytic degradation might be partially
responsible for degradation of PLLA-A-MOF because of the minor water adsorption by
the MOF before feeding into the extruder. The Mw of PLLA-S-MOF dropped by around
21% as compared to extruded PLLA. This can be ascribed to oxidative degradation plus
hydrolytic degradation due to the release of the moisture from the MOF during extrusion
processing [45].

Table 4.6 Molecular weight of PLLA, PLLA-A-MOF and PLLA-S-MOF
PLLA resin
Mn, Kda
Mw, Kda
PDI

A

83.8 ± 7.8

A

110.8 ± 3.5

A

1.3 ± 0.1

PLLA

PLLA-A-MOF
A,B

76.7 ± 0.4

A

105.0 ± 0.2

A

1.4 ± 0.0

B,C

67.6 ± 4.5

98.6 ± 3.2

B

A,B

1.5 ± 0.1

PLLA-S-MOF
C

55.6 ± 4.3
83.5 ± 2.0

C

B

1.5 ± 0.1

Note: Values in the same column with same capital superscript letters are not statistically
significantly different at α = 0.05

80

4.3.6. Carbon dioxide (CO2) and Oxygen (O2) Permeability
The selective permeability of a polymer or membrane is important for industrial
applications such as membranes, gas separation, packaging etc. Table 7 shows the CO2
and O2 permeability coefficients of the PLLA, PLLA-A-MOF and PLLA-S-MOF.
Similar CO2 and O2 permeability coefficients have been reported for PLA films [46].
Although there was no statistically significant difference in the CO2 and O2 permeability
coefficient among PLLA, PLLA-A-MOF and PLLA-S-MOF, Figure 4.8 shows that the
CO2 permeability coefficients of PLLA-S-MOF have high deviation from the average
values as compared to PLLA and PLLA-A-MOF. The low standard deviation in the
PLLA-A-MOF indicates good interfacial adhesion in the PLLA and activated metal
organic framework as supported by Fig 4.9. High standard deviation values of PLLA-SMOF could be ascribed to the presence of voids at the interphase of PLLA and saturated
MOF as observed in the SEM image (Fig 4.10 a) producing larger variations in the
barrier properties of the end films. Figure 4.10 b shows a collapsed metal organic
framework crystal, the crystal structure may have collapsed during the extrusion
processing.

81

PLLA
PLLA-A-MOF
PLLA-S-MOF

8

6

Permeability x 10

-17

, kg.m.m2.s-1.Pa-1

10

4

2

0
0

5

15

10

20

25

30

Time, h

Figure 4.8: Permeability coefficients of PLLA, PLLA-A-MOF and PLLA-S-MOF

Table 4.7 CO2 and O2 permeability coefficients of PLLA, PLLA-A-MOF and PLLA-SMOF
Permeability Coefficient x 10
CO2

-18

-2 -1

-1

(kg.m.m .s .Pa )
O2

A

PLLA

45.5 ± 2.72

PLLA-A-MOF

49.9 ± 4.08

PLLA-S-MOF

65.0 ± 2.11

A
A

A

6.00 ± 0.400

A

6.59 ± 0.843

A

6.07 ± 0.7.53

Note: Values in the same column with same capital superscript letters are not statistically
significantly different at α = 0.05

82

5 µm
Figure 4.9: SEM image of PLA-A-MOF fractured surface at 12 kV and 39 mm
working distance.

10 µm
Figure 4.10 a: SEM image of PLLA-S-MOF fractured surface acquired at 12 kV
and 47 mm working distance.

83

10 µm
Figure 4.10 b: SEM image of PLLA-S-MOF XRD disc surface acquired at 12 kV
and 39 mm working distance.

4.4. Conclusions
The presence of moisture in the Cu3BTC2 MOF at elevated temperatures during
melt compounding processes can significantly contribute to the degradation of the MOF
crystal structure. In addition, the adsorbed moisture in the MOF particles may also
accelerate the degradation of the PLLA matrix as observed in GPC studies. As
demonstrated in the tensile studies incorporating Cu3BTC2 MOF improves the flexibility
and tensile properties of PLLA-MOF matrices. Improvement in toughness and SEM
images of PLLA-A-MOF indicated good interfacial interactions between activated MOF
and PLLA. The MOF particles have a tendency to hinder the crystallization of the
polymeric matrix.

84

REFERENCES

85

REFERENCES

1. Auras R, Harte B and Selke S. An overview of polylactides as packaging materials.
Macromolecular Bioscience 2004; 4: 835–864.
2. Soto-Valdez H, Auras R and Peralta E. Fabrication of poly(lactic acid) films with
resveratrol and diffusion of resveratrol into ethanol. Journal of Applied Polymer Science
2011; 121: 970–978.
3. Ljungberg N and Wesslén B, Preparation and properties of plasticized poly(lactic acid)
films. Biomacromolecules 2005; 6:1789-1796.
4. Zhang L, Xiong C and Deng X. Miscibility, crystallization and morphology of poly(β
hydroxyl butrate)/poly(d,l lactide) blends. Polymer 1996; 37: 235-241.
5. Ray SS and Okamoto M. Polymer/layered silicate nanocomposites: a review from
preparation to processing. Progress in Polymer Science 2003; 28: 1539–1641.
6. Bhardwaj R and Mohanty AK. Modification of brittle polylactide by novel
hyperbranched polymer based nanostructures. Biomacromolecules 2007; 8: 2476-2484.
7. Rowsell JLC and Yaghi OM. Metal organic frameworks: a new class of porous
materials. Microporous and Mesoporous Materials 2004 73: 3–14.
8. Britt D, Tranchemontagne D and Yaghi OM. Metal organic frameworks with high
capacity and selectivity for harmful gases. PNAS 2008 105: 11623–11627.
9. Ferey G, Mellot-Draznieks C, Serre C, Millange F, Dutour J, Surble S and Margiolaki
I. A chromium terephthalate based solid with unusually large pore volumes and surface
area. Science 2005; 309: 2040–2042.
10. Mueller U, Schubert M, Teich F, Puetter H, Schierle-Arndt K and Pastre J. Journal of
Materials Chemistry 2006; 16: 626–636.
11. Chuck CJ, Davidson MG, Jones MD, Kociok-KÖhn G, Lunn MD and Wu S. Airstable titanium alkoxide based meta organic framework as an initiator for ring opening
polymerization of cyclic esters. Inorganic Chemistry Communications 2006 45: 6595–
6597.
12. Li J, Kuppler RJ and Zhou H. Selective gas adsorption and separation in metal
organic frameworks. Chemical Society Reviews 2009; 38: 1477-1504.
13. Rosi NL, Eckert J, Eddaoudi M, Vodak DT, Kim J, O’Keeffe M and Yaghi OM.
Hydrogen storage in microporous metal organic frameworks. Science 2003; 300: 11271129.

86

14. Ananthoji R, Eubank JF, Nouar F, Mouttaki H, Eddaoudi M and Harmon JP.
Symbiosis of zeolite-like metal–organic frameworks (rho-ZMOF) and hydrogels:
Composites for controlled drug release. Journal of Material Chemistry 2011; 21: 95879594.
15. Bordiga S, Regli L, Bonino F, Groppo E, Lamberti C, Xiao B, Wheatley PS, Morris
RE and Zecchina A. Adsorption properties of HKUST-1 toward hydrogen and other
small molecules monitored by IR. Physical Chemistry Chemical Physics 2007; 9, 2676–
85.
16. Alaerts L, Séguin E, Poelman H, Thibault-Starzyk F, Jacobs PA, Dirk E and De Vos
DE. A Probing the lewis acidity and catalytic activity of the metal organic framework
[Cu3(BTC)2] (BTC=Benzene-1,3,5-tricarboxylate). Chemistry A European Journal 2006;
12: 7353-7363.
17. Móilín molecular modelling software. Patrick McArdle and Rex Dark, NUI, Galway,
Ireland. More information can be found in: Patrick McArdle, Karen Gilligan, Desmond
Cunningham, Rex Dark and Mary Mahon Cryst Eng Comm, 6, 303-309 (2004).
18. Mahajan R, Burns R, Schaeffer M and Koros WJ. Challenges in forming successful
mixed matrix membranes with rigid polymeric materials, Journal of Applied Polymer
Science 2002; 86: 881–890.
19. Yuzay IE, Auras R, Soto-Valdez H and Selke S. Effects of synthetic and natural
zeolites on morphology and thermal degradation of poly(lactic acid) composites.,
Polymer Degradation and Stability 2010; 95: 1769-1777.
20. Kim S, Pechar TW and Marand E. Poly(imide siloxane) and carbon nanotube mixed
matrix membranes for gas separation. Desalination 2006 192: 330–339.
21. Adams R, Carson C, Ward J, Tannenbaum R and Koros W. Metal organic framework
mixed matrix membranes for gas separations. Microporous and Mesoporous Materials
2010; 131: 13-20.
22. Chung T-S, Jiang LY, Li Y and Kulprathipanja S. Metal organic framework mixed
matrix membranes (MMMs) comprising organic polymers with dispersed inorganic
fillers for gas separation. Progress in polymer science 2007; 32: 483–507.
23. Guo H, Zhu G, Hewitt IJ and Qiu S. “Twin copper source” growth of metal organic
framework membrane: Cu3(BTC)2 with high permeability and selectivity for recycling
H2. Journal of American Chemical Society 2009; 131: 1646–1647.
24. Elangovan D, Nidoni U, Yuzay I E, Selke SEM and Auras R. Poly(lactic acid) metal
organic framework composites: mass transport properties. Industrial & Engineering
Chemistry Research 2011; 50: 11136-42.

87

25. Elangovan D, Yuzay I, Selke S and Auras R. Poly(lactic acid) metal organic
framework composites: optical, thermal and mechanical properties. Polymer International
2012; 61: 30-37.
26. Inkinen S, Hakkarainen M, Albertsson A, Södergard A. Biomacromolecules 2011;12:
523–532.
27. Dorgan JR, Rheology of Poly(lactic acid), in Poly(Lactic Acid): Synthesis,
structures, properties, processing, and applications, edited by Auras R, Lim L, Selke S
and Tsuji H. Wiley Series Published Chapter 10: 125-139 (2010).
28. Schlichte K, Kratzke T and Kaskel S. Improved synthesis, thermal stability and
catalytic properties of the metal-organic framework compound Cu3(BTC)2. Microporous
and Mesoporous Materials 2004; 73: 81–88.
29. Huang L, Joshi KL, Van Duin ACT, Bandosz TJ and Gubbins KE. Molecular
dynamics simulation of thermal stability of Cu3(BTC)2 metal organic framework.
Physical Chemistry and Chemical Physics 2012; 14: 11327-11332.
30. Chui SSY, Lo S M-F, Charmant JPH, Orpen AG and Williams ID. A chemically
functionalizable nanoporous material [Cu3(TMA)2(H2O)3]n. Science 1999; 283: 11481150.
31. Biemmi E, Christian S, Stock N and Bein T. High-throughput screening of synthesis
parameters in the formation of the metal-organic frameworks MOF-5 and HKUST-1.
Microporous and Mesoporous Materials 2009; 117: 111-117.
32. Zhou H, Green TB and Joo YL. The thermal effects on electrospinning of polylactic
acid melts. Polymer 2006; 47: 7497-7505.
33. Mathew A, Oksman K and Sain M. The effect of morphology and chemical
characteristics of cellulose reinforcements on the crystallinity of polylactic acid, Journal
of Applied Polymer Science 2006; 101, 300–310.
34. Suryanegara L, Nakagaito A and Yano H. The effect of crystallization of PLA on the
thermal and mechanical properties of microfibrillated cellulose-reinforced PLA
composites. Composites Science and Technology 2009; 69: 1187–1192.
35. Perego G, Cella GD and Bastioli C. Effect of molecular weight and crystallinity on
poly(lactic acid) mechanical properties. Journal of Applied Polymer Science 1996; 59:
37–43.
36. Prestipino C, Regli L, Vitillo J and Bonino F. Local structure of framework Cu(II) in
HKUST-1 metallorganic framework: spectroscopic characterization upon activation and
interaction with adsorbates. Chemistry of Materials, 18, 1337-1346, 2006.

88

37. Küsgens P, Rose M, Senkovska I, Fröde H, Henschel A, Siegle S and Stefan Kaskel.
Characterization of metal-organic frameworks by water adsorption. Microporous and
Mesoporous Materials 2009; 120: 325–330.
38. Mustafa D, Breynaert E, Bajpe SR , Martens JA and Kirschhock CEA. Stability
improvement of Cu3(BTC)2 metal–organic frameworks under steaming conditions by
encapsulation of a Keggin polyoxometalate. Chemical Communications 2011; 47: 80378039.
39. Ghosh S, Ghosh-Bandyopadhyay S and Sain M. Composites in poly(lactic Acid):
synthesis, structures, properties, processing, and applications, edited by Auras R, Lim L,
Selke S and Tsuji H. Wiley Series. Chapter 17, 293-307 (2010).
40. Lim L, Auras R and Rubino M. Processing technologies for poly (lactic acid).
Progress in Polymer Science 2008; 33: 820–852.
41. Nakamura K, Kiyohara K, Takeda A and Okubo Y. US patent 0069192 (2006).
42. Okubo Y, Kiyohara K, Kawabe S, Takeda A , Suzuki T and Nakamura K. US patent
7569259 B2 (2009).
43. Wang Z-Y, Luo Y-F ,Ye R-R and Song X-M. Synthesis of novel biodegradable
material poly(lactic acid-trimesic acid) via direct melt copolycondensation and its
characterization. Journal of Polymer Research 2011; 18: 499–508.
44. Perego G, Cella G D and Bastioli C. Effect of molecular weight and crystallinity on
poly(lactic acid) mechanical properties. Journal of Applied Polymer Science 1996; 59:
37–43.
45. Carrasco F, Pagès P, Gámez-Pérez J, Santana O and Maspoch M. Processing of
poly(lactic acid): characterization of chemical structure, thermal stability and mechanical
properties. Polymer degradation and stability 2010; 95: 116-125.
46. Auras R, Harte B, Selke S and Hernandez R, Journal of plastic film and sheeting
2003; 19: 123–135.

89

Chapter 5: Toughening of poly(L-lactic acid) with Cu3(BTC)2 metal organic
frameworks crystals

90

Chapter 5: Toughening of poly(L-lactic acid) with Cu3(BTC)2 metal organic
framework crystals
Poly(L-lactic acid) (PLLA) and metal organic framework (MOF) composites were
prepared by melt extrusion of PLLA with 5, 10 and 20% w/w of activated Cu3(BTC)2
MOF. The morphology and stability of injection-molded samples were evaluated using
thermogravimetric analysis, differential scanning calorimetry (DSC), gel permeation
chromatography, X-ray diffraction, and scanning electron microscopy (SEM). The
composites showed improved toughness during the tensile tests as compared to the neat
PLLA matrix. The toughness mechanism of the composites was studied using SEM and
rheological studies. SEM images indicated that cavitation induced by debonding at the
interface of PLLA and MOF particles during the uniaxial stress were primarily
responsible for the improved toughness of the composites. The SEM images of the
composites, the solid like plateau observed in the PLLA composites during the parallel
plate rheology at low frequency, and the decrease in the cold crystallization enthalpy
during the DSC studies indicated good interactions between the PLLA chains and MOF
in the composites. The developed composites have potential for various applications,
which includes gas separation, energy and active packaging.

91

5.1 Introduction
High petroleum prices, corporate environmental awareness and favorable legislation have
provided an increased demand for bio-based and biodegradable polymers. Poly(lactic
acid), PLA, the main commercially available bio-based and compostable polyester
produced by the monomeric synthesis of L-lactic acid derived from renewable resources
such as corn, cassava, or sugar beets has successfully gained market acceptance for
consumer goods applications [1, 2]. PLA has similar mechanical, thermal and barrier
properties to poly(styrene) and poly(ethylene terephathalate). It has high perm-selectivity
to gases like (CO2/CH4) [3, 4] so it has potential to be used for gas separation. However,
the brittleness of PLA along with low heat deflection temperature and poor barrier
properties hinders its wide market applications [5-7].
In general, fillers such as talc, carbon black and/or zeolites are added to a
polymeric matrix to reduce the cost or to achieve desired properties like specific permselectivity. Mixed matrix membranes (MMMs), which traditionally consist of zeolite,
carbon molecular sieves or other porous particles embedded in a polymeric matrix, are
increasingly being used for improving mechanical properties, thermal properties and
perm-selectivity of polymers [8-10]. Although MMMs prepared from inorganic materials
like zeolites have been widely researched in the last few decades, the poor compatibility
between the inorganic fillers and organic polymeric matrix can lead to sub-micron size
holes in the MMMs [11-12]. Mahajan et al [11] studied MatrimidTM, a thermoplastic
polyimide, with 20% zeolite 4A; they grafted the polyimide chains on the zeolite using a
silane-coupling agent to improve compatibility between the zeolite 4A and MatrimidTM.

92

Metal organic frameworks (MOFs) are a class of porous crystalline compounds
+

2+

manufactured from metal ions (Cu , Cu

, Ag+, Zn2+, Co2+, K+, etc.) bridged together

by organic ligands forming multi-dimensional structures. MOFs offer a promising
alternative to zeolites and other molecular sieves in the production of MMMs since they
have high surface area, selective sorption, gas separation potential and storage capacity.
However, high cost and poor mechanical properties of the MOF reduces possibility of
commercial applications of these compounds. Incorporating MOFs in polymers is one of
various solutions to contain the cost of membranes and achieve the desired mechanical
properties. The hybrid inorganic-organic composition of the MOF compounds may offer
better compatibility with organic polymeric materials as compared to zeolites and other
inorganic molecular sieves. Various researchers have synthesized MMMs using different
MOFs and polymers to improve the permeability and selectivity of the matrix [12-15].
Ploegmakers et al [13] studied polyimide-copper benzene 1,3,5 tricarboxylate MOF
MMMs for ethylene and ethane separation. They observed that 20% addition of
Cu3(BTC)2 MOF in the polyimide increased the selectivity to ethylene/ethane of
polyimide from 4.1 to 7.1. Scanning electronic microscopy images suggested good
compatibility between polyimide and Cu3(BTC)2, and the glass transition temperature
o

(Tg) of the polyimide decreased from 345 to 329 C with the addition of 20% Cu3(BTC)2.
Elangovan et al [16] reported that PLLA and Cu3(BTC)2 MOF were compatible as
determined by contact angle measurement offering improved composite characteristics
like mechanical properties and selective barrier properties as compared to PLLA.

93

We have previously observed that strong polymer filler interactions played an
important role in improving toughness of PLA-MOF composites [17]. Rheological
studies can be helpful in understanding the complex microscopic interaction of polymeric
chains with filler particles, which ultimately affects the macroscopic properties of the
composites. Thus, the main purpose of this work was to understand the effect of the
Cu3(BTC)2 MOF on the toughness, viscoelastic properties, morphology and stability of
the PLLA composites.
5.2 Methodology
5.2.1 Materials
Poly(L-lactic acid) (PLLA) resin grade 4043 D, 98% L-lactide, with weight average
molecular weight (Mw), number average molecular weight (Mn) and polydispersity index
(Mw/Mn) of 111 kDa, 84 kDa and 1.3; respectively, was provided by NatureWorks LLC
(Blair, NE, USA). Basolite
2 -1

1500-2100 m .g

TM

C300 MOF [(Cu3(BTC)2] with surface area ranging from

and particle size ranging from 5 to 30 μm was purchased from Sigma

Aldrich (St Louis, MO, USA).
5.2.2 Sample preparation
o

PLLA resin was dried at 80 C for 4 h and and Basolite

TM

C 300 MOF [(Cu3(BTC)2]

o

was activated at 200 C for 24 h using a vacuum oven at 0.1 MPa. PLLA, PLLA-5%
MOF, PLLA-10% MOF and PLLA-20% MOF composite samples were processed using
a vertical co-rotating twin-screw micro-compounder (DSM Research, Geleen, The

94

Netherlands) attached to an injection molder. The extruder has 150 mm screw long
3

screws with L/D ratio of 18. The volume of the barrel is ~15 cm . The material was
o

processed at 190 C using 5 min cycle time at ~1 MPa (140 psi) injection pressure. The
o

transfer cylinder and mold temperatures were set at 195 and 65 C, respectively. Dog
bone tensile bars, flexural samples, and X-ray diffraction (XRD) discs of PLLA, PLLA5% MOF, PLLA-10% MOF and PLLA-20% MOF composites were prepared and stored
o

in a desiccator at room temperature (~23 C) until further testing.

5.2.3 Characterization
5.2.3.1 Tensile test
Tensile properties were evaluated according to ASTM D638-10 using a Universal Tensile
Machine model UTS SFM 20 from United Calibration Corporation, (Huntington Beach,
CA, USA). The machine was equipped with a laser extensometer. Injection molded
dogbone samples with a gauge length of 25.4 mm were stored at room temperature in a
dessicator over desiccant for at least 40 h prior to testing. The samples were then tested at
o

room temperature (~23 C) using a 453 kg load cell at 0.023 kg of preload. PLLA, PLLA5% MOF, PLLA-10% MOF and PLLA-20% MOF were tested at a crosshead speed of
-1

-1

50.8 mm.min (2 in.min ).

5.2.3.2 Rheology
Dynamic frequency sweep tests of PLLA and PLLA composites were performed using
o

AR 2000 advanced rheometer from TA-Instruments (New Castle, DE, USA) at 175 C

95

using 25 mm diameter parallel plates with a gap of 1100 μm and frequency ranging from
-1

0.01 to 100 rad.sec . The strain used for the frequency sweep tests was selected as 2%,
which is in the linear viscoelastic range of the material as determined by strain sweep.
5.2.3.3 X-Ray diffraction (XRD)
X-ray diffraction spectroscopy was performed on XRD discs of PLLA, PLLA-5% MOF,
o

PLLA-10% MOF and PLLA-20% MOF stored at 23 C using a Bruker D8 advance X-ray
diffractometer (Bruker AXS GmgH, Karlsruhe, Germany) at 40kV, 40mA (1,600W)
using Cu Kα radiation ( = 1.5418 Å). The studies were conducted using a 1.2 mm
primary beam slit and 2.0 mm detector slit. The X-ray scans were carried out at speed of
0.02 degrees per second. Data was collected in triplicates.
5.2.3.4 Differential scanning calorimetry (DSC)
A DSC Q100 (TA instruments) was used to perform DSC analysis using
o

o

-1

heat/cool/heat cycles from 0 to 180 C at a rate of 10 C.min . The melting temperature
(Tm) and the associated heat enthalpy (ΔHm) were calculated from the first heating cycle.
The glass transition (Tg) temperature, cold crystallization onset (Tco), cold crystallization
peak (Tcc) and enthalpy of cold crystallization (ΔHc) were calculated from the second
heating cycle to eliminate any thermal history, which may have been introduced during
processing or storage. The samples were run in triplicates.
Consequently, the percent crystallinity of the samples was estimated using
equation (1):

96

X c  % 

H m  H c
100
c
H m 1  x 

(3)
c

where ΔHm is enthalpy of fusion; ΔHc is the enthalpy of cold crystallization; ΔH m is
c

enthalpy of fusion of pure crystalline PLA; ΔH m = 93.1 J/g [18]; and x is the mass
fraction of the MOF in the MMM.
5.2.3.5 Thermogravimetric analysis (TGA)
Thermogravimetric analysis of Cu3(BTC)2 MOF, PLLA, PLLA-5% MOF, PLA10% MOF and PLA-20% MOF was performed using a TGA 2950 (TA-Instruments, New
-1

Castle, DE, USA) under nitrogen flow of 100 ml.min . Samples weighing between 5 and
o

10 mg were heated at a rate of 10 C.min

-1

o

from 25 to 700 C. The data was analyzed

using Universal Analysis software version 2000 (TA Instruments, New Castle, DE,
USA). The TGA samples were run in triplicate.
5.2.3.6 Weight average molecular weight
To study the effect of processing conditions and MOF content on the stability of
the PLLA, gel permeation chromatography (GPC) analysis was performed on the PLLA
resin, extruded PLLA and PLLA-MOF composites using a GPC by Waters Inc. (Milford,
MA, USA). Calibration was done using polystyrene standard materials with a molecular
3

6

weight ranging from 2.9 x 10 to 3.64 x 10 Da using a third order polynomial equation.
The analyses were performed at a flow rate of 1 mL.min-1 and runtime of 45 min at 35°C
-1

using the Mark-Houwink corrected constant K = 0.000174 (mL·g ) and a = 0.736 for

97

dilute PLLA solution in tetrahydrofuran (THF)[19]. The instrument was equipped with a
Waters 1515 isocratic pump, Waters 717 autosampler, a series of Waters Styragel
Columns (HR4, HR3 and HR2) and Waters 2414 refractive index. Approximately 20 mg
of specimen were dissolved in 10 mL of HPLC grade THF with 99.99% purity (PharmcoAaper, Brookfield, CT, USA). The solution was then filtered using a 0.45 μm filter.
5.2.3.7 Scanning electron microscopy (SEM)
To understand the morphology of the composites, gold coated samples sputtered
using an Emscope SC 500 (Emscope Laboratories Ltd., Ashford, UK) were analyzed
using a JEOL JSM 6610 LV (JEOL, Japan) scanning electron microscope (SEM) at 12kV
accelerating voltage. The microscope was equipped with a tungsten filament.

5.2.3.8 Data analysis
Tukey's HSD (Honestly Significant Differences) tests were performed to determine if the
means were significantly different from each other at 95% confidence interval (α = 0.05).
The analyses were conducted using SAS 9.0 Software (SAS Institute Inc., Cary, NC,
USA).

5.3. Results and discussion
5.3.1 Tensile test/Toughness
Craze formation is primarily responsible for the brittle failure of PLA under
tensile stress. The low entanglement density of amorphous glassy PLA favors crazing
over shear yielding leading to a brittle fracture [20-22]. The toughness of the PLA can be
improved by various methodologies such as the addition of impact modifying rubber

98

particles, inorganic particles, blending, plasticization, etc. [22-26]. In heterogeneous
matrix the organic/inorganic particles or other fillers serve as a stress concentrators [2730]. Good interfacial interactions between the particles and the matrix are a prerequisite
to achieve better mechanical properties. Under uniaxial stress, the triaxial stress
generated at the interface of the filler particles and the organic matrix can promote
debonding and cavitation. Debonding at the interface releases the triaxial stress and
cavitation triggered matrix-shearing can dissipate the energy leading to the plastic
deformation and improved toughness of the composite materials. A schematic
representation of the cavitation process is presented in Figure 5.1.

99

Figure 5.1: Toughening mechanism of PLLA-MOF composites adapted from references
[28, 30]. Step I: polymer MOF composite, step II: debonding at the interface of the
polymer-MOF interface under tensile stress, step III: plastic deformation.

100

Table 5.1 Tensile properties of PLLA, PLLA-5% MOF, PLLA-10% MOF and PLLA-20% MOF
Material

Tensile Strength (MPa)

PLLA

76.9 ± 1.5

PLLA-5% MOF

58.9 ± 0.8

PLLA-10% MOF

52.4 ± 1.2

PLLA-20% MOF

44.5 ± 0.1

A
B
C
D

Elongation at break (%)
A

3.6 ± 1.5

16.9 ± 3.2

3.0 ± 0.3

B

3.0 ± 0.2

C

9.8 ± 3.9
5.4 ± 1.4

Modulus of Elasticity (GPa)

2.8 ± 0.1

A,C

A

A,B
A,B

2.6 ± 0.2

B

3

Toughness (MJ/m )
1.70 ± 1.06
8.49 ± 1.45
4.33 ± 1.86
1.86 ± 0.51

Note: Values in the same column with same capital superscript letters are not statistically significantly different at α = 0.05

101

A
B
C
A,C

Stress strain graphs of PLLA, PLLA-5% MOF, PLLA-10% MOF and PLLA-20%
MOF composites can be observed in Figure 5.2. Brittle fracture was observed for the
PLLA matrix without any plastic deformation. However, PLLA-MOF composites
showed considerable plastic deformation especially for PLLA-5% MOF composites.
Tensile force generated stress concentration at the interface of the matrix and the
Cu3(BTC)2 MOF led to debonding initiated cavitation process at the interface of MOF
particles and PLLA matrix as observed in SEM micrographs represented in Figure 5.3 b
to d [29-31]. The stress generated by the tensile force at room temperature was diffused
around the voids, generating local plastic deformation followed by strain softening. The
decrease in the yield stress of the composites can be ascribed to the dilative stress
component, which decreases the yield stress or the onset of the plastic deformation [32,
33]. The yield stress decreased with the increase in the MOF concentration from 5 to 20%
due to increased dilatation with increased MOF concentration under tensile stress [3335]. Parson et al [36] also observed increased dilatation with increase in CaCO3
concentration from 10 to 20% in an HDPE matrix. Wu et al [37] studied toughening of
polymers using rubber particles. The authors concluded that interparticle distance (matrix
ligament thickness) is a crucial factor for the toughening effect due to cavitation. This
effect was primarily due to yielding of the ligament. We observed decrease in the
elongation of the composites with increase in the concentration from 5 to 20 %. The
matrix ligaments present between the cavities are primarily responsible for the increased
toughness. The increase in the concentration of the MOF particles decreased the distance
between the cavities, which may have worked as a fracture initiating flaw / cluster

102

constraining the plastic deformation [35]. The bigger size cavities are easily visible in
microscopic images of PLLA-20% MOF composition in Figure 5.3d.

PLLA
PLLA-5% MOF
PLLA-10%MOF
PLLA-20% MOF

Figure 5.2: Stress-strain graph of PLLA and PLLA MOF composites

103

Several researchers have studied surface modified or coated inorganic particles
[24, 28, 31]. Meng et al [31] fabricated PLA composites with nano-titania particles and
nano-titania particles coated with polycaprolactone (PCL) in various concentrations
ranging up to 40 percent. The authors reported significant improvement in the toughness
of the PCL coated nano-titania composites due to debonding at the interface and
cavitation. Improved toughness was also attributed to the even distribution of stress
around the interface of the matrix and uniformly dispersed fillers in the heterogeneous
systems. The maximum strain at break of 64.7% was achieved at 26.5% wt. percent
loading of coated TiO2. On the other hand, in PLA uncoated TiO2 composites, the TiO2
particles agglomerated, leading to insignificant improvement in the mechanical
properties. Zuiderduin et al [28] studied the effect of CaCO3 and steric acid modified
CaCO3 particles on the toughening of polypropylene. Steric acid modified CaCO3
particles dispersed well and provided good mechanical properties, while partial
agglomeration was observed in CaCO3 particle especially at higher concentration, which
led to brittle transition. Above 40 wt % CaCO3 particles, brittle failure was observed due
to agglomeration. Glassy thermoplastics undergo plastic deformation in pure shear [38].
In the plastic deformation region, the decrease in the tensile strength with the increase in
the strain could be related to the increase in the shear flow with the increase in the void
size.

104

a

b

100 µm
c

50 µm
d

50 µm

50 µm

Figure 5.3: Tensile fractured cross section of a) PLLA, b) PLLA-5% MOF c) PLLA10% MOF, d) PLLA-20% MOF

5.3.2 Rheological studies
The storage modulus (G’), loss modulus (G’’), and tan δ (G”/G’) of PLLA, PLLA-5%
MOF, PLLA-10% MOF, and PLLA-20% MOF as a function of oscillatory frequency are
represented in Figure 5.4 a, b and c, respectively. The storage modulus of PLLA follows
typical entangled polydisperse melt. A secondary plateau or terminal shoulder was

105

observed in the G’ of the PLLA-MOF composites at low frequency. This solid like
behavior can be ascribed to the three dimensional network formation [21, 40] caused by
bridging of the strong polymeric chains between the MOF particles due to strong PLLACu3BTC2 MOF interactions [17]. These topological restraints may have contributed
towards the immobilization of the polymeric chains in the PLLA-MOF composites,
limiting their ability to relax at low frequency. This observation was also supported by
DSC studies discussed later in this chapter. At high frequency the polymeric chains were
able to overcome these restraints as represented in Figure 5.5 [40].
Tan delta (G”/G’) of PLLA and PLLA composites as a function of frequency
sweep is represented in Figure 5.4c. The tan δ showed a decreasing trend with increasing
frequency in the case of PLLA. However, mild peaks were observed for the PLLA-MOF
-1

composites around the frequency of 0.5 rad.s .
Figure 5.4d represents the complex viscosity (η*) of the PLLA, PLLA-5% MOF,
PLLA-10 % MOF and PLLA-20 % MOF composites. The decrease in complex viscosity
of the composites as compared to PLLA may be attributed to the free volume change and
the decrease in entanglement. Similar observations were made by other researchers
during the fabrication of composites with modified inorganic particles [41, 42]. Luo et al
[41] studied composites of PLA and TiO2 functionalized with lactic acid. They observed
decrease in the complex viscosity with the addition of the functionalized TiO2. The
authors attributed the decrease in the complex viscosity to the decrease in entanglement
of linear polymeric chains. Lin et al [42] observed a 40% decrease in the complex

106

viscosity of PLA-2.5% hyperbranched polyester amide blend as compared to PLA,
further decrease in the complex viscosity was observed with increasing hyperbranched
polyester amide content up to 15 percent, which was attributed to the increase in the free
volume and decrease in entanglement.

PLA
PLA-5% MOF
PLA-10%MOF
PLA-20%MOF

Figure 5.4: (a) Storage Modulus (G’) of PLLA and PLLA MOF composites as a function
of oscillatory frequency

107

PLA
PLA-5% MOF
PLA-10%MOF
PLA-20%MOF

Figure 5.4: (b) Loss Modulus (G”) of PLLA and PLLA MOF composites as a function of
oscillatory frequency

108

PLA
PLA-5% MOF
PLA-10%MOF
PLA-20%MOF

Figure 5.4: (c) Tan delta curves of PLLA and PLLA-MOF composites

109

PLA
PLA-5% MOF
PLA-10%MOF
PLA-20%MOF
10000

1000

100
0.01

1

0.1

10

Figure 5.4: (d) Complex viscosity of PLLA and PLLA MOF composites

110

100

10000

G’ = G”

1000

100

10
10

100

1000

10000

Figure 5.4: (e) Cole-Cole plot of PLLA and PLLA MOF composites

111

100
80

60

40
PLA
PLA-5% MOF
PLA-10%MOF
PLA-20%MOF

20

100

1000

10000

Figure 5.4: (f) Van Gurp Palmen plot of PLLA and PLLA MOF composites

112

Figure 5.5: Schematic representation of the configuration of polylactide MOF
composites a. at rest b. low shear rates c. at high shear rates adapted from reference [40]

113

A Cole-Cole Plot of the PLLA, PLLA-5% MOF, PLLA-10% MOF and PLLA20% MOF are represented in Figure 5.4 e. G’ and G” represents the elastic and viscous
parts of the complex viscoelastic fluids. At low frequencies, the viscoelastic properties
are very sensitive and indicative of the underlying molecular architecture of the structure.
At 175oC PLLA primarily demonstrated viscous behavior than the elastic behavior.
However, with the addition of MOF particles the elastic component of the PLLA
increased at low frequency with increasing MOF content due to favorable interactions
between PLLA matrix and the MOF crystals.
The van Gurp-Palmen plot of PLLA, PLLA-5% MOF, PLA-10% MOF and
PLLA-20% MOF composites is represented in Figure 5.4 f. The high phase angle of the
composites at low frequency indicates that the composites were free of percolation.
PLLA curve approaching phase angle close to 90o also indicates primarily viscous
behavior. The addition of Cu3BTC2 MOF crystals provides minor improvement in elastic
behavior, which increases with the increase in the MOF concentration from 5 to 20%.
5.3.3 X-Ray Diffraction (XRD)
Diffraction patterns of PLLA, PLLA-5% MOF, PLLA-10 % MOF and PLLA-20% MOF
composites are represented in Figure 5.6. The wide PLLA hump suggests amorphous
nature of injection molded PLLA samples. In case of composites we can see the
crystalline peaks of metal organic framework at the top of amorphous PLLA background.
During extrusion process PLLA composites retained their face centered cubic (FCC)
crystal structure. In our previous study [17], we observed that the adsorption of water by
MOF particles before extrusion were detrimental to the final properties of the composites.

114

(400)
(331)
(422)
(333)
(440)

(220) (222)

(111)

(200)

a

PLLA-20%MOF

b

PLLA-10%MOF

c
PLLA-5% MOF

d

PLLA

5

10

15

20

25

30

35

40

2 Theta (degrees)

Figure 5.6: X-ray diffraction patterns of PLLA-MOF composites

115

Detailed analysis of the effect of heat and moisture on the Cu3(BTC)2 MOF crystal
structure during extrusion with PLLA was presented in chapter 4.
5.3.4 Differential Scanning Calorimetry (DSC)
The average crystallization onset temperatures for PLLA, PLLA-5% MOF, PLLA-10%
MOF and PLLA-20% MOF observed in the DSC studies were 103.8, 113.8 115.1 and
o

115.2 C, respectively. The shift in the onset of the cold crystallization temperature of the
PLLA-MOF composites to higher temperatures with the increase in MOF content can be
attributed to the good interactions between the polymeric chains and MOF crystals,
which hindered the mobility of the chains. Such interactions between the polymeric
chains and MOF crystals support the finding of the increase in the shear storage modulus
in the rheology studies in the terminal region as observed in Figure 5.4 a. Table 5.2
represents the detailed DSC analysis of PLLA and its composites. We can also observe
that the cold crystallization enthalpy decreased with the addition of the MOF. The
decreased mobility of the polymeric chains can be ascribed to strong interactions between
the Cu3BTC2 MOF crystals and PLLA. There was no significant change in the Tg of the
composites as compared to PLLA, which indicates that the addition of the MOF crystals
does not provide any flexibility to the chain.
5.3.5 Thermogravimetric analysis (TGA)
The TGA thermographs for activated MOF, PLLA, PLLA-5% MOF, PLLA-10% MOF
and PLLA-20% MOF are represented in Figure 5.7. The activated MOF has 5% w/w
moisture content, mostly adsorbed during the loading of sample into the machine. The
116

organic component of the framework, benzene tricarboxylate, starts degrading at around
325oC. The average onset of thermal degradation of PLLA, PLLA-5% activated MOF
and PLLA-10% activated MOF and PLLA-20% MOF is at 324.5, 322.5, 320.5 and
o

315.8 C, respectively. The increase in the concentration of MOF decreases the onset of
o

thermal degradation of the composites. We observed Td for PLLA around 385 C,
o

whereas the PLLA–MOF composites have Td around 500 C. This could be ascribed to
the conversion of the remaining benzene tricarboxylate groups present in the MOF to CO
and CO2, as observed by Huang et al in their ReaxFF reactive molecular dynamics
(RMD) simulation studies [45]. They observed that Cu3(BTC)2 MOF has good thermal
o

stability up to 300 C, at higher temperatures the structural collapse of MOF has been
simulated [45]. The difference in the final plateau for the PLLA, PLLA-5% MOF, PLLA10% MOF and PLLA-20% MOF is mainly attributed to the difference in the metal
content in the composites.

117

Figure 5.7: Themogravimetric analysis of PLLA, Cu3(BTC)2 MOF and PLLA- MOF
composites.

118

Table 5.2 Detailed DSC analysis information of PLA and its composites
o

Cold Crystallization

Tg ( C)
Sample

o

o

Tco ( C)
PLLA

60.8 ± 0.1

PLLA-5% MOF

60.9 ± 0.0

A
A

103.8 ± 0.2
113.8 ± 0.2

PLLA-10% MOF 62.8 ± 0.0B

115.1 ± 1.1

PLLA-20% MOF 62.2 ± 0.3C

115.2 ± 0.4

Δ Hc (J/g)

Tcc ( C)
A
B
B
B

119.1 ± 0.1
129.9 ± 0.4
131.2 ± 1.2
129.7 ± 0.1

Melting

A
B
B
B

19.6 ± 0.9
3.6 ± 0.1
5.6 ± 1.4
6.7 ± 0.3

A

B
B,C
D

o

o

Tm ( C)

Δ Hm (J/g)

A

147.9 ± 0.2A

21.9 ± 0.5

B

150.8 ± 0.1B

6.9 ± 0.3

B

152.9 ± 1.0B

11.0 ± 0.7

B

152.2 ± 1.3B

15.0 ± 2.5

Tmo ( C)
143.3 ± 0.1
145.5 ± 0.1
145.8 ± 0.8
145.4 ± 0.5

Note: Values in the same column with same capital superscript letters are not statistically significantly different at =0.05

119

A

B
C
D

Table 5.3 Molecular weight of PLLA, PLLA-5%-MOF, PLLA-10% MOF and PLLA-20%-MOF
PLLA control
Mn, kDa
Mw, kDa
PDI

A

71.9 ± 1.2

A

103.0 ± 0.7

A, B

1.4 ± 0.0

PLLA-5% MOF

PLLA-10% MOF

A

A

72.8 ± 3.1

73.4 ± 0.6

A

A

102.7 ± 1.2

102.2 ± 0.2

A, B

B

1.4 ± 0.0

1.4 ± 0.0

PLLA-20% MOF
A

71.7 ± 3.8

B

107.1 ± 1.7
A

1.5 ± 0.1

Note: Values in the same column with same capital superscript letters are not statistically significantly different at α = 0.05

120

5.3.6 Gel Permeation Chromatography (GPC)
GPC analysis was performed on PLLA, PLLA-5% MOF, PLLA-10% MOF and PLLA20% MOF. Table 5.3 shows the Mw, Mn, and PDI data of PLLA, PLLA-5% MOF,
PLLA-10% MOF and PLLA-20% MOF. No statistically significant difference in the
PLLA and PLLA MOF composites were found, indicating that the presence of MOF does
not degrade PLA.
5.4 Conclusions
The PLLA-hybrid MOF composites were prepared by a melt extrusion process. The
uniformly dispersed MOF particles in the polymer matrix increased the toughness of the
PLLA matrix by a debonding initiated cavitation process. Large plastic deformation was
observed for PLLA-5 wt. % of Cu3(BTC)2 MOF. Favorable interfacial interactions
necessary between Cu3(BTC)2 MOF and PLLA for improved toughening were further
endorsed by rheological, microscopy and thermal analysis (DSC). Strong interactions
between MOF particles and PLLA restrict the mobility of the polymeric chains. The
MOF crystals retained their structure during extrusion processing, as observed in the
XRD studies. Gel permeation chromatography indicated that processing of PLLA with
MOF did not have any significant effect on the molecular weight of the polymer in the
final MMMs compositions.

121

REFERENCES

122

REFERENCES
1. Auras R, Harte B and Selke S. An overview of polylactides as packaging materials.
Macromolecular Bioscience 2004; 4: 835-864.
2. Soto-Valdez H, Auras R, Peralta E. Fabrication of poly(lactic acid) with resveratrol
and the diffusion of resveratrol into ethanol. Journal of Applied Polymer Science
2011; 121: 970-978.
3. Weber CJ, Haugaard V, Festersen R, and Bertelsen G. Production and applications of
biobased packaging materials for the food industry. Food Additives and
Contaminants 2002; 19: 172-177.
4. Almenar E, and Auras R. Permeability, Sorption and diffusion of pure PLA in
Poly(lactic acid) synthesis, structures, properties, processing, and applications. Edited
by Auras R, Lim LT, Selke SEM, and Tsuji H. Wiley Series 2010.
5. Auras R, Harte B, Selke S and Hernandez R. Mechanical, physical and barrier
properties of poly(lactide) films. Journal of Plastic Film and Sheeting 2003; 19: 123135.
6. Auras R, Harte B and Selke S. Effect of water on the oxygen barrier properties of
poly(ethylene terephthalate) and polylactides films. Journal of Applied Polymer
Science 2004; 92: 1790-1803.
7. Teng CC, Ma CCM, Cheng BD, Shih YF, Chen JW and Hsiao YK. Mechanical and
thermal properties of polylactide-grafted vapor-grown carbon nanofiber/polylactide
nanocomposites. Composites Part A: Applied Science and Manufacturing 2011; 42:
928-934.
8. Chung TS, Jiang LY and Kulprathipanja S. Mixed matrix membranes (MMMs)
comprising organic polymers with dispersed inorganic fillers for gas separation.
Progress in Polymer Science 2007; 32: 483-507.
9. Husain S and Koros WJ. Mixed matrix hollow fiber membranes made with modified
HSSZ-13 zeolite in polyetherimide polymer matrix for gas separation. Journal of
Membrane Science 2007; 288: 195-207.
10. Pechar TW, Kim S, Vaughan B, Marand E, Tsapatsis M, Jeong HK and Cornelius CJ.
Fabrication and characterization of polyimide-zeolite L mixed matrix membranes for
gas separation. Journal of Membrane Science 2006; 277: 195-202.
11. Mahajan R and Koros WJ. Mixed matrix membrane materials with glassy polymers.
Part 1. Polymer Engineering and Science 2002; 42: 1420-1431.

123

12. Adam R, Carson C, Ward J, Tannenbaum R, and Koros W. Metal organic framework
mixed matrix membranes for gas separations. Microporous and Mesoporous
Materials 2010; 131:13-20.
13. Ploegmakers J, Japip S and Nijmeijer K. Mixed matrix membranes containing MOFs
for ethylene/ethane separation Part A: membrane preparation and characterization.
Journal of Membrane Science 2013; 428: 445-453
14. Perez EV, Balkus KJ, Ferraris JP and Musselman IH. Mixed matrix membranes
containing MOF-5 for gas separations. Journal of Membrane Science 2009; 328: 165173.
15. Basu S, Cano-Odena A and Vankelecom IFJ. Asymmetric Matrimid®/[Cu3(BTC)2]
mixed-matrix membranes for gas separations. Journal of Membrane Science 2010;
362: 478-487.
16. Elangovan D, Yuzay I, Selke S, and Auras R. Polymer International 2012; 61: 30-37.
17. Kathuria A, Abiad MG, and Auras R. Deterioration of metal organic framework
crystal structure during fabrication of poly(L-lactic acid) mixed matrix membranes.
Polymer International 2013.
18. Inkinen S, Hakkarainen M, Albertsson A, Södergard A. From lactic acid to
poly(lactic acid) (PLA): Characterization and analysis of PLA and its precursor.
Biomacromolecules 2011; 12: 523–532. (2011).
19. Dorgan JR, Rheology of Poly(lactic acid), in Poly(Lactic Acid): Synthesis, Structures,
Properties, Processing, and Applications, ed by Auras R, Lim L, Selke S and Tsuji H.
Wiley Series Published Chapter 10: 125-139 (2010).
20. Grijpma DW, Pennings AJ. (Co)polymers of L-lactide Mechanical Properties,
Macromol. Chem. Phys. 195, 1649- I663 (1994).
21. Berger LL. Relationship between Craze Microstructure and Molecular Entanglements
in Glassy Polymers 1. Macromolecules 1989; 22: 3162-3167.
22. Rahul Bhardwaj and Amar K. Mohanty. Modification of Brittle Polylactide by Novel
Hyperbranched Polymer-Based nanostructures Biomacromolecules 2007, 8, 24762484.
23. Zhao Q, Ding Y, Yang B, Ning N and Fu Q. Highly efficient toughening effect of
ultrafine full-vulcanized powdered rubber on poly(lactic acid) (PLA). Polymer
Testing 2013; 32: 299-305.
24. Chan CM, Wu J, Li JX, Cheung YK. Polypropylene/calcium carbonate
nanocomposites. Polymer 2002; 43: 2981-2992.

124

25. Ljungberg N, Wesslen B. Preparation and properties of plasticized Poly(lactic acid)
films. Biomacromolecules 2005; 6: 1789-1796.
26. Jin HJ, Chin IJ, Kim MN, Kim SH, and Yoon JS. Blending of poly(L-lactic acid) with
poly(cis-1,4-isoprene). Eur. Polym. J 2000; 36: 165-169.
27. Chow TS. Tensile strength of the filled polymers. Journal of Polymer Science:
Polymer Physics 1982; 20: 2103-2109.
28. Zuiderduin WCJ, Westzaan C, J. Hue´tink J, Gaymans RJ. Toughening of
polypropylene with calcium carbonate particles. Polymer 2003; 44: 261-275.
29. Kim GM, and Michler GH. Micromechanical deformation processes in toughened
and particle filled semicrystalline polymers. Part 1: Model representation for
micromechanical deformation processes. Polymer 1998; 39:5689-5679.
30. Kim GM, and Michler GH. Micromechanical deformation processes in toughened
and particle filled semicrystalline polymers. Part 2: Model representation for
micromechanical deformation processes. Polymer 1998; 39:5699-5703.
31. Meng B, Tao J, Deng J, Wu Z, and Yang M. Toughening of polylactide with higher
loading of nano-titania particles coated by poly(ε-caprolactone). Materials Letters
2011; 65: 729-732.
32. Edited by Brostow W, Performance of Plastics, Hanser (SPE) Page 34.
33. Lazerri A, Thio YS and Cohen RE. Volume strain measurements on
CaCO3/polypropylene particulate composites: The effect of particle size. Journal of
Applied Polymer Science 2004; 91: 925-935.
34. Lazerri A and Bucknall CB. Applications of dilatational yielding model to rubber toughened polymers. Polymer 1995; 36: 2895-2902.
35. Argon AS and Cohen RE. Toughenability of polymers. Polymer 2003; 44: 60136032.
36. Parsons EM, Boyce MC, Parks DM and Weinberg M. Three-dimensional large-strain
tensile deformation of neat and calcium carbonate-filled high-density polyethylene.
Polymer 2005; 46: 2257-2265.
37. Wu S. A generalized criterion for rubber toughening: The critical matrix ligament
thickness. Journal of Applied Polymer Science 1988; 35: 549-561.
38. Maxwell MA, and Yee AF. The effect of strain rate on the toughening mechanisms of
rubber-modified plastics. Polymer Engineering and Science 1981; 21: 205-211.
39. Sarvestani AS, and Picu CR. Network model for the viscoelastic behavior of polymer
nanocomposites. Polymer 2004; 45: 7779-7790.

125

40. Havet G and Isayev AI. A thermodynamic approach to the rheology of highly
interactive filler-polymer mixtures. Part II, Comparison with polystyrene/nanosilica
mixtures. Rheologica Acta 2003 42: 47-55.
41. Luo YB, Li WD, Wang XL, Xu DY, and Wang YZ. Preparation and properties of
nanocomposites based on poly(lactic acid) and functionalized TiO2. Acta Materialia
57 (2009) 3182–3191.
42. Lin Y, Zhang KY, Dong ZM, Dong LS and Li YS. Study of hydrogen bonded blend
of polylactide with biodegradable hyperbranched poly(esteramide). Macromolecules
2007, 40, 6257-6267.
43. Kinloch AJ, Young RJ. Fracture behaviour of polymers Applied Science Publishers
Ltd, London GB 1983.
44. Edited by RJM Smit. Toughness of heterogeneous polymeric systems a modeling
approach 1998.
45. Huang L, Joshi KL, Van Duin ACT, Bandosz TJ and Gubbins KE, Physical
Chemistry and Chemical Physics 2012; 14: 11327-11332

126

Chapter 6. Mass transfer properties of PLLA- Cu3(BTC)2 MOF

127

Chapter 6. Mass transfer properties of PLLA- Cu3(BTC)2 MOF
6.1 Introduction
Poly(lactic acid) (PLA) has increased its market presence due to competitive resin
prices and biodegradability especially in the packaging and the textile industries.
Permeability of various gases and water molecules plays an important role in the
selection of packaging materials. PLA is a poor barrier material for CO2 and water vapor
as compared to traditional petroleum based polymers like poly(vinyl chloride),
poly(ethylene terephthalate) and poly(propylene)

[1-3]. Thermal and mechanical

properties are significantly impacted by isomeric composition of the PLA. However, it
has been reported that the stereo isomeric content of PLA (i.e., PLLA or PDLA) does not
significantly impact its water permeability [4, 5]. Tsuji and Tsuruno reported that
PLLA/PDLA blend films with only stereocomplex crystallites showed lower water
permeability coefficients than PLLA or PDLA polymers [6].
Metal organic frameworks (MOFs), a class of micro-porous materials, have
attracted a lot of attention due to their high sorption properties, catalysis and high surface
area [7-9]. MOFs can host various molecules in their pores, depending on their
interaction. The host-guest interactions between MOFs and gaseous molecules, water
vapor or organic molecules have been credited to: (i) the available metal sites which
interact/co-ordinate with the gases or water molecules and (ii) organic linkers which can
physisorb gases due to secondary forces [10-12]. Careful selection of a MOF for a
particular application is important to obtain the desired results. The ability of a particular
MOF to host a particular species of gas strongly depends on the type of the functional

128

groups attached to the organic linkers or metal species. In this research, copper benzene
1,3,5 tricarboxylate [Cu3(BTC)2] MOF was selected due to its suitable interactions with
CO2 [13].
Optimum presence of CO2 in the head space of a package can inhibit the growth
of microorganisms. However, a high concentration of CO2 in packaged fresh produce is
always expected due to their respiration process, in which fresh produce consumes
oxygen and releases CO2. A high concentration of CO2 can encourage the fermentation
process generating compounds such as acetaldehyde, ethanol, etc, which can decrease the
acceptability of the product [14-15]. Therefore, it is important to maintain an optimum
level of CO2 in the headspace by avoiding its accumulation. Membranes with high
permeability to CO2 can help to prolong the shelf life of the fresh produce. The water
vapor transmission rate (WVTR) of a packaging material is crucial to maintain the shelf
life of various food and pharmaceutical products [16-17]. The shelf life of dry products
like crackers and cereal is compromised if the moisture content in the package crosses the
critical moisture content level. On the other hand, in certain products like fresh produce,
it is crucial to maintain a high water activity to maintain the shelf life. Oxidative changes
in the food can produce biochemical changes leading to deterioration of food product
such as food chips, coffee, etc. compromising the sensory quality of the product [18-19].
Trans-2-hexenal is an aroma compound present in various fruits [20-22]. The
retention of trans-2-hexenal provides an impression of fresh odor of the fruits. Corbo et
al [23] studied the shelf life of sliced apples using hexanal and trans-2-hexenal. The

129

inclusion of these aromatic compounds in the packaging atmosphere considerably
depressed yeast growth. Joo et al [24] reported production of antimicrobial film using
trans-2-hexenal encapsulated in cyclodextrin, and the reduction of Alternaria solani
growth due to the release of around 0.08 L trans-2-hexenal/L air.
It has been demonstrated that presence of mesoporous or microporous fillers such
as zeolites [25-26], carbon molecular sieves [27] and metal organic frameworks (MOFs)
[28-29] can improve the gas permeability and separation properties of a polymeric
matrix. MOFs have been primarily in focus for over a decade for their ease of synthesis,
high surface area, gas adsorption and selectivity [30]. The hybrid organic-inorganic
compositions of the MOFs make them good candidates for polymer matrix based
MMMs, as such hybrid compounds can offer defect free MMMs and good thermal
stability [31-33].
Cu3(BTC)2 MOF has been investigated for its affinity with CO2 and H2O [3439]. Torrisi et al [34-35] reported that the aromatic organic linkers (benzene tri
carboxylate) present in the Cu3(BTC)2

MOF can interact with CO2 due to strong

quadrupole-quadrupole interactions. They studied the van der Waals interactions between
CO2 and the aromatic structures present in the Cu3(BTC)2 MOF ligands. Two classes of
functional group were selected for substitution on the benzene rings: (i) electrophilic
halogen group such as C6H2F4, C6H5Cl, C6H5Br and (ii) nucleophilic methyl group
such as C6H5CH3, C6H4(CH3)2, etc. It was observed that the halogen substitution of the
benzene rings destabilize the π-quadrupole interactions. The destabilization effect was

130

ascribed to the charge withdrawal effect of hydrogen substituted group. Methyl and
polymethyl substitution on the benzene ring increased the CO2 affinity.
Kusgen et al [36] characterized Cu3(BTC)2 MOF using water adsorption. They
suggested that water adsorption follows two steps: (a). axial coordination of the water
molecules to the polar hydrophilic copper sites, and (b). filling of non-polar hydrophobic
sites due to organic linkers at higher pressure. Castillo et al [37], studying the interaction
of various hydrocarbons, gaseous and water molecules using Monte Carlo molecular
simulation, found that Cu3(BTC)2 MOF has strong affinity towards water molecules
compared to other gaseous molecules in the following order CO2 > O2 > N2. This
indicates that during an application water molecules will compete with other gases for
available sites. Supronowicz et al [38] recently calculated the adsorption energies of
H2O, CO2, and O2, among other gaseous molecules, using ab initio density functional
theory. Adsorption enthalpies for H2O, CO2, O2 calculated in this study were -53.8, -20.1
and -5.7 kJ/mol. These last three studies [36-38] indicate good van der Waals interactions
of Cu3(BTC)2 MOF with water and CO2 molecules.
The purpose of this study was to understand the effect of the presence of
Cu3(BTC)2 MOF on the permeability and perm-selectivity of PLLA for packaging and
other MMM industrial applications. The mass transfer properties of PLLA- Cu3(BTC)2
MOF based membranes to water (H2O), carbon dioxide (CO2), oxygen (O2) and trans-2-

131

hexenal were evaluated by assessing the permeability to these compounds for PLLAMOF mixed matrix membranes.
6.2. Methodology
6.2.1 Materials
Poly(L-lactic acid) (PLLA) grade 4043 D with 98% L-lactide content was provided by
NatureWorks LLC (Minnetonka, MN, USA). PLLA resin has an average molecular
weight (Mw), number average molecular weight (Mn) and polydispersity index (Mw/Mn)
of 111 kDa, 84 kDa and 1.3, respectively. Basolite

TM

C300 metal organic framework
2 -1

[Cu3(BTC)2] with surface area ranging from 1500-2100 m .g and particle size ranging
from 5-30 μm was purchased from Sigma Aldrich (St Louis, MO, USA). Trans-2hexanal with at least 95% purity was supplied by SAFC, US.
6.2.2 Fabrication of membranes
o

PLLA resin was dried for 4 h at 80 C and Basolite

TM

C 300 MOF was activated at

o

200 C for 24 h using vacuum oven. PLLA and PLLA-5% MOF, PLLA-10% MOF and
PLLA-20% MOF composite samples were processed using a vertical co-rotating twinscrew micro-compounder (DSM Research, Geleen, The Netherlands) attached with an
injection molder. The extruder has 150 mm screw long screws with L/D ratio of 18. The
3

o

volume of the barrel was approximately 15 cm . The material was processed at 190 C
using 5 min cycle time at ~1 MPa (140 psi) injection pressure. The transfer cylinder and
o

mold temperatures were set at 195 and 65 C; respectively. XRD discs of PLLA, PLLA-

132

5% activated MOF, PLLA-10% activated MOF and PLLA-20% activated MOF
composites were prepared and stored in a desiccator at room temperature (~23 oC).
The films used for the permeability studies were prepared using a PHI 30 ton
compression molding machine (City of industry, CA, USA) with 12” x 12” plate size.
The injection molded samples prepared using the twin screw DSM micro-compounding
o

instrument were compressed at 170 C and ~1.0 MPa for 5 min.

6.2.3 Mass transfer properties
6.2.3.1 Carbon dioxide transmission rate (CO2TR)

Carbon dioxide transmission rate (CO2TR) of the PLLA, PLLA-5% MOF, PLLA-10%
MOF and PLLA-20% MOF were measured as per ASTM F2476 05 using an Permatran

®

o

4/41 (MOCON Inc., Minneapolis, MN, USA) at 23 C and 0% RH using 100% CO2 . N2
was used as a carrier gas at flow rate of 50 sccm. The tests were performed using an
exam time of 30 minutes. The transmission was measured using an aluminum mask with
2

3.14 cm area. The average value of the last ten stable points in steady state was used to
calculate the permeability values using equation 6.1. The data was analyzed for at least
three samples.

gas permeability 

(6.1)
transmission rate (kg/m2 .s) x thickness (m)
partial pressure (Pa)

133

6.2.3.2 Water vapor transmission rate (WVTR)
The water vapor transmission rates of the PLLA, PLLA-5% MOF, PLLA-10% MOF and
®

PLLA-20% MOF were measured according to ASTM F1249 using a Permatran W3/31
o

(MOCON Inc., Minneapolis, MN, USA) at 23 C and 100% RH using N2 gas as a carrier
at flow rate of 100 sccm. The tests were performed using exam time of 30 minutes. The
2

transmission was measured using aluminum masks with 3.14 cm area permeability. The
average value of last ten stable points was used to calculate the permeability values using
equation 6.1. The data was analyzed for at least three samples.
6.2.3.3 Oxygen transmission rate (OTR)
The oxygen transmission rates of the PLLA, PLLA-5% MOF, PLLA-10% MOF and
PLLA-20% MOF were measured as per ASTM D3985-05 using an Oxtran

®

2/21

o

(MOCON Inc., Minneapolis, MN, USA) at 23 C and 0% RH using 100% as permeant
and 98 % N2/ 2% H2 as carrier gas at flow rate of 20 sccm of the test gas and 10 sccm
flow rate of carrier gas. The tests were performed using an exam time of 30 minutes. The
2

transmission was measured using aluminum masks with 3.14 cm area. The average
value of the last ten stable points was used to calculate the permeability values using
equation 6.1. The data was analyzed for at least three samples.
6.2.3.4 Aroma transmission rate

134

The aroma vapor transmission rate of trans-2-hexanal was measured using a modified
ASTM E-96 cup method at 23 ± 0.1 °C, 50 ± 2% RH and partial pressure difference of
1436 Pa. The aroma vapor permeability coefficients of the PLLA and PLLA-20% MOF
were calculated from the slope of the weight loss from the cup with respect to time using
equation 6.2.

aroma permeability 

slope (kg/s) x thickness (m)
partial pressure difference (Pa) x area (m2 )

(6.2)

6.2.4 Data analysis
Tukey's HSD (Honestly Significant Differences) tests were performed to determine if the
means were significantly different from each other at a 95% confidence interval (α =
0.05). The analyses were conducted using SAS 9.0 Software (SAS Institute Inc., Cary,
NC, USA).

6.3. Results and discussion
6.3.1 Carbon dioxide permeability coefficient
Carbon dioxide permeability coefficients of PLLA, PLLA-5% MOF, PLLA-10%MOF
and PLLA-20% MOF are presented in Table 6.1. There were no statistically significant
difference in the permeability coefficients of PLLA, PLLA-5% MOF and PLLA-10%
MOF. However, with the addition of 20% Cu3(BTC)2 MOF in the PLLA matrix, we
observed ~ 38% increase in the permeability coefficient as compared to neat PLLA.
Favorable interaction energies between the Cu3(BTC)2 MOF and CO2 might be
responsible for the increased rate of transportation of the CO2 through PLLA-Cu3(BTC)2
135

MOF [29, 37-38]. Basu et al [28] studied CO2/CH4 and CO2/N2 gas separations using
polyimide-Cu3(BTC)2 MOF based MMMs. The authors reported an increase in the
permeability and selectivity of membranes with increase in the filler loading due to the
strong quadrupole moment of CO2 and poor electrostatic interactions of N2 and CH4
molecules with the Cu3(BTC)2 MOF.

Table 6.1 CO2 and O2 permeability coefficients of PLLA, PLLA-5% MOF, PLLA-10%
MOF and PLLA-20% MOF

Permeability Coefficient x 10
-2 -1

-18

Permselectivity

-1

(kg.m.m .s .Pa )
CO2

αCO2/O2

O2
A

PLLA

45.54 ± 2.72

PLLA-5% MOF

49.85 ± 4.08

PLLA-10% MOF

42.89 ± 3.13

PLLA-20% MOF

63.03 ± 1.86B

A
A

A

7.59

A

7.56

A

6.94

A

10.26

6.00 ± 0.40
6.59 ± 0.84
6.18 ± 0.88
6.14 ± 0.87

Note: Values in the same column with same capital superscript letters are not statistically
significantly different at α = 0.05

6.3.2 Oxygen permeability coefficient

136

The oxygen permeability coefficient of PLLA, PLLA-5% MOF, PLLA-10%MOF and
PLLA-20% MOF are presented in Table 6.1. There were no statistically significant
differences in the permeability coefficients of PLLA and PLLA-Cu3(BTC)2 MOF based
MMM, which can be ascribed to poor van der Waals interactions of O2 with Cu3(BTC)2
[37].
The perm-selectivity of PLLA (αCO2/O2) improved from 7.6 to 10.3 with the
addition of 20% MOF crystals. The increased perm-selectivity can be attributed to the
favorable interactions between MOF crystals and CO2 gas than O2 gas.

6.3.3 Water vapor permeability coefficient
Water vapor permeability coefficients of PLLA, PLLA-5% MOF, PLLA-10% MOF and
PLLA-20% MOF are presented in Table 6.2. The presence of MOF did not affect the
permeability coefficients of PLLA. As permeability is a product of solubility and
diffusivity, (equation 6.3), The relatively strong interactions (>40KJ/mol) [25, 26] of H2O
molecules with Cu3(BTC)2 MOF might have hindered the diffusivity of the water
molecules resulting in unchanged permeability of the PLLA-MOF MMMs by increasing
the water sorption in the PLLA-MOF membranes as indicated by Elangovan et al [40]
and reducing the diffusion coefficient.

Permeability = Diffusion x Solubility

137

(6.3)

Table 6.2 Water vapor permeability coefficients of PLLA, PLLA-5% MOF, PLLA-10%
MOF and PLLA-20% MOF

Permeability Coefficient x 10

-14

-2 -1

-1

(kg.m.m .s .Pa )

H2O
PLLA

3.29 ± 0.31

A,B

PLLA-5% MOF

3.72 ± 0.16

PLLA-10% MOF

3.13 ± 0.09

PLLA-20% MOF

3.28 ± 0.23

A
B
A

Note: Values in the same column with same capital superscript letters are not statistically
significantly different at α = 0.05

6.3.4 Trans-2-hexenal permeability coefficient
Trans-2-hexenal permeability coefficients of PLLA and PLLA-20% MOF are presented
in Table 6.3. The permeability coefficient of the PLLA-20% MOF membranes increased
by 60% as compared to the PLLA matrix. The coordination of trans-2-hexenal with the
unsaturated copper atoms might have contributed to increased permeability of MMMs.
To the best of author’s knowledge, there is no published literature to date which discusses
the interactions of trans-2-hexanal with Cu3(BTC)2 MOF, any other MOF or mixed
matrix membranes.

138

Table 6.3 Tran-2-hexenal permeability coefficients of PLLA and PLLA-20% MOF

Permeability Coefficient x 10

-13

-2 -1

-1

(kg.m.m .s .Pa )

Tran-2-hexenal
PLLA

2.92 ± 0.16

PLLA-20% MOF

4.70 ± 0.24

A
B

Note: Values in the same column with same capital superscript letters are not statistically
significantly different at α = 0.05

6.4 Conclusions

The mass transfer properties of PLLA- Cu3(BTC)2 MOF based membranes to water
(H2O), carbon dioxide (CO2), oxygen (O2) and tran-2-hexenal were evaluated. The CO2
and trans-2-hexanal permeability coefficient of PLLA-20% MOF increased by around
38% and 60% respectively as compared to PLLA. However, there was no change in the
O2 and water vapor permeability coefficients of PLLA with the addition of MOF. The
MMM permeability coefficient of various gases and organic molecules is strongly
influenced by the kind of interactions between the microporous materials and permeants.
The perm-selectivity of PLLA-20% MOF (αCO2/O2) improved from 7.6 to 10.3.

139

REFERENCES

140

REFERENCES

1. Auras RA, Harte B, Selke S and Hernandez RJ. Mechanical, physical and barrier
properties of poly(lactide) films. Journal of Plastic Film and Sheeting 2003; 19: 123135.
2. Bao L, Dorgan JR, Knauss D, Hait S, Oliveira NS and Maruccho IM. Gas permeation
properties of poly(lactic acid) revisited. Journal of Membrane Science 2006; 285:
166-172.
3. Auras R, Harte B and Selke S. An overview of polylactides as packaging materials.
Macromolecular Bioscience 2004; 4: 835-864.
4. Lehermeier HJ, Dorgan JR and Way JD. Gas permeation properties of poly(lactic
acid). Journal of Membrane Science 2001; 190: 243-251.
5. Almenar E, and Auras R. Permeability, Sorption and diffusion of pure PLA in
Poly(lactic acid) synthesis, structures, properties, processing, and applications. Edited
by Auras R, Lim LT, Selke SEM, and Tsuji H. Wiley Series 2010.
6. Tsuji H and Tsuruno T, “Water Vapor Permeability of Poly(L-lactide)/Poly(D-

lactide) Stereocomplexes,” Macromol. Mater. Eng., vol. 295, no. 8, pp. 709–715, Jul.
2010.
7. Eddaoudi M, Moler DB, Li H, Chen B, Reineke TM, O’Keeffe M and Yaghi OM.
Modular chemistry: secondary building units as a basis for the design of highly
porous and robust metal-organic carboxylate framework . Accounts of Chemical
Research 2001; 34: 319-330.
8. Rosi NL, Eckert J, Eddaoudi M, Vodak DT, Kim J, O’Keeffe M and Yaghi OM.
Hydrogen storage in microporous metal-organic frameworks. Science 2003; 300:
1127-1129.
9. Rowsell JLC and Yaghi OM. Strategies for hydrogen storage in metal-organic
frameworks. Angewandte Chemie 2005; 44: 4670-4679.
10. Prestipino C, Regli L, Vitillo JG, Bonino F, Damin A, Lamberti C, Zecchina A,
Solari PL, Kongshaug KO and Bordiga S. Local Structure of Framework Cu(II) in
HKUST-1 Metallorganic Framework:  Spectroscopic Characterization upon
Activation and Interaction with Adsorbates. Chemistry of Materials 2006; 18: 13371346.
11. Wang Z and Cohen SM. Post synthesis covalent modification of a neutral metalorganic framework. Journal of the American Chemical Society 2007; 129: 1236812369.

141

12. Eddaoudi M, Kim J, Rosi N, Vodak D, Wachter J, O’Keeffe M and Yaghi OM.
Systemic design of pore size and functionality in isoreticular MOFs and their
application in methane storage. Science 2002; 295: 469-472.
13. Millward AR and Yaghi OM. Metal-organic frameworks with exceptionally high
capacity for storage of carbon dioxide at room temperature. Journal of the American
Chemical Society 2005; 51: 17998-17999.
14. Allende A, Aguayo E and Arte´s F. Microbial and sensory quality of commercial
fresh processed red lettuce throughout the production chain and shelf life.
International Journal of Food Microbiology 2004; 91: 109– 117.
15. Brent A. Mrozinski. Dissertation: Acetaldehyde Scavengers for Poly(ethylene
terephthalate): Chemistry of Reactions, Capacity, and Modeling of Interactions, The
University of Toledo.
16. Brody AL, Strupinsky ER, and Kline LR. Active packaging for food applications
2001, Technomic Publishing Co. Inc. PA, USA.
17. Ooraikul B and Stiles ME 1991. Modified atmosphere packaging of food. Ellis
Horwood Limited
18. Ragnarsson JO and Labuza TP. Accelerated shelf-life testing for oxidative rancidity
in foods-A review. Food chemistry 1977; 4: 291-308.
19. Vermeiren L, Devlieghere F, Beest MV, Kruijf ND and Debevere J. Developments in
the active packaging of foods. Trends in food science and technology 1999; 3: 77-86.
20. Talens P, Escriche I, Martı´nez-Navarrete N, Chiralt A. Influence of osmotic
dehydration and freezing on the volatile profile of kiwi fruit. Food Research
International 2003; 36: 635-642.
21. Angerosa F, d’Alessandro N, Basti C and Vito R. Biogeneration of volatile
compounds in virgin olive oil: their evolution in relation to malaxation time. Journal
of Agricultural and Food Chemistry 1998; 46: 2940-2944.
22. Seuvre A-M, Philippe E, Rochard S, Voilley A. Retention of aroma compounds in
food matrices of similar rheological behaviour and different compositions. Food
Chemistry 2006; 96: 104–114.
23. Corbo MR, Lanciotti R, Gardini F, Sinigaglia M and Guerzoni ME. Effects of
hexanal, trans-2-hexenal, and storage temperature on shelf life of fresh sliced apples.
Journal of Agriculture and Food Chemistry 2000; 48: 2401-2408.
24. Joo MJ, Merkel C, Auras R and Almenar E. Development and characterization of
antimicrobial poly(L-lactic acid) containing trans-2-hexenal trapped in cyclodextrins.
International Journal of Food Microbiology 2012; 153: 297-305.

142

25. Sen D, Kalıpcılar H and Yilmaz L. Development of polycarbonate based zeolite 4A
filled mixed matrix gas separation membranes. Journal of Membrane
Science 2007; 303: 194– 203.
26. Ahn J, Chung WJ, Pinnau I and Guiver MD. Polysulfone/silica nanoparticle mixedmatrix membranes for gas separation. Journal of Membrane Science 2008; 314: 123133.
27. Bertelle S, Gupta T, Roizard D, Vallières C and Favre E. Study of polymer–carbon
mixed
matrix
membranes
for
CO2 separation
from
flue
gas Desalination 2006; 199: 401-402.
28. Adams R, Carson C, Ward J, Tannenbaum R and Koros W. Metal organic framework
mixed matrix membranes for gas separations. Microporous and Mesoporous
Materials 2010; 131: 13-20.
29. Basu S, Cano-Odena A and Vankelecom IF. Asymmetric Matrimid®/[Cu3(BTC)2]
mixed-matrix membranes for gas separations. Journal of Membrane Science 2010;
362: 478−487.
30. Chui SSY, Lo SM-F, Charmant JPH, Orpen AG and Willaims ID. A chemically
functionalizable nanoporous material [Cu3(TMA)2(H2O)3]n. Science 1999; 283: 11481150.
31. Kathuria A, Abiad M and Auras R. Deterioration of metal-organic framework crystal
structure during fabrication of poly(L-lactic acid) mixed-matrix membranes. Polymer
International 2013; 62: 1144-1151.
32. H. Li, M. Eddaoudi, M. O’Keeffe, O.M. Yaghi, Design and synthesis of an
exceptionally stable and highly porous metal–organic framework, Nature 402 (1999)
276-279.
33. Rowsell JLC, O.M. Yaghi, Metal–organic frameworks: a new class of porous
materials, Microporous and Mesoporous Materials 2004; 73: 3-14.
34. Torrisi A, Mellot-Draznieks C and Bell RG. Impact of ligands on CO2 adsorption in
metal-organic frameworks: First principles study of the interaction of CO2 with
functionalized benzenes. I. Inductive effects on the aromatic ring, The Journal of
Chemical Physics 2009; 130, 194703: 1-13.
35. Torrisi A, Mellot-Draznieks C and Robert G. Bell. Impact of ligands on CO2
adsorption in metal-organic frameworks: First principles study of the interaction of
CO2 with functionalized benzenes. II. Effect of polar and acidic substituents,
Inductive effects on the aromatic ring. The Journal of Chemical Physics 2010; 132,
044705:1-13.

143

36. Küsgens P, Rose M, Senkovska I, Fröde H, Henschel A, Siegle S and Stefan Kaskel.
Characterization of metal-organic frameworks by water adsorption. Microporous and
Mesoporous Materials 2009; 120: 325–330.
37. Castillo JM, Vlugt TJH and Calero S. Understanding water adsorption in Cu3(BTC)2
metal organic frameworks. The Journal of Physical Chemistry C 2008; 112: 1593415939.
38. Supronowicz B, Mavrandonakis A and Heine T. Interaction of small gases with the
unsaturated metal centers of the HKUST‑ 1 metal organic framework. The Journal of
Physical Chemistry 2013; 117: 14570-14578.
39. Grajciar L, Bludsky O and Nachtigall P. Water Adsorption on coordinatively
unsaturated sites in CuBTC MOF. The Journal of Physical Chemistry Letters 2010; 1:
3354-3359.
40. Elangovan D, Nidoni U, Yuzay I E, Selke SEM and Auras R, Industrial and
Engineering Chemistry Research 50: 11136-42 (2011).

144

Chapter 7 Conclusions and recommendation for future work

145

Chapter 7 Conclusions and recommendation for future work

7.1 Conclusions
This dissertation research work was divided into four parts: (a). synthesis and
characterization of Cu3(BTC)2 metal organic framework (MOF), (b). effect of the
processing parameters on the stability of Cu3(BTC)2 MOF crystals during fabrication of
poly(L-lactic acid) (PLLA)-Cu3(BTC)2 MOF based mixed matrix membranes, (c).
understanding the effect of MOF crystals on the toughening mechanism of PLLA , and
(d). evaluation of mass transfer properties of PLLA)-Cu3(BTC)2 mixed matrix
membranes.
7.1.1 Synthesis and characterization of Cu3BTC2 MOF
Cu3(BTC)2 MOF was synthesized using a microwave assisted synthesis process.
The characterization of the Cu3(BTC)2 MOF was performed using X-ray diffraction
studies, scanning electron microscopy, thermo-gravimetric analysis and BET surface area
studies. A highly crystalline chemical structure was observed during X-ray diffraction,
with average BET surface area of 1236 ± 103 m2/g. The MOF crystals had particle sizes
ranging from 5-30 µm as observed from SEM. Thermo-gravimetric studies indicated that
particles were generally stable up to 300oC.
7.1.2 Stability of Cu3BTC2 MOF crystals during fabrication of mixed matrix
membranes.
Poly(L-lactic acid) was melt compounded with 5% of activated Cu3(BTC)2 MOF
and 5% of water saturated Cu3(BTC)2 MOF. During melt compounding of Cu3(BTC)2
MOF with PLLA, the presence of moisture in the water saturated Cu3BTC2 MOF

146

significantly contributed to the deterioration of the MOF crystal structure. The water
released from the saturated MOF particles during extrusion contributed to the degradation
of the PLLA matrix due to hydrolysis of PLLA as observed in GPC studies. The weight
average molecular weight of PLLA-5% saturated MOF dropped to 83.5 kDa as compared
to 110.8 kDa for PLLA resin. Cu3BTC2 MOF increased the toughness of PLLA-MOF.
Improvement in toughness, increase in the CO2 permeability coefficient, and SEM
images of PLLA-5% saturated MOF indicated good interfacial interactions between
activated MOF and PLLA. The activated MOF particles retained their structure and
morphology. The presence of 5% activated MOF hindered the crystallization of PLLA
due to strong interfacial interactions between PLLA and Cu3BTC2 MOF as observed in
the SEM images.
7.1.3 Effect of MOF crystals on the toughness of PLLA
Large plastic deformation was observed for PLLA-5 wt. % of Cu3BTC2 MOF.
The toughness of PLLA-5% activated MOF composites improved by approximately
500%. The uniformly dispersed MOF particles in the polymer matrix increased the
toughness of PLLA matrix by debonding initiated cavitation process. The improvement
in the toughness decreased with further increase in the MOF concentration from 5% to
20% due to decrease in the interparticle distance of MOF incorporated in the PLLA
matrix. Rheological studies indicated the composites demonstrated solid like behavior
due to strong interactions between PLLA and MOF, the elastic component of the PLLA
matrix improved with the addition of the MOF crystals and the composites were free of
percolation. Favorable interfacial interactions necessary between Cu3(BTC)2 MOF and
PLLA for improved toughening were further endorsed by rheological, microscopy studies

147

and thermal analysis (DSC studies). Strong interactions between MOF particles and
PLLA restricted the mobility of the polymeric chains.

7.1.4 Mass transfer properties of PLLA
CO2 permeability of PLLA-20% wt. Cu3(BTC)2 MOF mixed matrix membranes
increased from 45.54 to 63.03 kg.m.m-2.s-1.Pa-1 as compared to neat PLLA. The permselectivity (αCO2/O2) of PLLA-20% MOF improved from 7.6 to 10.3, respectively. The
permeability coefficient of tran-2-hexenal increased by 60% with the addition of 20%
MOF. Permeability coefficients of various gases and organic molecules are strongly
influenced by the kind of interactions between the microporous materials, matrix and
permeants. These composites can be used for packaging application in which the CO2
evolved over time or entrapped in the headspace can affect the quality and shelf life of
the food product. The increased CO2 permeability can be helpful for gas separation. For
example, the calorific value of the methane produced in the biogas plant can be improved
by removing CO2.
7.1.5 Overall conclusions
The brittle nature of PLLA has hindered its mass scale applications. This
dissertation research work helped in better understanding the toughening mechanism of
PLLA, PLLA-MOF interface and the effect of the MOF content on the functional
properties of these composites. This work also helped in understanding the stability of the
Cu3(BTC)2 MOF crystal structure during extrusion processing. Mass transfer properties
indicated that the presence of Cu3BTC2 MOF improved tran-2-hexenal permeability and
CO2/O2 perm-selectivity of PLLA-20% MOF composites.

148

7.2 Recommendations for future work
7.2.1 Chemical modification of metal organic framework
Depending on the chemistry of the functional groups present in a MOF the
permeability and selectivity of the membranes is strongly influenced. Post synthesis
modification of MOF is one of the possible routes which can help in further improving
the mass transfer properties and perm-selectivity and high separation efficiency of
membranes.
7.2.2 Modeling mass transfer properties
The work described in chapters 4 and 5 improved the scientific understanding of
the morphology and interface of MOF and PLLA mixed matrix membranes. Chapter 6
helped in understanding the mass transfer properties of PLLA-MOF MMMs. However,
the mechanism of mass transfer is still poorly understood. Thus, additional work in this
area by evaluating the mass transfer properties of these membranes to other compounds
and utilizing theoretical models to understand the mass transfer properties can further
improve the scientific understanding of these MMMs.
7.2.3 Environmental impact
Metal organic frameworks consists of transition metal ions bridged together using
organic linkers. The environmental impact of the use of these compounds has not been
evaluated or understood as a packaging material. Due to the presence of metal ions, these
microporous materials can have negative impacts on the environment. Further work is
needed to study their impact if used for packaging applications. Comparative studies with
other metal free microporous species such as microporous organic polymers (MOPs),
MOFs synthesized using alkali metals can be conducted.

149

7.2.4 Shelf life studies
Further, work is needed to understand and determine if such composites can be
utilized in real life packaging applications. Designing an actual package and running
shelf life studies in the lab on relevant products can be helpful in evaluating and
understanding the performance of these materials for real packaging applications.

150