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This is to certify that the

thesis entitled

Chemical Modification of Polymer Surfaces Using
Sulfonation to Improve Adhesion Properties.

presented by

Himanshu Asthana

has been accepted towards fulﬁllment
of the requirements for

MEL—degree in Chemical Engineering

(Department of Chemical Engineering)

KW
Dr. Lawrence T. Drzal MW

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—___

CHEMICAL MODIFICATION OF POLYNIER
SURFACES USING SULFONATION TO EVIPROVE

ADHESION PROPERTIES

By

I-IIMANSHU ASTHANA

A THESIS

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

MASTER OF SCIENCE

Department of Chemical Engineering

1993

ABSTRACT

CHEMICAL MODIFICATION OF POLYMER SURFACES USING
SULFONATION TO IMPROVE ADHESION PROPERTIES
BY

Himanshu Asthana

The nature of chemical changes occuring on the surface of polymers has been
investigated in the thesis. Polypropylene and polystyrene were investigated in this
study. XPS and FTIR Spectroscopy were used in conjunction with each other to
determine the quantitative and qualitative changes occuring on the surface due to
sulfonation.

It was observed that the surfaces of both polymers reach a saturation limit.
Polypropylene is saturated in almost 3 minutes and polystyrene in the ﬁrst minute itself.
The polypropylene data indicates the formation of several chemical species which mainly
includes sulfonic acid as well as a conjugated system of doubly bonded carbons in the
backbone. On the other hand polystyrene mainly results in sulfonic acids at the para
position of the aromatic ring along with some unsaturation in the aliphatic carbons in the

backbone.

Dedicated to my family who put me on the right path and
guided me through all this inspite of being half a world

away

iii

ACKNOWLEDGEMENTS

I would like to thank Dr. L.T. Drzal for the guidance and motivation that he
provided me during the course of this research. The advice and infra-structure provided
by Mr. MJ. Rich and Mr. Brian Rook in the laboratory during the course of
experimentation helped me immensely in my work. I would take this opportunity to
thank Mr. Dan Hook who imparted me with the knowledge on instrumentation and
analytical techniques which went a long way in completing the objectives of the presented
work. Mr. Brian Erickson who was actively involved with me in the project was very
helpful and encouraging and I thank him for all the intellectual and moral support besides
other things. A work of this nature cannot be accomplished without active
encouragement from well-wishers. In this respect I appreciate the help offered by Sanjay

Padaki, Rik ter Veen and Subramanyam Iyer.

iv

List of Tables

TABLE OF CONTENTS

....... viii

List of Figures ......... x
Chapter 1 . Introduction. ......... 1
1.1 NatureofForees ......1
1.1.1 Van—der Waals Forces ......... 1

1.1.2 Polar Forces ......... 4

1.1.3 Electrostatic Forces ......... 4

1.2 Mechanism of Adhesion ......... 5
1.2.1 Chemical Reaction Theory ' ......... 6

1.2.2 Mechanical Theory ......... 6

1.2.3 Electrostatic Theory . ......... 7

1.2.4 Diffusion Theory ......... 7

1.3 Work on Polymer Surfaces ......... 8
Chapter 2. Surface Modiﬁcation Techniques. ........ 11
2.1 Available Techniques ........ 11
2.1.1 Flame Treatment ........ 13

2.1.2 Acid-etching Treatment ........ 13

2.1.3 Corona and Plasma Treatment ........ 14

2.1.4 Chemical Modiﬁcation Techniques ........ 16

2.2 Sulfonation - ....... . 17
2.2.1 Sulfonating agents ........ 18

2.3 Sulfurtrioxide Molecule ........ 21
2.3.1 Solid Sulfurtrioxide ........ 21

2.3.1 Liquid Sulfurtrioxide ........ 23

2.3.2 Gaseous Sulfurtrioxide ........ 23

2.4 Oleum or FW* ........ 24
Chapter 3. Sulfurtrioxide Generator. ........ 25
3.1 Principle of Generation ........ 25
3.1.1 The Reactor ........ 25

3.1.2 Vapor Filter ........ 27

3.1.3 Surge Tank ........ 27

3.1.4 Storage Tank ........ 28

3.1.5 Piping and Valves
3.1.6 Pumps
3.1.7 Controls

3 .2 Operation of the Reactor . . .
3.2.1 Oleum Circulation Patterns . . .
3.2.2 803 Circulation Patterns . . .
3.3 Sulfonating Chamber . . .

Chapter 4. Measurement and Control of Sulfurtrioxide

Concentration. . . .

4.1 External Parameters Affecting S03 Concentration . . .
4.2 Techniques used to measure 803 Concentration . . .

4.2.1 Volumetric Gas Sampling . . .
4.3 The pH Model . . .
4.4 Relation between pH and VGS Models . . .

Chapter 5. Experimental Methods and Protocols.

5.1 X-Ray Photoelectron Spectroscopic Analysis . . .
5.2 Fourier-Transform Infra-red Analysis . . .
5.3 Contact Angle Analysis . . .
5.4 Materials . . .
5.5 Reaction Conditions and Scheme . . .

Chapter 6. Sulfonation of Polypropylene. . . .

6.1 Nature of Polypropylene . . .
6.2 FTIR-Spectroscopy Observations . . .
6.3 FTIR Results . . .
6.3.1 Band Region 1170 cm’1 and 1450 cm‘1 . . .

6.3.2 Band Region 1600—1750 cm‘1 . . .

6.3.3 Band Region 3000-3500 cm‘1 . . .

6.4 Discussion of Results . . .
6.4.1 Proposed Reaction Scheme . . .
6.4.1a Formation of C,D-Sultones . . .

6.4. lb Formation of Alkene Sulfonic acids

6.4.1c Formation of Hydroxy Sulfonic acids . . .

vi

..... 28
..... 29
..... 29

..... 33
..... 33
..... 33
..... 34

..... 38

..... 38
..... 39
..... 39
..... 41
..... 42

..... 46

..... 46
..... 48
..... 51
..... 51
..... 53

..... 70
....72
..... ‘73

6.5

6.6
6.7
6.8
6.9

6.4. 1d Reaction of B-Sultone with SO3 ........

73

6.4.1e Formation Of Ketones
XPS Observations and Results
6.5.1 Quantitative analysis of XPS Results
6.5.2 Qualitative analysis of XPS Results
Contact Angle Results
Degradation of Polypropylene
Discussion
Conclusions

Chapter 7. Sulfonation of Polystyrene.

7.5
7.6
7.7

Nature of Polystyrene

FTIR-Specroscopy Results

Discussion of Results

7.3.1 Proposed Reaction Scheme

XPS Observation and Results

7.4.1 Quantitative Analysis of XPS Results
7.4.2 Qualitative Analysis of XPS Results
Contact Angle Results

Discussion

Conclusions

Chapter 8. Conclusions.

Bibliography.

vii

.75
.76
.76
.81

Table 2. 1

Table 2.2

Table 6.1

Table 6.2

Table 6.3

Table 6.4

Table 6.5

Table 6.6

Table 6.7

LIST OF TABLES

A comparison of sulfuric acid versus sulfurtrioxide as

sulfonating agents [10]. ........ 20

Comparison of the pr0perties of various polymeric

forms of S03 [11]. ........ 22

The table shows the wavenumbers and the probable

chemical species responsible for their presence. ........ 57

The table shows the data for atomic concentration
for polypropylene sulfonated for 1 thru 5 minutes as

obtained from XPSA. ........ 77

The data presented above shows the respective atomic
ratios using the data from table 6.2. Notice the saturation

levels reached at 4 minutes. ........ 79

The table shows the percentage contribution of the
different peak regions after deconvolution of the
Cls peak. Notice the contribution from the peak

in 284.3-284.7 region decreases as treatment time increases.

The table illustrates the peaks resulting due to
deconvolution of oxygen peak. The contribution

‘ of the 531.8-532.5 eV region is maximum. This is
also the region where most of the O=S signal falls. ......

The table shows the contact angle obtained on
sulfonated polypropylene using water as a probe liquid.
Note that at 1 minute treatment the contact angle

does not change signiﬁcantly. .......

The data shows the atomic concentration comparison of
ﬂame treated, acid etched, 1 minute sulfonated and
untreated popypropylene. Note that the major impact

viii

. 84

".86

.88

Table 7. 1

Table 7.2

Table 7.3

Table 7.4

ﬂame treated and acid etching has been oxidation of the surface. 93

The table illustrates the relevant peak positions and the

probable chemical species associated with them. ........ 98

The table shows the data for atomic concentration
for polystyrene sulfonated for 1 thru 5 minutes as
obtained from XPS. ....... 109

The data presented in the table shows the respective

atomic ratios using the data from Table 7.2. Notice the

constant saturation levels reached in the ﬁrst minute

of treatment itself. The ratios stay constant throughout. . . . . 111

The table shows the contact angles obtained on
sulfoanted polystyrene using water as a probe liquid.
Note the saturation achieved in the ﬁrst minute of treatment itselﬂl3

Figure 1.1

Figure 2.1

Figure 2.2

Figure 2.3
Figure 3.1a

Figure 3.1b

Figure 3.2

Figure 3.3

Figure 4.1

Figure 4.2

Figure 4.3

LIST OF FIGURES

The ﬁgure illustrates a liquid drop resting on a

solid surface. 0 is the contact angle. ......... 8
Figure 2.1(A) illustrates the structure of oleum while

2.1(B) illustrates the structure of sulfuric acid.

Oleum is sulfuric acid saturated with sulfm'trioxide. ....... 20
Polymeric form of solid SO3 . ........ 22
Possible canonical forms of SO3 molecule. ........ 23
Flow—sheet of the gas-phase SO3 generator.

(Contd. ﬁgure 3.1b) ........ 31
Schematic showing the SO;4 ﬂow pattern in the

sulfonating chamber. ( Contd. from 3.1a). ........ 32
A schematic ﬁgure of the sulfonating chamber

showing positions of the manifolds. ........ 34
Schematic of the sampling port for S03 gas. ........ 36
The ﬁgure illustrates the variation of

vapor pressure of SO, and oleum with respect

to temperature [11]. ........ 43
The ﬁgure illustrates the experimentally determined

vapor pressure and the actual vapor pressure of 803

in the reactor [25]. ........ 44
The calibration curve showing the relationship

between pH and percentage SO3 obtained from V68 [25]. . 45

Figure 5.1
Figure 5.2

Figure 6.1
Figure 6.2a

Figure 6.21)

Figure 6.2::

Figure 6.2d

Figure 6.2e

Figure 6.2f

Figure 6.3a

Figure 6.3b

Figure 6.3c

Figure 6.3d

Figure 6.3e

Figure 6.4

. polypropylene and unsulfonated polypropylene.

The ﬁgure illustrates the repeating unit for polypropylene. . 52

The ﬁgure illustrates the repeating unit for polystyrene. polymer. 52
The ﬁgure illustrates the repeating unit of polypropylene. . 55

The ﬁgure illustrates the IR spectrum of unsulfonated
polypropylene. ........ 59

The ﬁgure illustrates the IR spectrum of polypropylene
sulfonated for 1 minute followed by neutralisation
with ammonium hydroxide. ........ 59
The ﬁgure illustrates the IR spectrum of polypropylene

sulfonated for 2 minute followed by neutralisation
with ammonium hydroxide. ........ 60
The ﬁgure illustrates the IR spectrum of polypropylene

sulfonated for 3 minute followed by neutralisation
with ammonium hydroxide. ........ 60
The ﬁgure illustrates the IR spectrum of polypropylene

sulfonated for 4 minute followed by neutralisation
with ammonium hydroxide. ........ 61
The ﬁgure illustrates the IR spectrum of polypropylene

sulfonated for 5 minute followed by neutralisation
with ammonium hydroxide. ........ 61
The difference spectra between 1 minute sulfonated

........ 62

The difference spectra between 2 minute sulfonated
polypropylene and unsulfonated polypropylene. ........ 62
The difference spectra between 3 minute sulfonated
polypropylene and unsulfonated polypropylene. ........ 63 .
The difference spectra between 4 minute sulfonated
polypropylene and unsulfonated polypropylene.

The difference spectra between 5 minute sulfonated
polypropylene and unsulfonated polypropylene.

The ﬁgure illustrates the formation of conjugated

system of double bonds as a result of sulfonation. ......... 69

xi

Figure 6.5

Figure 6.6

Figure 6.7

Figure 6.8

Figure 6.9 .

Figure 6.10

Figure 6.11

Figure 6.12

Figure 6.13

Figure 6.14

Figure 6.15

Figure 6.16

The ﬁgure shows the formation of secondary alcohol
species in the ﬁrst minute of treatment. The incorporation
of the sulfonic acid starts after the second minute of sulfonation. 70

The ﬁgure illustrates the rearrangement of B-sultones
to C,D—sultone. ........ 71

The ﬁgure illustrates the rearrangement of B-sultone
to alkene sulfonic acids followed by neutralisation
with ammonium hydroxide. ........ 72

The ﬁgure shows the rearrangement of B-sultone
to hydroxysulfonic acid. ........ 73

The ﬁgure shows the reaction of SO, with B-sultone

to make pyro sultone. This can rearrange to give

several products. Rearrangement to hydroxy sulfonic acid -
has been shown above. ' ........ 74

The ﬁgure shows the possible route of formation

of ketone species. The formation of ketone was

explained by Ihata et.al. on the basis of degradation

due to VIS light. ........ 75

The plot shows the contribution of different
elements as sulfonation progresses. The contribution
of carbon decreases as sulfonation time increases. ........ 78

The ratios achieve a saturation value at almost 3 minutes
treatment. ........ 81

The ﬁgure shows the contribution of different peaks
in the Cls envelope. The contribution of C-C linkage
decreases with increasing time. ........ 85

The ﬁgure illustrates the contribution of the different
deconvoluted peaks in the DIS peak. ........ 87

The ﬁgure shows the change in contact angle of water
on polypropylene with increasing time of sulfonation.
Note the saturation reached on the surface at about 2 minutes. . 89

The ﬁgure illustrates the IR spectra of degraded

polypropylene. The spectra was collected by preparing
a KBr disk with degraded material scraped off the surface

xii

Figure 7.1
Figure 7.2a

Figure 7.2b
Figure 7.2e

Figure 7.2d

Figure 7.2e

Figure 7.2f

Figure 7.33
Figure 7.3b
Figure 7.3e
Figure 7.3d
Figure 7.3e

Figure 7.4

of sulfonated polypropylene.

........ 91

The ﬁgure illustrates the repeating unit for polystyrene. ..... 96

The ﬁgure illustrates the IR spectrum of unsulfonated polystyrendDO

The ﬁgure illustrates the IR spectrum of polystyrene
sulfonated for 1 minute followed by neutralisation
with ammonium hydroxide.

The ﬁgure illustrates the IR spectrum of polystyrene
sulfonated for 2 minute followed by neutralisation
with ammonium hydroxide.

The ﬁgure illustrates the IR spectrum of polystyrene
sulfonated for 3 minute followed by neutralisation
with ammonium hydroxide.

The ﬁgure illustrates the IR spectrum of polystyrene
sulfonated for 4 minute followed by neutralisation
with ammonium hydroxide.

The ﬁgure illustrates the IR spectrum of polystyrene
sulfonated for 5 minute followed by neutralisation
with ammonium hydroxide.

The difference spectra between 1 minute sulfonated
polystyrene and unsulfonated polystyrene.

The difference spectra between 2 minute sulfonated
polystyrene and unsulfonated polystyrene.

The difference spectra between 3 minute sulfonated

polystyrene and unsulfonated polystyrene.

The difference spectra between 4 minute sulfonated
polystyrene and unsulfonated polystyrene.

The difference spectra between 5 minute sulfonated
polystyrene and unsulfonated polystyrene.

0000000

The ﬁgure illustrates the formation of aromatic sulfonic

acids on reaction with S03.

xiii

The para position is the
most probable site of attack due to ease of steric reasons.

. 107

Figure 7.5

Figure 7.6

Figure 7.7

The ﬁgure illustrates the formation of alkene species
during sulfonation of polystyrene. ....... 108

The ﬁgure illustrates the atomic concentrations of various

elements present in polystyrene as a result of sulfonation.
Notice the constancy achieved in all the samples. ....... 110

The plot illustrates the variation of contact angle of water
on sulfonated polystyrene as time of sulfonation is varied. . . . 114

xiv

Introduction

 

Chapter 1

 

Adhesion is deﬁned as the state in which two dissimilar surfaces are held together
by interfacial forces. Since it is an interfacial phenomenon the forces on the surface play

a very important role in determining the nature and strength of adhesion.

1. 1 NATURE OF INTERFACIAL FORCES

The forces at the interface system may be Long Range Forces or Short Range
Forces. It has been shown that most of the inter-atomic forces are short range forces.
If it were not so then the energy of a given portion of a system would depend on its
macroscopic size as well [1]. However, there are several forces which are long range.
These forces can act across the interface and are decisive in determining the adhesive
strength. Broadly these forces may be classiﬁed as follows:

- Van-der Waals forces.

Polar forces.

Electrostatic forces.

1.1.1 Van-der Waals Forces

Van der Waals forces account for the general attractive interaction between atoms
in the neutral state [1,2]. The van der Waals interactions are a result of three different
contributions which are related to each other. A neutral molecule at any given time is
in a state of agitation which leads to randomly orienting dipole-dipole interaction. These

1

2

interactions were ﬁrst described by Keesom and are called Keesom Interactions. They
vary inversely as the sixth power of the distance between the dipoles and are described

by the following equation [1].

Zu‘
3ché

 

em»)... = -

where, p. is dipole-dipole interaction, units of Debye
T is the temperature of the system, units of Celsius
x is the separation between the entities, units of cm.

6 is the potential energy, units of ergs.

Another possible interaction in an atom is due to a randomly oriented dipole and. an
induced dipole (if the atom or molecule is polarizable). These were described by Debye
and are called Debye Interactions. They also vary inversely as the sixth power of the
distance between the dipole and induced dipole . The equation describing them is as

follows [1]

where, u is the dipole-moment in units of Debye.
a is the polarizability in units of cm3.

x is the separation in units of cm.

3
As has been pointed out, for Keesom and Debye Interactions, it is important that the

molecules have permanent dipole moments. All the forces account for attractive (or
repulsive) interactions in molecules which can be polarized. However, it is a universal
observation that there is a general attractive interaction which exists in atoms and
molecules which are not inherently charged and do not have a dipole moment associated
with them. London, in 1930 showed the existence of a more general kind of electrical
force beMeen molecules which is always attractive and arises due to the fact that (even)
neutral atoms and molecules constitute a system of oscillating charges. The positive
necleus and the negative electron cloud result in a ﬂuctuating dipole which leads to
ﬂuctuating dipole-induced dipole interactions. These are called London-van der Waals

or Dispersion Interactions and are described by the following equation [1]

“1“2
6 [(lth1)+(1/hV2)]

 

e(x) = -% x

where, hr, and hv2 are the characteristic energies of atoms 1 and 2.

a, and or; are polarizability of atoms 1 and 2 in cm3

Like the Debye and Keesom Interactions these also vary inversely as the sixth
power of the distance between the molecules. To sum up, the interactions are
characterised by the general equation as follows.

Debye, Keesom, Dispersion Interactions a a“

Where d is the distance between the molecules.

1.1.2 Polar Forces

Another class of forces which contributes to adhesion is Polar Interactions [1,2].
These are result of an actual elecnon(ic) reconﬁguration of the species involved in
adhesion. Interactions of the Hydrogen-bonding type are also included in this class of
forces. To encompass all interactions which are of electron acceptor-electron donor type,
the term Polar Interactions along with other acceptor-donor interactions are called Lewis
Acid-Base Interactions. These are speciﬁc in nature vis-a-vis the chemical species under
consideration and are essentially asymmetrical. The interactions of this class are short-
range in comparison to the Dispersive forces which have been discussed previously
which are a result of Long-Range Interactions.
1.1.3 Electrostatic Forces

Electrostatic Interactions play an important role in Polar media where it can be
said with conﬁdence that there are very few entities which do not carry a surface charge
of electrical origin [2]. A charged surface experiences a force in an electric ﬁeld similar
to the situation when a ﬁeld is induced by the relative motion of such a surface. In both
cases there is a slip between the layer and the medium [1] the result of which may be
measured in terms. of the charge density. The extent of Electrostatic Interactions is
indicated by the Electra-Kinetic Potential or {—Potential. It is the potential of entities
measured at the slipping plane by electro-kinetic methods. The {-Potential is not directly
related to the non-polar component of the surface energy. Also, it should be kept in
mind that {-Potential is developed entirely within the ﬂuid region. Electrophoresis and
electro-osmosis are among common applications of Electrostatic Interactions. The

mathematical expression for {-Potential or Helmholtz potential is as follows [18].

C: 411:an
e

r

where, 11 is the ﬂuid viscosity
5, is the dielectric constant of the ﬂuid

in is the electrophoretic mobility of the particle.

The nature of Electrostatic Interactions is different from Dispersive Interactions.
Pauling has classiﬁed Electrostatic interactions as a broad class of bonding. They include
ion-ion, ion—dipole, ion-induced dipole etc kind of interactions [44]. In this class of
interactions, an actual transfer of electrons within the atomic species involved. For e.g. ,
in NaCl (sodium chloride or common salt) the metallic sodium ion actually donates the
electron to the non-metallic chlorine atom. The main point to note in electrostatic
interactions is that the interaction of an atom of a deﬁnite electronic structure is
independent of the proximity of other atoms around it. Usually, there is an appreciable
difference in the electronegativities of the atomic species involved in this kind of

interaction.

1.2 MECHANISM OF ADHESION
As has already been pointed out earlier, adhesion is the resistance to separation
of a joint by application of external force. The magnitude of adhesion is a function of

several factors like the interfacial forces manisfested as the surface energy, work of

6
adhesion, bulk and surface layer mechanical properties, joint geometry etc. [3,4].

Several theories have been proposed to explain the mechanism of adhesion [3]. They can
broadly be classiﬁed as follows.

- Chemical Reaction Theory

- Mechanical (Hooking) Theory

- Electrostatic Theory

- Diffusion Theory
All the forces outlined above play an important role in determining the levels of
adhesion.
1. 2.1 Chemical Reaction Iheozy

The Chemical Reaction Theory proposes that the strength of joints is determined
mame by interfacial reactions. Whenever there is an actual chemical reaction across the
interface the probability of forming a bond of high strength is large. The presence of
polar groups at or across the interface contributes to the bond strength as it can lead to
an actual electronic exchange. However, if only the van-der Waals forces act across the
interface a joint of a lower strength is produced because electron exchange is not
possible. The main criticism of this theory is that it relies entirely on interfacial
reactions to which there is little direct and evidence.
1.2.2 Mechanical Theory

The Mechanical (Hooking) Theory emphasizes the Interlocking of adhesive and
the adherend at the interface. However, roughness cannot be the sole criterion for
adhesion as there are systems which involve adhesion between very smooth surfaces.

But, it may be an important mechanism in adhesion of porous materials.

1.2.3 Electrostatic Theory

The Electrostatic Theory is based on the premise that the adhesive joints actlike
a capacitor which becomes charged due to the contact of two different substrates. The
electrical double layer of the capacitor imparts strength to the joint. The main evidence
of this phenomenon is in case of pressure-sensitive adhesives where electron emission can
occur when a joint is broken. However, the main argument against this explanation is
that the phenomenon of fracture is used to explain the phenomenon of joining. In most
cases the nature of the original uncharged surfaces is nOt necessarily the same as that of
charged fractured surfaces. The theory also does not explain the adhesion of conductive
materials.

1.2.4 Diﬁiuion Theory

The Diffusion Theory is especially applicable to polymers. It postulates that
adhesion is not a result of surface contact alone. There is (inter)-diffusion of polymers
across the interface which imparts joint strength to the system. However, this cannot
explain adhesion in the broadest sense as in most non-polymeric solids where the
probability of diffusion will be very low at the temperatures and time frame under
consideration.

None of the mechanisms outlined above offers a complete picture of adhesion f
independently. For a holistic understanding it is important that each possible mechanism
evaluated and then understood. However, one fact that clearly emerges is that forces at
the surface play a crucial role in adhesion. If we can control or at least modify these
surface forces then we can tailor a surface to our adhesion requirements. Several surface

modiﬁcation techniques are available towards this end.

8
1.3 WORK ON POLYMER SURFACES

Work on polymer surfaces assumes an important academic and industrial
application because of the increasing application of polymers. The most common
parameter used to deﬁne this work is the Thennodynamic or Reversible Work of Adhesion
[46]. It is most commonly denoted by W A and is deﬁned as the change in free energy
when the materials are brought in contact and is same under equilibrium conditions as
the energy change when material interface is disrupted. However, the situation is far.
from a real situation where inelastic deformations occur during breaking of a joint
resulting in loss of energy. Thus, at best one can get a direct correlation between the
thermodynamic work of adhesion and the practical work of adhesion [46]. One of the
earliest equation proposed for work of adhesion is due to Young. The equation is a force

balance on a liquid drop placed on a solid surface as shown in Figure 1.1.

Ylv

Liquid phase Vapor phase
Yd Ylv
Solid phase

0

 

>

 

Figure 1.1: The ﬁgure illustrates a liquid drop resting on a solid surface. 0 is the

contact angle.

The equation is as follows.

WA'= Yrs/(.1 + c056)

There have been attemps for a long time to quantify and separate the various components
of work of adhesion based on the forces responsible for them. Approximately, there are
two components, the dispersive and non-dispersive (or polar) component. The work
related to this area has mainly been empirical in nature. The following equation was

widely used by Hansen and Kaelble in their work.

WA Wad+WAP

2(t,,“'t,“')"2 + 2037!)":

There was agreement on the dispersive cemponent of the work of adhesion but
there were various schools of thought which did not agree entirely with the non-
dispersive component termed polar in the above equation. One of the latest theory
prevalent is due to Fowkes [47]. He proposed that besides the dispersive component all
the other interactions are of acid-base types. That is, there is an interaction which

actually involves the electrons in the system. The equation was modiﬁed as follows.

10

E

A WAd + WAA-B
WA" + ﬂ-AHAQN

where,

f is factor which converts enthalpy to free-energy.

N is the number of accessible functional sites.

AH“ is the exothermic enthalpy per mole of acid-base adduct formation

at the interface.

However, there is controversy on this equation too regarding the interpretation and the
various assumptions which are valid while using this equation. For e. g. , Fowkes used
the value of f as unity in all his work while several investigators do not agree on this
assumption [47]. Thus, at present the work of adhesion on polymer surfaces is still not
well understood. More work at the fundamental level needs to be done before the nature
of polymer surfaces can be well understood and yield a better understanding about the

nature of work on polymer surface.

Surface Modiﬁcation Techniques

 

Chapter 2

 

2. 1 AVAILABLE TECHNIQUES

There are several commercial and laboratory-scale techniques available for
altering the surface in order to enhance adhesion capabilities. However, it must be
remembered that none of the techniques can be applied universally. The detailed
discussion hme shall be limited to polymer surfaces . Each has its own limitations and
thus the choice of the surface modiﬁcation technique is of utmost importance. Some of

the currently available techniques available currently are :

Flame treatment

Acid-etching

Corona discharge and Plasma treatment

Chemical treatment

Before each of them is discussed individually, it is important to familiarise ourselves
with two interrelated concepts which play an important role in Adhesion Science. They
are the concept of Weak Boundary Layer and The Interphase [4].

In the most general sense an adhesive joint is referred to as a three layered
structure. Two adherend layers enclosing a layer of an adhesive. However, it may be
that the adherends and the adhesives individually consist of more than one layer (each)

[5]. This complicates the problem several fold, as now the interface between layers will

11

12

play a role in determining the adhesion strength. The extent of adhesion also depends on
the mechanism of stress transfer. This is governed by the stress concentration proﬁle
between any two adjoining layers. For maximum adhesion the stress transfer should be
as smooth as possible. However, if we have multiple interfaces then there will be
multiple stress concentration proﬁles which implies that the problem is complicated
extensively. This surface region whose characteristics are different from the bulk is
termed as The Interphase.

It was proposed by Bikerman that even though “the failure may appear to occur
interfacial, in reality the failure always occurs cohesively in a weak layer on one of the
species involved in adhesion [23]. This weak layer where the bond fails is the Weak
Boundary Layer. He further proposed that WBLs are associated with each kind of
material. Bikerman’s proposition was based on empirical observations. He said that a
crack can propogate either between two molecules of the adhesive or two molecules of
the substrate or] between the substrate molecule and adhesive molecule. Thus, the
probability of the crack propagating along the interface is 1/ 3 I The above reasoning
shows that based on purely statistical grounds it is improbable that the crack propogates
along the interface only.

Weak‘Boundary Layers are associated with all classes of materials. On metals
they are usually oxides while in polymeric materials they are usually low molecular
weight chains which tend to migrate on the surface. The problem in polymers is
complicated because of the impact of functional, structural and morphological factors [4] .
The accepted view is that the mechanical properties of WBLs are poor in comparison to

those of the bulk. Thus, it becomes important that for the purposes of adhesion all

13

WBLs be removed and work is carried out on a surface whose properties are as close
to that of the bulk as possible. This also leads to simpliﬁcation of the stress
concentration proﬁle. Some of the surface treatments affect the polymer surface to that
end.
2.1.1 Flame Treatment

Flame treatment is a common surface modiﬁcation technique which ﬁnds several
applications in the industry. It consists of subjecting the polymer to an appropriate.
ﬂame. The purpose is essentially to oxidize the top few layers of the polymer surface.
The chemical effects of ﬂame treatment have been studied by several investigators in
detail [44,45]. It has been conclusively shown that a major impact of ﬂame treatment
is the enhancement of surface energy and polarity of the surface. The details and
mechanism about the chemical changes are not very clear at this stage. The only
conclusive fact which emerges from the literature is that oxidation of the surface is an
important change. Various investigators have correlated the polarity to the differently -
oxidized chemical species on the surface. For e.g. , Garbassi et.al. [44] have attributed
the increase in polarity to the carbonyl functionality introduced on the surface. The
reaction is supposed to proceed via a radical mechanism [44,45] which react and
rearrange to form different chemical species. Briggs et. al have shown that the depth of
modiﬁcation is in the range 40 A < d < 90 A. The overall impact of ﬂame treatment
is the improvement in adhesion properties of the polymer.
2.1.2 Acid-etching Treatment

Acid-etching is another method directed at removing the WBLs and in the process

also modifying the chemical structure on the surface [6, 21]. A controlled mixture of

l4
sulfuric acid and chromic acid is often used to etch the polymer surface. It has been

found that in the process of etching there is an actual chemical modiﬁcation of the
polymer surfaces as well. Mostly, it is characterised by the incorporation of sulfonic
acid groups on. the surface of the polymer attached to the carbon of the polymer chain.
This leads to an enhanced polarity on the surface and thus increased adhesion. The main
limitation of this technique is that it may lead to the destruction of surface of the
polymer. The harsh environment of the acid treatment leads to chain scission and a
modiﬁed topography. Investigation by Blais et al [21] shows that after treatment there
is a marked surface roughness that appears on the surface. This can also be responsible
for improved adhesion. The various sites on the polymer can react with the acid resulting
in various chemical moieties. If these chemical moieties are stable in the given chemical,
morphological and physical environment then a chain with a modiﬁed region is obtained.
However, there are chances that the new chemical moities are unstable energetically
which is the motivation for them to leave the system and thus they break away. These
are the scission products which are mainly aldehydes and ketones [21]. This is also the
reason that controlled modiﬁcation is sometimes not possible in acid-etching.
2.1.3 Corona and Plasma Treatment

Corona discharge and plasma treatments lead to cleaning and chemical
modiﬁcation of the surface which leads to an enhanced adhesion [7]. The "Plasma" is
deﬁmd as an ionized gas with an equal density of positive and negative charges. The
highly energetic chemical environment produced by plasmas is utilized in chemical
modiﬁcations of the materials. Free radical chemistry is the proposed mechanism of

reaction. Plasmas may be generated from various gases depending on the speciﬁc

15

requirement. Oxygen, Ammonia, Argon are some of the popular plasmas. However, due
to the complex nature of the plasmas generated, at times it can be difﬁcult to control
chemistry on the surface. The main effects of plasma and corona treatment are [7]:
2.1.3a Cleaning

This is a major effect of Plasma on the surface. Most of the other processes leave
behind a layer of contamination of organic nature. The main advantage of using Plasma
is its capability to remove molecular layers from polymer surfaces. Since this is the
actual layer of the polymer the complications arising due to Weak Boundary Layers are
removed. Thus, this gives a reproducible surface and reproducible bonds.

The problem with using other cleaning techniques such as solvent wash is the non-
volatile contents in them. It is almost impossible to obtain solvents without any non-
volatile content in them. For purposes of adhesion it is well-known that even the
slightest contamination can be detrimental.
2.1.3b Ablation

This is important for cleaning and removing contaminants and Weak Boundary
Layers from the surface. These become a part of the surface during fabrication and
handling procedures. Ablation is especially important because it generates a surface
topography which can enhance adhesion due to mechanical bonding and increase the
surface area for chemical interactions.
2.1.3c Cross-linking

Cross-linking is the observed phenomenon in Noble gas plasma when they are
used in ultra-clean environment. the free radicals are created when the excited atoms

attack the polymer surface resulting in the breakage of C-C, C-H bonds. These radical

16

species combine with each other and form stable moieties. Due to the radical migration
on the chain crosslinking, branching etc can occur. Crosslinking is known to give
improved heat resistance and cohesive strength [7].

2. 1 . 3d Surface Chemisry Modiﬁcation

The chemical nature of the surface is modiﬁed such that its capability to interact
with the adhesive is enhanced. This is achieved by using an appropriate plasma which
can add polar groups on the surface. For example, it would be expeced that an Oxygen
plasma would give a surface which has extensive oxygen functionalities. The new
chemical moieties formed are more polar in nature which leads to improved adhesion.
Due to these polar groups the polymer surface has an increased surface energy which
leads to improved wetting and thus adhesion.

Studies have shown that the plasma treatment effects only the top 20 - 200 A of
the surface. This makes it important that the surface is clean before treatment which in
turn demands that the treatment be long enough for removal of all kinds of Weak
Boundary layers. A
2.1.4 Chemical Modiﬁcation Techniques

Fluorination and Chlorination are two examples of chemical modiﬁcation
techniques. However, both of these have their limitations. Fluorination involves the
attachement of a ﬂuorine atom to the Carbon backbone of the polymer. Since ﬂuorine
is a highly electronegative element, this leads to bonds which have very high energies.
This in turn leads to bond breakages and thus a loss in mechanical and barrier
properties over a period of time. The ﬁnal product in ﬂuorination of organic polymers

is formation of a ﬂuorinated carboxylated product ( -FC=O ) [22,23] which are formed

17

after exposure to oxygen. The process of Fluorination is hazardous as one of the by-
products is Hydroﬂuoric acid which is highly corrosive. It involves exposing the
polymer to a mixture of ﬂuorine gas (0. 1-20%) along with a carrier gas such as argon
or nitrogen. Oxygen level has to be minimised as it leads to the undesirable carboxylated
by-product. This is one of the main reason the the process has proven to be difﬁcult to
control [8]. On the other hand, chlorination is a slow reaction which can be promoted

by Ultra-Violet light (UV). This makes it a difﬁcult process to use at industrial level.

2.2 SULFONATION

One common factor that emerges from above discussion is that a chemical
technique will be industrially attractive if it can offer a repeatable controlled reaction.
Sulfonation as a surface modiﬁcation technique has this potential. Moreover, unlike most
other techniques it has an added advantage that it offers control over two adhesion
parameters. One is the concentration of the chemical species and secondly the depth of
modiﬁcation achieved. These two variables in conjunction with each other offer a
powerful tool to tailor the surface in a controlled manner. Sulfonation is also an
attractive option for improving the barrier properties, electrical (surface) properties and '
metallization‘ of polymer surfaces [8]. The barrier properties involve the reduction of
permeability of polymers to liquids and gases. A detailed investigation can be obtained
from reference [9]. ‘ For example, sulfonation has been successfully tested in fuel tanks
where diffusion of gasoline out of tanks has been a major problem. It leads to a loss in
shape due to swelling and ultimately the life of the tanks is reduced. For barrier against

gases also sulfonation has been employed successfully.

1 8
2.2.1 Sulfonating Agents

There are several sulfonating agents in use. The most popular ones are the
sulfurtrioxide complexes of organic compounds. The generic reason for the formation
of comlex is that sulfur atom in S03 is a strong electron acceptor (Lewis acid) which has
a strong tendency to combine with Lewis bases to form complexes [10]. The stability
of the complex depends proportionally on the strength of the base used. At the time of
sulfonation the complex dissociates and free 80; is released for sulfonation. The base -
forms a salt with the acid according to the following reaction.

RH + SO3 -------- .> RSO3H.Base
Some of the common sulfonating agents which have been used for studying the chemistry
of sulfonation have been brieﬂy described below.
(2.2.1a) S03-pyridine complex.

This can be prepared in several ways. The most common being the direct
reaction of SO3 with pyridine. The complex is stable and insoluble in cold water and
cold alkali but decomposes when the temperature increases [10].

(2. 2. 1b) S03 -dioxane complex.

This is one of the earliest sulfonating agents to have been prepared and used to
study the detailed chemistry of sulfonation [14]. There are several factors involved in
synthesising this reagent [10] and the preparation should be a careful process. Care has
to be taken that the adduct be prepared just before use as on long standing the solid can
decompose violently even at room temperature. It has been employed largely for

sulfonating alkenes and alcohols.

19
(2.2.1c) S0, triethyl phosphate. ,

The best way to prepare this complex is to add freshly distilled SO, to the
phosphate at 15 °C. It is the electron-rich phosphoryl oxygen which reacts with ‘ SO,
exothermically to make a 1:1 adduct. Other adducts are also possible depending on the
extent of reaction [10]. Depending on the substrate to be sulfonated the ratio of SO, to
phosphate has to be controlled.

(2. 2. 1d) Halosulfonic Acids.

The most common acid in this class is chlorosulfonic acid. This is a strong acid
(ClSO,H). In effect it is an HCl complex of SO, and more suitable represented as
SO,.HC1. It is prepared by direct reaction of the two compounds [10]. At the time of
reaction S0, is freed as a chemical moiety and HCI is liberated as a gas. This is a
popular reagent for sulfating alcohols.

(2.2. 1e) Sulfuric Acid and Oleum

H2S04.xH,O is the most popular sulfonating agents in practise. A comparison
with respect to some important factors has been shown in Table 2.1 [10]. Although the
commercial availability of both of these is great, the main shortcoming is the hazard

which can be produced by them.

20

ii I ‘3“
Ho—s—o—s—OH HO-IS|=O

o

(A) (B)

Figure 2.1: Figure 2.1(A) illustrates the structure of Oleum while 2.1(B) illustrates
the structure of Sulfuric acid. Oleum is Sulfuric acid saturated with Sulfurtrioxide.

Oleum and Sulfuric acid are extremely hygroscopic and react hygroscopically with
water. Thus, any contact with water must be avoided. Due to their oxidising nature
they should not be contacted with combustible materials.

Table 2.1: A comparison of Sulfuric acid versus Sulfurtrioxide as Sulfonating agents

 

 

 

 

 

 

 

[10].
Factor Compared Sulfuric Acid SulfurTrioxide
Reaction Rate Slow ~ Instantaneous
Heat Input Requires heat for Strongly exothermic '
completion , reaction
Extent of reaction Partial Complete
Side reactions Minor Sometimes extensive
Viscosity of Reaction Low Sometimes High
Mixture
Boiling Point 290-317 °C 42-44 °C
Solubility in halogenated Very low Miscible
solvents

 

 

 

 

 

As is evident from the table above almost all the factors except heat

21

side reactions and viscosity of reaction mixture make sulfurtrioxide a better and a more
viable choice as the sulfonating agent. Moreover, if the reaction is carried out in a

controlled manner then the abovesaid shortcomings can be overcome.

2.3 SULFURTRIOXIDE MOLECULE

The sulfurtrioxide molecule has a very complicated chemistry [10]. The
complication arises from the fact that Sulfur is a strongly electron-deﬁcient atom while
oxygen is an electron-rich center. Thus, the molecule has a Lewis acid site as well as
a Lewis base site. This I imparts it an Amphoteric Character. The sulfur in S0, has a
strong tendency to increase the number of electrons to eight, ten or even twelve at times
[10]. The amphoteric character is also responsible for the ease with which it can
polymerize and its behaviour as a sulfonating agent. The electron-deﬁcient sulfur atom
attacks the basic sites while the electron-rich oxygen atoms attack the acidic sites.
Sulfurtrioxide can exist in solid, liquid and gaseous forms.
2. 3. 1 Solid Sulﬁtrtrioxide

Sulfurtrioxide in the solid ' state has various polymeric forms which are alpha,

beta and gamma form. The gamma form is the most desirable and exists when SO, is
absolutely pure and anhydrous [l 1]. Alpha form is highly crystallized and difﬁcult to
work with. It has highly stable asbestos-like crystals which do not melt easily. A
comparison of properties of the various polymeric forms is given in the table which

follows [11].

22

i ii i
O—S—O—S—O—S—O
II II ll
0 O 0
Figure 2.2: Polymeric form of solid S0,

The ﬁ-form is the metastable form ( m.p. 32.5°C) and is easy to melt. On the other
hand a-form is highly stable ( 62.3°C). Solid samples do not have a well-deﬁned
melting point due to an undeﬁned mixture of a-form and B-form. Attempts to melt this
mixture without prior expertise can lead to a sudden increase in vapour pressure ( of

80,) which can be risky.

Table 2.2. Comparison of the properties of various polymeric forms of SO,[11]

 

Equilibrium Melting point 16.8 °C 32.5°C 62.3°C
Heat of fusion 7.5 kJ/mol 12.1 lemol 25.9 kJ/mol
Heat of Sublimation 49.8 kJ/mol 54.4 kJ/mol 68.2 kJ/mol
Vapor pressure
0°C 45 mm Hg 32 mm Hg 5.8 mm Hg
25°C - 433 mm Hg 344 mm Hg 73 mm Hg
50 °C 950 mm Hg 950 mm Hg 650 mm Hg

75 °C 3000 mm Hg 3000 mm Hg 3000 mm Hg

 

 

 

 

23

2. 3. 2 Liquid Sulfurtrioxide

In the liquid state sulfurtrioxide is available in the stabilised and unstabilised
forms. The stabilised form contains very low quantities of chemicals which inhibit the
formation of asbestos-like Beta (mp. 32.5°C) form and Alpha (mp. 62.3°C) crystals
which are the high melting forms. Theoretically, in the stabilised form all the liquid SO,
must be in the Gamma form. However, the presence of moisture destroys the inhibiting.
action of the stabiliser and with time straight-chain Beta 'form and the highly cross-linked
stable Alpha form is formed. It is advisable that the liquid be stored at 35-41°C. Due
to the possibility of formation of the different polymeric forms there is always a
possibility of a sudden increase in vapor pressure on heating the liquid.
2.3.3 Gaseous Sulﬁtrtrioxide

In gaseous state SO, exists as a monomeric form. Its proposed structure is
planar, triangular and symmetrical. It is a resonance hybrid in which all the Oxygen
atoms are equivalent. However, the exact distribution of electrons in a Sulfurtrioxide
molecule is uncertain[10]. The possible canonical forms of 80, are shown in Figure

2.3 [10].

:Q: ”:0
é'g 9::3
:6: :0

Figure 2.3: Possible canonical forms of 80,.

24
2.4 OLEUM OR FUMING SULFURIC

An easier and more acceptable way to handle SO, is in the form of Oleum.
Oleum is the term employed to describe fuming sulfuric acid and consists of- SO,
dissolved in 100% sulfuric acid. It is speciﬁed in terms of percent strength. Say, 20%
oleum would mean 20% SO, dissolved in 80% sulfuric acid on a weight basis. On
exposure to the atmosphere 80, tends to escape and combine with the moisture to form
an acid mist, thus it must be avoided as much as possible. This has been the choice in
the studies carried out in the presented thesis. The strength and operating conditions
have been delineated in the context. Gaseous 80, was generated from oleum with a
specially designed SO, generator, the details of which have been outlined in the following

chapter.

Sulfurtrioxide Generator

 

Chapter 3

 

In order to use Sulfonation Process for surface treatment it is imperative that the
generation of SO, is a controlled and a repeatable process. In keeping with the industrial
norms it is important that it be a safe process. This chapter delineates the details of the
Gas-phase SO, generator provided by Coalition Technologies Inc. , Midland ( Patent #

4,915,912, Figure 3.1a,b )

3.1 PRINCIPLE OF GENERATION

The source of pure gaseous SO, is the gas-phase generator provided by Coalition
Technologies Limited, Midland [13]. The operation is based on the principle that if
oleum (fuming sulfuric acid) is controllably agitated with air/ nitrogen it will release its
excess SO, . In the generator agitation is controlled and governed by forced-ﬂow of air
by means of a gas-pump which is operated when gaseous Sulfurtrioxide is required.
The generator has several units which assist in controlled SO, production. The details
of each individual component have been delineated in the following passages.
3.1.1 The Reactor

The reactor is the heart of the system. It is a Glass-vessel which has Raschig
Ring packings and appropriate ﬁtting arrangements on the cover to facilitate the ﬂow of
the reaction ﬂuid. The reaction ﬂuid is Oleum (of known concentration) which is pumped
continously in a 3/8 inch steel tubing closed loop through the reactor vessel. For

purposes of generating SO, gas, air is pumped into the reactor through a 1/2 inch steel

25

26

tubing by the gas pump. The temperature is controlled by a heating jacket which in turn
is controlled by a Series 965 Watlow Temperature Controller. The desired temperature
for the reactor can be set and controlled from the Controller.

In order to minimise the contact of active Oleum with atmospheric moisture, the
sealing arrangement on the reactor (and all other relevant components of the SO,
generator) has been elaborately designed. The mouth of the (reactor) glass—vessel is
covered by a steel-sheet with appropriate ﬁttings for tubing arrangement. The whole
assembly upper is made intact and leak free by a teﬂon ring held in place by an
overlaying rubber ring and a hose clamp arrangement. This ensures that the
concentration of oleum is not depleted due to intrusion of atmospheric moisture which
tends to convert oleum into sulfuric acid. These features are also important for safety,
handling and repeatibility during experimentation.

Since the reactor conﬁguration demands maximum liquid-gas contact in order to
enrich the gaseous phase in SO, , it has been provided with Raschig ring packings. The
advantages of using packings are as follows [12].

- The packings are unaffected by the acid environments.

-The lower pressure—drop of a packed bed reactor reduces load on the liquid

pump.

- Packings provide an increased surface area for attaining of gas phase

equilibrium.

- Residence time of the liquid in the packed bed is short. This is particularly

important in this case as the liquid is thermally sensitive.

27

The reactor is equipped with a pressure gauge which senses the pressure in the
reactor. If at anytime there is an overpressure, a safety valve on the surge tank will
release the pressure. ’

3. 1. 2 Vapor Filter

Inspire of the best efforts and arrangement to eliminate moisture leaking into the
system, it is not possible to eliminate it completely. The SO,-air mixture generated in
the' reactor has moisture content and other impurities associated with it. In order to
purify the outgoing gas which has to be used for sulfonation a vapor ﬁlter has been
provided in the generator. It consists of a glass vessel of the same dimensions as the
reactor and is ﬁlled with ﬁber glass wool. The moisture and the particulate solids from
the gas mixture are trapped by the wool and rendered clean.

3.1.3 Surge Tank

The outlet of the vapor ﬁlter is connected to a surge tank via a four-way valve
which governs the direction of the ﬂow of the gas mixture. The dimensions of the surge
tank are same as the reactor and it is equipped with a pressure relief valve and a vent
valve. In case of a minor over-pressure condition, the vent valve can be used to release
the excess pressure. However, if due to an accident the pressure rises beyond control,
the pressure - relief valve will release the pressure to the enviroment. It is set at an
overpressure limit of 3psi. The outlet of the surge tank is connected to the inlet of the

gas PumP-

28
3.1.4 Storage Tanks

The oleum is stored in three storage tanks which are provided in the generation
unit itself. This facilitates the alteration in the concentration of active oleum in the
reactor to the desired level. The stored oleum can be transferred to the reactor through
3/ 8 inch lines by a liquid pump. Also, it is possible to transfer oleum from the reactor
to the storage tanks by an appropriate arrangement of the valves. There are three storage
tanks in the unit which are of the same dimension as the reactor. A common inlet and.
an outlet line connects the liquid pump, reactor and storage tanks. Each tank is isolated
from the main lines by a one-way valve._ Since oleum can freeze under ambient
conditions it has to be kept warm. The temperature of the storage tank area is
maintained, by a forced air heated convection system.

3.1.5 Piping and Valves

The generator unit has a relatively simple piping layout. Appropriate in-line
valves 3/8 inch stainless steel tubing with compression ﬁttings valves have been placed .
according to the requirements. The valves and the tube lines are color coded for the
purposes of convenience and Chemical Engineering norms [13].

All the gas lines are 1/2 inch dia. and those handling liquids are 3/8 inch dia.
The valves and lines which handle liquid oleum or SO, are color coded RED. For gas
(mixture) the color is YELLOW. In addition valves on each storage tank are used to
vent the gas mixture from the storage tanks when they are being ﬁlled or emptied of the

liquid are coded BLACK.

29

On the surge tank there are two additional valves. There is a pressure relief valve
which is set at fail open (FO) condition and there is a green vent valve. They can be
used to release excess pressure which may develop in the system.

3.1.6 Pumps .

The SO,-generator unit has two pumps installed in it. One is a liquid pump and
the other is a gas pump. The liquid pump is an M-7 Micropump. It has beenidesigned
such that all the parts which come in contact with oleum are made up of teﬂon. This
allows a smooth operation and also signiﬁcantly reduces maintenance costs due to the
highly corrosive and reactive environment of SO, . To prevent contaminants from
entering the pump a 140 micron ﬁlter has been installed at the inlet port of the pump.
The other pump is a Metal-Bellows gas pump with a capacity of 15 liters per minute.
It has the capability of handling SO,-air mixtures. Unlike the liquid pump it is not in
continous operation and used used only when S0, is required for sulfonation.

3.1.7 Controls

There are three control panels on the SO, generator. They are the Temperature
Control, Main Control and Pump Control [13]. The Temperature Controller allows
control of the temperature of the reactor and the storage tanks area. The reactor
temperature is especially important because it directly affects the concentration of SO,
generated. As the temperature of the system is increased the amount of SO, generated
also increases. The temperature of the cabinet must be maintained above the freezing
point of the oleum stored (depending on the concentration of oleum). A j-type

thermocouple is used in both the controls for temperature sensing.

30

The main control panel has all the operation controls of the generator. It consists
'of four push button switches and a timer (with display). Three of the push buttons
control the operation of the pumps while the fourth controls the timer.

The pump control panel has a DC power supply and a relay that supplies power
to the liquid pump. The mode of operation of the liquid pump (speed and direction) is
also controlled from this panel. The pump can be operated in reverse as well as forward
direction. The speed control knob allows the pumping rate to vary according to the
needs.

A schematic ﬂow-sheet of the lay out of generator has been presented in Figures
3.1a and 3.1b in the following pages. To facilitate the understanding of the diagram the
ﬂow-sheet has been split into two ﬁgures. The main unit which generates the gas has
been shown in ﬁgure 3.1a and the unit with sulfonating chamber set-up is shown in

ﬁgure 3.1b.

31

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3—.n 8:»... .358 c.8395» 5m 83...?» 05 he 335.32% ”26 9.5»:—

       

SE .525,

39855
0:. 88m

o>_a> .33.». 62820

 

and. ome 05 oh

 

32

35 sec .380 a .855 «5.283 2.. a 8:2. 2.: .om 2: «:52... 3352.8 32” 2&3

 

 

 

588m 05 oh

 

 

 

iXT 2&6
, tom mama—8am meta—6:5

 

 

 

 

 

 

 

 

863m 05 Seen—

 

3305-55 3:... were

E I 33.6
88:2

 

 

 

 

33

3.2 OPERATION OF THE REACTOR
The main purpose of the reactor is to generate pure gaseous SO, under

controlled conditions at a ﬁxed level over a range of concentration. To achieve this it
is important that in addition to the external conditions like temperature etc. , the nature
of the oleum producing S0, is always same.
3. 2. 1 Oleum Circulation Patterns

To maintain consistency of the gaseous mixture being generated the concentration
and activity of the oleum source must be kept constant. To do this it is circulated
continously. In the generator this is done by pumping the oleum (in a closed loop through
the reactor) continously through the liquid pump via the 140 micron ﬁlter installed at
the inlet of the pump. When SO, is required, the gas pump can be switched on to force
air in the reactor which absorbs the excess SO, from the ﬁrming sulfuric.
3. 2.2 SO, Gas Circulation Patterns

As has been discussed above, in order to generate SO, , air in the lines is pumped
into the reactor containing active oleum. The pumping operation results in release of 80,
from fuming sulfuric acid. The gas mixture generated is stripped of impurities
(including moisture) by the vapor ﬁlter. The gas then goes into the surge tank from
where it is recycled into the reactor via the gas pump. This process is continued until
an SO, -saturated gas mixture is obtained. This cycle is called Internal Circulation. The
gas line connecting the vapor ﬁlter to the surge tank has a 4-way valve which can be
positioned to direct the ﬂow of SO, into the sulfonating chamber which lies in-between

the vapor ﬁlter and the surge tank. It is in this chamber that the substrate under

34

investigation is sulfonated. During sample sulfonation, gas ﬂows from the vapor ﬁlter
into the surge tank through the sulfonating chamber and is said to be in External
Circulation. There are distinct advantages of using SO, in external circulation mode as
against a one point usage and disposal of the excess. Firstly, there is conservation of the
unused portion of the gas. This is recycled into the system and in subsequent contact
with oleum it is saturated again. Secondly, it is easier to maintain a. constant
concentration of the product gas in this manner. This results in constant experimental
conditions. The major shortcoming of this procedure is the moisture carryover in the

system during sulfonation.
3.3 SULFONATING CHAMBER

The sulfonating chamber is a stainless steel parallelopiped box with manifolds at

two of its narrow sides. The dimensions of the box are 15 x 15 x 1.75 inches and the

Inlet s03 to the
l Chamber
I Outlet 303 to the
Reactor

Figure 3.2: A schematic ﬁgure of the sulfonating chamber showing positions of
the manifolds.

 

 

 

 

35
steel is 1/4 inches thick. Both the manifolds are welded to the narrow sides of the box

and are made of stainless steel. All the ﬁttings on the manifolds are l/ 2 inch swagelok
stainless steel ﬁttings. Figure 3.2 is a sketch of the manifold showing a lay out of the
inlet and outgoing tubes.

The sheet to be sulfonated is placed inside the chamber on a support which
ensures that the sheet is not touching the sides of the chamber. The sheet must be as taut
as possible so that the ﬂow of gas mixture is maintained uniform ensuring that
sulfonation conditions do not change while in the middle of the run. The open side of
the chamber through which the sheet and the support is placed inside is a highly
machined surface. The lid which closes the side is also highly machined and has a Buna
-N rubber O-ring which comes in direct contact with the opening on the box. This has
proved to be a good arrangement for creating and maintaining a vacuum (of 300
microns) in the chamber before sulfonating. '

The inlet line to the sulfonating chamber carries the SO, gas mixture from the
reactor via the vapor ﬁlter in a 2 inch teﬂon line braided with stainless steel. Teﬂon is
an inert material and thus does not degrade in contact with S0, . Stainless steel braiding
provides strength to the tube assembly. The same port is also the inlet for nitrogen gas
which is used to create an inert atmosphere in the chamber before sulfonating and purge
the system after sulfonation is over. Also located on the main lines which carry SO, (or
nitrogen) into the sulfonating chamber is a sampling port for monitoring the SO,
concentration (see Figure 3.3). The sampling port is made up of stainless steel. Silicon
septum (coated with teﬂon on one side) is inserted in the port and made secure by a

teﬂon stopper. An air-tight syringe with 18 gauge needle is used for sampling purposes.

36.

The gas mixture is pulled out of the lines while sulfonating. The contents of this syringe
are then released into an erlenmeyer ﬁtted with a glass adaptor. The erlenmeyer contains
water. The SO, of the gas mixture reacts with water to form acid. The strength of this

acid is indicative of the strength of the gaseous mixture.

 

 

Figure 3.3: Schematic of the sampling port for SO, gas.

The second manifold is the outlet line of the sulfonating chamber and it recycles
the gas mixture back into the reactor. At this port a T-arrangement has been installed,
one arm of which is connected to the manifold and the other arm to the vacuum Pump.
The rubber hose connecting the vaccuum pump to the manifold (through the 1/ 2 inch
ball-valve) has a Liquid nitrogen trap. The installation of this trap system is for the
purpose of trapping any contaminants, especially residual SO, from going into the pump

and to prevent back streaming of pump oil into the system.

37
The successful operation of the generator has been veriﬁed by checking as to

whether it can generate SO, in a controlled and repeatable manner. For achieving this
end the ﬁrst requirement is that one must have a conﬁdent method of measuring the gas

concentration. This is the topic of discussion of the following chapter.

Measurement And Control of Sulfurtrioxide

 

Chapter 4

 

The process of surface treatment using sulfonation in order to be industrially and
commercially viable must have strict control over the amount of SO, generated under a
given set of conditions. The gaseous product should be of constant composition and must
be repeatable under the stated conditions. However, considering that SO, (and Oleum)
are highly reactive chemical species the system has to be so designed that the inﬂuence
of external parameters is minimised while offering maximum control on the parameters

that control the amount of SO, generated.

4. 1 EXTERNAL PARAMETERS AFFECTING SO, CONCENTRATION

There are several external parameters which strongly affect 80, concentration.
The most important is moisture which is extremely detrimental to the concentration of
measurable SO,. Moisture immediately combines with SO, to form sulfuric acid when
brought in contact with Oleum or S0,. Thus, even trace amounts of moisture must not
be admitted into the system. In order to achieve this, the storage tanks were blanketed
with dry nitrogen gas. The surge tank and the vapor ﬁlter were also ﬂushed with
nitrogen. In order to minimise the damage due to moisture during actual use of the
machine, all the lines, manifolds, valves etc. were heated up to 110 °C for at least 5
hours to remove all the moisture absorbed on the surface. The sulfonating chamber was
subjected to a vaccuum of 300 microns to reduce moisture from the polymer ﬁlm being

sulfonated.

38

39

Another important parameter which controls the amount of SO, generated is the
temperature at which the oleum is maintained. The vapor pressure of gaseous SO,
generated is highly sensitive to the temperature of the oleum reactor. In order to
maintain a constant temperature, the reactor has been provided with ‘a heating jacket and

a controller through which the required temperature can be maintained (3; 1°F).

4.2 TECHNIQUES USED TO MEASURE SO, CONCENTRATION

An accurate and reproducible method to measure the concentration of SO, in the
gas mixture is a prerequisite for the successful operation of the process. The details of
the technique used are described in the following discussion.

4. 2.1 Volumetric Gas Sampling

This technique is based on the principles of chemical titration. That is, at the
end-point of the titration reaction, the numbers of equivalents of both species reacted is
same. Now, it is known that S0, is highly susceptible to reaction with water to form
sulfuric acid. Thus, if we assume a complete reaction between SO, gas and water the
concentration of the resulting acid should be identical to the concentration of SO, gas.
This principle has been used in the present study in order to quantify the SO, gaseous
concentration.

The SO,-gas mixture was trapped in a specially designed glass tube called a
Volumetric Gas Sampler (V GS) of 210 ml by installing it in the external lines just before
the sulfonating chamber. All the lines and valves were heated to 110°C to drive out all
the absorbed moisture before being used. In order to minimise the effect due to external

moisture the lines were thoroughly ﬂushed with dry nitrogen at a rate of 34 l/min for 2

 

40

minutes so that all the atmospheric air was replaced with dry nitrogen. At this stage the
'4-way valve was switched to the external loop mode so that SO, gas could be introduced
in the gas sampler. After 5 minutes the gas ﬂow was stopped, stop-cock on the gas
sampler shut off and the V68 removed from the system. In order to create a slight
vaccuum for facilitating the introduction of water the sampler was placed in freezer for
10 minutes. Deionised water was introduced in the sampler gradually by the action of
vaccum. The amount of deionised water supply was always in excess so that air could
not enter the system and also so that all the SO, molecules react. As soon as water
entered the gas sampler it became cloudy due to sulfuric acid mist. The reaction is as
follows.
80, + H20 ------ > H2804

The glass sampler was shaken thoroughly to carry out the reaction to completion and to
ensure that all the SO, molecules are reacted. The resultant acid was transferred to a
ﬂask and titrated against sodium hydroxide (N aOH) of known concentration with
phenolphthanalein as an indicator. The neutralisation reaction resulted in formation of
a salt. The proposed reaction sequence is as follows.

H2SO4 + 2 NaOH ----- > Na,SO4 + 2H,O
The reaction end-point was determined as soon as the reaction mixture achieved a pink
coloration. Also, at the end-point the number of equivalents of NaOH reacted equals the
number of equivalents of H2SO4 reacted.
Or, in other words

err = N2Vz

41.
where, N, is the normality of NaOH, V1 is the volume of NaOH used and .

N 2 is the normality of H2SO4, V2 is the volume of H280, used.
Now, notice that according to the stoichiometric relation two moles of NaOH are used
for each mole of H2S04 employed. Thus, the following relationship can be derived.
Volume of S0, = ( NM," VNmH/Z) ( 22.4 l/g-mol)

Thus, Percentage volume of SO, = (Volume of SO,)/(Volume of VGS)

4.3 THE pH MODEL

There are certain inherent limitations in the technique outlined above. Firstly,
introducing water to carry out its reaction with SO, to make an acid was difﬁcult and
user dependent. The SO, vapour had a tendency to escape from the gas sampler thus
giving erroneous results. Also, the VGS, gas lines etc had to be washed and dried out
each time they were used. To reduce the sampling time and to ensure a stricter control
over repeatability, the pH of the resulting acid was measured instead of carrying out
actual titrations each time. The theoretical derivation of the equation to be employed is
based on the deﬁnition of pH. The pH of a reagent is deﬁned as the negative log (base
10) of the hydrogen ion concentration of the system. The reagent in the system under
consideration is sulfuric acid which was prepared by reacting DI-water with S0, gas.

The details of the relation are as follows.

503 + H20 = H2804
H280, = 2H+ + so,-2
[ H2304] = [ HU/2

Now, [ H”] = 10'?"

 

42
Thus, [80,] = [H2804] = [H+]/2 = IO'PH/Z

Number of moles of S0, = [ S0,] ( Volume of acid)
Number of moles of SO, = [ S0,] ( Vmo)
Volume of S0, = (22.4 l/g—mole) ( Vmo) row/2

Percentage 80, = (Volume of SO, )/( Total Volume of gas mixture)

Now, volume of water employed was kept constant in each run (20ml) and the
total volume of gaseous mixture was constant at 100 ml. Therefore,

Percentage SO, = (224 ”0""
The pH was measured using Corning Model M-250 pH/ISE meter which had an accuracy
of 10.001.

Now, the amount of SO, released from a given concentration of SO, is dependent
on the temperature. Figure 4.1 shows the variation of vapor pressure of SO, and oleum
with respect to temperature. The oleum in the reactor was known to be of 30% strength.
The plot of experimentally calculated vapour pressures and the actual vapour pressures
shown in Figure 4.2 indicate a variable difference between the experimental values and
actual values [25]. Thus, this method although time saving and reproducible did not

prove to be accurate.

4.4 RELATION BETWEEN pH AND VGS MODELS.

Although the‘VGS model was time consuming it was accurate. On the other hand
the pH method was time saving and reproducible but inaccurate. Thus, an attempt was
made to relate pH values to the VGS values [25]. The results are shown in Figure 4.3

[25]. This served as the master curve for all the work presented in this thesis.

43

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E” :
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E103
% :
o .
8'5-
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ExperimrtalData
0 ...4T...-,....,
25 30 35 4O 45
Reactor Temperature (C)

Figure 4.2: The figure illustrates the experimentally determined vapor pressure and

the actual vapor pressure of SO, in the reactor [25] .

45

 

Percent S03 from VGS

 

 

 

 

Figure 4.3: The calibration curve showing the relationship between pH and

percentage SO, obtained from VGS [25].

Experimental Methods and Protocols

 

Chapter 5

 

The characterisation of the surface of modiﬁed polymers is of utmost importance
to determine the chemical changes occuring on the surface. Several spectroscopic
techniques were employed for this purpose. Also, in order to ensure that the modiﬁed
surface is essentially identical in every run, or in other words for repeatability of the
experiments strict experimental protocols were followed. This chapter delineates the

spectroscopic methods and the experimental protocols followed.

5.1 X-RAY PHOTOELECTRON SPECTROSCOPY

X-ray Photoelectron Spectroscopy is a surface-sensitive technique which is
ﬁnding increasing applications in surface analysis of polymers. The technique is
powerful because besides giving qualitative atomic information it can provide quantitative
data and chemical information. For example, typically a complete data set would consist
of the kinds of atoms present on the surface, percentage of each of them and also the
chemical environment of the atoms. That is, the oxidation state of each of the element
present in the system.

X-rays are the primary beam which are impinged on the surface of the material
being analysed under ultra-high vaccuum conditions. The interaction of X-rays with the
atoms of the material causes the ejection of the core electron(s) imparting them kinetic
energy in the process. This is called the Photo-ionization process. The binding energy

of the core electrons is characteristic of the element from which they originate and since

46

47

the X-ray photons are non-energetic, the kinetic energy of the photo-electrons is also
unique to each element (and its corresponding oxidation state). The governing equation

for XPS in its simplest form is an energy balance.

Ex: ’1” "Eb

where, EK is the kinetic energy of the photo-electron

hp is the energy of the primary beams
B, is the binding energy of the core electrons
The photo-electrons are collected and sorted according to their energy, thus
yielding the spectrum. However, there are certain inherent limitations of this technique.
The most important one being the requirement of Ultra-high vaccuum (UHV). The
signal quality and quantity is greatly reduced in the absence of UHV as the photo-
electrons collide with gas phase molecules on their way to the detector thus distorting the
signal. Another major problem, especially with polymers is charge build-up on the
surface. As the electrons leave the surface of the sample, there is a net charge build-up
due to decreasing number of electrons on the sample causing an increasing resistance to
the electronsfrom leaving the surface. This can shift the signal substantially. One way
to alleviate this problem is to ﬂood the sample with low-energy electrons which do not
affect the signal character. The third problem is that the X-rays are difﬁcult to focus.
This may lead to an. unacceptable spatial resolution of the sample under investigation.
The maximum depth from which the signal is collected can range from 2-5 nm. A

detailed explanation of the process is given in reference [15].

48
All XPS experiments were carried out on a Perkin-Elmer PHI 5400 XPSA system

using an Al Ka (1486.6 eV) monochromatic source (PHI 10-410). Spectra were
collected at a base pressure of approximately 10'9 Torr and electron take-off
angle of 65° using a position sensitive detector on a 180° hemispherical analyser set at

89.9 eV for wide scan and 35.5 eV for narrow scans of the elemental regions.

5.2 FOURIER TRANSFORM INFRA-RED SPECTROSCOPIC ANALYSIS

Fourier Transform Infra-red Analysis involves the use of Infra-red region of
Electro-magnetic spectrum to analyse the chemical information about the species under
investigation. The infra-red region of the spectrum lies at wavelengths longer than visible
light and less than microwaves. That is, 400 nm < Am < 800 nm ( 1 nm = 10'9m).

The principle of operation involves the absorption of energy by molecules/chemical
entities which promotes them to a higher energy state. All compounds which have
covalent bonds absorb energy at various frequencies of electromagnetic radiation in the
IR region. The absorption of energy is a quantized process. The energy absorbed (8.36-
41.8 kJ/mole) puts the molecule in a higher energy state which increases the amplitude
of the stretching, bending vibrationalmotions of the bonds in molecules. However, since
the presence of dipole-moments is a prerequisite for energy absorption not all molecules
can absorb IR energy. A

Each molecule has a different set of bonds. That is, a single kind of bond may
be present in two different molecules but the fact that they do not have the same chemical
environment implies that the natural frequency of this bond will be different. Thus, no

two structures can have exactly the same IR absorption pattern/ IR spectrum. This is the

49

reason that IR can be used to quantify molecular information.
IR rays impinging on a sample can be reﬂected, absorbed, transmitted or scattered.

Mathematically, this is expressed as,

Io= I.+I.+II+I.

where, I0 intensity of the incident beam.
1, is intensity of the reﬂected beam.
1, is intensity of the absorbed beam.
It is intensity of the transmitted beam.

I is intensity of the scattered beam.

Any of the above beams can be the sampling beam for the purposes of analysis, the ﬁnal
choice depending on the nature of sample, geometry of the sample etc. For opaque
samples where polymer sample has a low transmittance Internal Reﬂectance Spectroscopy
(or Attenuated Total Reﬂectance) is used. This technique is a contact sampling method
involving a crystal with a high refractive index and low IR absorption. The main
advantage of IRS is that spectra of opaque samples can be obtained. In the present study
KRS-S (Thalliurn Bromide-Iodide) crystal was used with angle of analysis being 45°.
ATR MTCN mode with MCTN detector was employed. The depth of penetration of

i the beam is given by the following equation.

50

-_1
DD = (.241?) ( nlzsinze - 1222) 2
where, D1) is the depth of penetration in microns.

It is the wavelength in microns.
n,‘ is refractive index of the crystal.
n2 is the refractive index of the material.

0 is the angle of the face of the crystal.

IR spectra were recorded on a Perkin-Elmer 1800 FTIR Spectrometer with MCTN
detector. The sample chamber was purged with dry and carbon dioxide removed air for
10 minutes before spectra were recorded. An ATR attachment with KRS-S crystal
(Thalliurn Bromide-Iodide) internal reﬂectance element was used for analysis. The crystal
had 45° endface angles and 50x50x2 mm dimension. The sample strips were placed on
either side of the crystal and ﬂushed securely with pads. The sample signal was ratioed
against KRS-S crystal reference. 500 scans were collected for each sample.

In order to study the material degraded by sulfonation polypropylene samples were
prepared with noninfrared absorbing KBr matrix. The material was scraped off the
surface of polypropylene with a clean razor blade. This was thoroughly mixed with KBr

and a disk made out of it under high pressure.

51
5.3 CONTACT ANGLE ANALYSIS

Contact Angle is a measure of degree of wettability of liquid on a solid surface.
The concept of contact angle is widely employed to estimate the trends in wettability and
thus adhesion. A contact angle of 0° indicates spontaneous wetting while 180° indictes
non-wetting condition. That is, if the angle is 0° the situation is most favourable for
adhesion while 180° is the most unfavorable.

For the present work, contact angle analysis was used to complement the
information provided by XPS and observe the trends on the nature of changes occuring
on the surface. The angles were measured using a Raine-Hart Goniometer Model 100-00.
Water was used as the probe liquid. 5 uL drops were placed manually on surface of the
polymer under investigation with a Precision Microliter Pipette (Pipetteman). The angle

was read from the calibrated eye-piece.

5.4 MATERIALS

One of the main objectives of the present work was to characterise the effect of
sulfonation on an aliphatic polymer and on an aromatic polymer. For this purpose,
polypropylene was chosen as the model for aliphatic system and polystyrene was chosen
as a model for aromatic system. Both the polymers were 0.03 inches thick ﬁhns and used
as available from a commercial source.

The repeating unit of polypropylene is shown in Figure 5.1. As is clear from the
ﬁgure, it is characterised by primary, secondary as well as a tertiary carbon. Tertiary
carbon is the most probable site for sulfonation due to the highly electrophilic nature of

sulfonation reaction. Details will be discussed in Chapter 6 (Sulfonation of

52
Polypropylene)

CH,

sen-ca

Figure 5.1: The figure illustrates the repeating unit for polypropylene.

The repeating unit for polystyrene has been depicted in Figure 5.2. It is
characterised by two secondary carbons of aliphatic nature and a phenyl ring. Due to the
high electron density on this ring ortho and para position are the most probable sites for
sulfonation. More details on this aspect have been presented in Chapter 7 (Sulfonation

of Polystyrene).

{CH—CHzﬁ—
0

Figure 5.2: The ﬁgure illustrates the repeating unit for polystyrene polymer.

53

Gaseous sulfurtrioxide of known concentration was generated from a specially designed
generator the details of which have already been given in Chapter 3. The concentration

of SO, used in the following work was 1% ( :l: 0.2% ) SO,-air mixture.

5.5 REACTION CONDITIONS AND SCHEME
All the reactions were carried out under ambient conditions. To clean the surface-

of the polymers prior to sulfonation, they were treated with a commercially available
detergent ’Micro’. All the sheets were washed and rinsed in deionised water thoroughly
and the samples were then air-dried. In order to minimise the effect of moisture and
to ensure the repeatability of the reaction , the following procedure was followed in each
run.

- Micro-washed sheets thoroughly washed in DI-water and air-dried were placed in
the sulfonating chamber and ﬂushed with dry Nitrogen (34 l/min) for 5 minutes

- A vaccuum of about 300 microns was applied to the sulfonating chamber (with the
sheet placed inside) for 10 minutes. This removes the moisture to the maximum
possible extent. However, it is not possible to remove the monolayer of water
which is adsorbed on the surface of the polymer.

- The vaccuum was broken by purging and ﬂushing the system with dry nitrogen
(34 l/min) for 5 minutes for bringing it to atmospheric conditions again. The unit
cannot be operated at sub-atmospheric conditions, since a vaccuum on the reactor
will seriously disturb the SO,equilibrium in the reactor.

— The sheets were sulfonated for the desired time.

54

- The system was then ﬂushed with dry nitrogen (34 l/min) for 5 minutes before
opening the sulfonating chamber. This ensures that all the residual SO,/H,SO4 is
purged out of the system through the vent tube.

- The sheets were neutralised in 5% aqueous solution of Ammonium Hydroxide
(NH40H). This was prepared by mixing the commercially available 1 part volume
of 30% NH40H with 6 parts volume of DI- water.

- The neutralised sheets were placed in excess of DI-water for 5 minutes.

- The sheets were then washed thoroughly in running DI-water and air-dried.

The details of the reactions and chemical changes occuring on the surface are presented

in the following Chapters ( 6 and 7).

Sulfonation of Polypropylene

 

Chapter 6

 

Polypropylene has wide ranging industrial and daily life applications. However,
like most thermosplastics polypropylene also has a low-energy surface. It has been
shown by investigators that surface modiﬁcation of polypropylene using sulfonation is
a viable technique to improve its adhesion properties [25]. The focus of present
discussion is to follow the chemical changes that occur on the surface of polypropylene
as a result of sulfonation.

6.1 NATURE OF POLYPROPYLENE '

Polypropylene is an aliphatic polymer. In other words, it does not have any
phenyl rings in its structures. The repeating unit of polypropylene is shown in Figure

6.1.

CH,

{ea—ca

Figure 6. 1: The ﬁgure illustrates the repeating unit of polypropylene.

As can be seen in Figure 6.1 it is characterised by primary, secondary as well as a
tertiary carbon atom. Thus, if this system can be characterised as to its reactivity to
sulfonation, then predictions about other polymer systems which contain these kind of

carbons can be made.

55

56

It has been documented in the literature sulfonation is an electrOphilic reaction.
The sulfur in sulfurtrioxide behaves as a Lewis acid and oxygen atoms behave as a Lewis
base. The molecule is planar, triangular and symmetrical [10]. The S-O bond is short
(1.4198 A) and all the oxygen atoms have been shown to be equivalent indicating a
resonance hybrid structure. The high reactivity of sulfurtrioxide is due to the electron
deﬁcient nature of sulfur which tries to increase the number of electrons in its outermost
shell to ten or even twelve [10] thus supporting the electrophilic nature of the reactions
of sulfurtrioxide.

Due to the highly electrophilic mechanism of sulfonation, the tertiary carbon is
most susceptible to attack during sulfonation followed by secondary and primary
respectively. This is because tertiary carbon is bonded to three other carbons secondary
to two and primary to a single carbon atom. This makes the electron density on tertiary
carbon maximum which will drive the reacting SO, molecule preferentially towards this
site. However, sulfurtrioxide is such a reactive species that reaction with secondary or
primary cannot be ruled out.

To investigate the nature of chemical species various analytical techniques were
employed the details of which are in the preceding chapter. The following results are
for polypropylene sulfonated with 1% SO,-air mixture (by volume) followed by
neutralisation with 5 % aqueous solution of ammonium hydroxide unless stated otherwise.
The observations and results from each of them will be discussed in detail in the

following passages.

 

57
6.2 FTIR-SPECTROSCOPY OBSERVATIONS.

Several investigators have used Infra-red technique to determine the chemical
species on the top 1-2 pm of the sulfonated surface [ 26-30]. Based on work by Ihata
et. al. [30], Cameron et. al. [28,29], Gibson et. al. [35] and reference 31 and 34 the

following table has been compiled for ready reference in the following discussion.

Table 6. 1: The table shows the wavenumbers and the probable chemical species

 

 

responsible for their presence.
Peak Position ( cm") Possible chemical species
-CHOH
1167 O=S=O stretch
1045 O=S =0 stretch
1385 -NH of ammonium
1425 O=S=O stretch
~ 1600 Conjugated C=C stretch
1680-1620 Unconjugated C=C stretch, water
1700-1750 -C=O ( ketones, carboxylic acid)
3030 -CH=CH- stretch
_ 3220 -NH of NH,
3450 -OH ( water, alcohol, acid)

 

 

 

 

58
Figure 6.2 shows the IR spectra (in transmission mode) of Polypropylene sulfonated from

1 thru 5 minutes in 1% SO,-air mixture followed by neutralisation with ammonium
hydroxide. Figure 6.3 is the spectra of polypropylene subtracted from sulfonated
polypropylene (in absorbance mode) which shows the changes occuring on the surface
as treatment time is increased from 1 minutes to 5 minutes. The major bands observed
after sulfonation are the bands that appear in 1170 cm‘1 region, a band in 1350 cm‘1
region, a band in 1450 cm" region, a band in 1600-1750 cm"region, band in 3400 cm".
region and in 3200 cm‘1 region.

Bands at 1170 cm" and 1450 cm" are attributed to S=O vibrations . The exact
location and nature of these bands depends on the groups attached [31]. Also, in the
region 1390-1430 cm‘1 the contribution can be from -NH of NH,+1 [34]. As the time of
sulfonation is increased not only the the intensity of these bands increase but also subtle
changes start to occur within the envelope. There is a possibility that additional peaks
start emerging in this region. The essential nature of the compounds which yield peaks .
in this region is the same, i.e. they contain S=O linkage but the nature of group which
carries this chemical species starts to vary which leads to the belief that certain other
compounds besides sulfonic acids are being formed [ 31 ]. Also, the intensity of each
of these bands increases as the time of sulfonation is increased.

A review of the past literature work shows that formation of sulfonic acids is the
major chemical change effected due to sulfonation on the polymer surfaces [26-30].
Sulfonic acids are characterised by two peak positions. One is a strong band near 1175
cm'1 and the other is a medium intensity band near 1055 cm‘1 [31]. In the present work

these band positions have been ﬁxed at 1167 cm‘1 and 1045 cm". But, since changes .

59

100

 

 

 

90: ‘T‘V: 4_ v
i
801?
70{—

60{—

Transmittance

50{-

 

40,—-

 

 

 

‘ r- 1 '1 r 1 L
30 III IITTIIrITWIIII TTTﬁjTTTIIII

. ' I .
4000 3 500 3000 2500 2000 l 500 1000 650

 

Wavenumber (cm- 1)
Figure 6.2a: The ﬁgure illustrates the IR spectrum of unsulfonated polypropylene.

90

go €:—-\f" C=C

705-
60g-
50%—
40%—
30%—
20%-
10%-
051 1 I 1'11”?!

M T T T ‘ f T T I T—l' T j I Y I I I

. I . . I . I .
4000 3500 3000 2500 2000 1500 1000 650

 

 

-CI-IOH

 

Transmittance

 

 

 

 

1

Wavenumber (cm-1)

Figure 6.20: The figure illustrates the IR spectrum of polypropylene sulfonated for
1 minute followed by neutralisation with ammonium hydroxide.

901‘
80%
70%
60%
50%-
40%—
30%-
-2o%—
10%-
OE ..1 1" l +' l l

I T I r I I I T T T I T T I j T V V I r

' ' ' I ' I '
4000 3500 3000 2500 2000 1500 1000 650

 

 

 

 

Transmittance

 

 

 

 

 

 

Wavenumber (cm- 1)

Figure 6.2c: The ﬁgure illustrates the IR spectrum of polypropylene sulfonated for

2 minutes followed by neutralisation with ammonium hydroxide.

 

90:
80%
70%
60%
50%—
40%-
30%—

 

 

 

Transmittance

 

20 —}- O=S=O

10 .1...
3 l 1 l l l l

O r t r I r r I I v I r r I f r r 7 I r I I I V T 1

. ' I ' ' I ' ’
4000 3500 3000 2500 2000 1500 1000 650

 

 

 

Wavenumber (cm' 1)
Figure 6.2d: The ﬁgure illustrates the IR spectrum of polypropylene sulfonated for

3 minutes followed by neutralisation with ammonium hydroxide.

61

 

Transmittance

 

 

 

 

 

3 ' l l r l 1 l
O I I . I . . I l I I r T ‘ I T r Y. I . I I I T w w

. I . r .
4000 3500 3000 2500 2000 1500 1000 650

Wavenumber (cm' 1 )
Figure 6.2e: The ﬁgure illustrates the IR spectrum of polypropylene sulfonated for

4 minutes followed by neutralisation with ammonium hydroxide.

90
80 —j
70 i
60%

 

Transmittance
.5
o
n.

 

20%-

 

 

o:— O 0
1 j S

‘ l l l l l . l

O I I I . I I I I r I I T I n 7 I f T M I I I . I I l I I I

4000 3 500 3 000 2500 2000 1500 1000 650

 

 

 

Wavenumber (cm' I)

Figure 6.2f: The ﬁgure illustrates the IR spectrum of polypropylene sulfonated for

5 minutes followed by neutralisation with ammonium hydroxide.

62

 

02%
018%
0I6%
014%
012%

01-%
008%

I—
006%-
: 4 I I
O-l , I Y I I I I I I Y Iqﬁ T“ I

ﬁ
004%
IT I

I
4000 3500 3000 2500 2000 1500 1000 650

-CHOH

Absorbance

 

 

 

 

 

Wavenumber (cm'l)

Figure 6.33: The difference spectra between 1 minute sulfonated polypropylene and

unsulfonated polypropylene.

 

03‘

025{—

c>

no

1L1 l
I

O

u

(I)

u

0

Absorbance
F’
G
I
l

S3
p...
I].
I

 

 

 

 

0.05 ——
. C:
C=C)
“ l J - l
O F T I - T I T ﬁr Y T Y f‘] T T Y

' I I
4000 3500 3000

 

 

 

I W I T I T

I . .
2500 2000 1500 1000 650

Wavenumber (cm- 1)

Figure 6.3b: The difference spectra between 2 minute sulfonated polypropylene and

unsulfonated polypropylene.

63

0.6 .

 

O=S=O

_o
A
.1.

 

 

Absorbance
0
w
l

 

F’ .
N
1 l l l l

 

 

 

 

 

l V l l

I l T r 7 I I . 7 I T I Y Y T I T l ‘

TY. II' ‘1“'
4000 3500 3000 2500 2000 1500 1000 650

 

Wavenumber (cm'l)

Figure 6.3c: The difference spectra between 3 minute sulfonated polypropylene and

unsulfonated polypropylene.

0.6

 

q
q
05....
.
-
.4

0.4 d} '
'1

 

.I

0.3 «‘—
1

0.2 -} K

. =0

 

Absorbance

 

 

 

 

 

 

__J l

f f ‘v

o-‘ 1...
1000650

T I I
4000 3 500 3 000 2 500 2000 l 500

Wavenumber (cm' 1) .
Figure 6.3d: The difference spectra between 4 minute sulfonated polypropylene and

unsulfonated polypropylene.

0.6

 

I
0.5 +-

04—:-
1
0.3 '1-
3
0.2:—

01-}

 

Absorbance

 

 

    
   

 

l ‘ l

C=C
.. | J l '
O 1 . l 1 I l 1 I I I I i ’ I I I I l I I # I

Y I . r I
4000 3 500 3 000 2500 2000 1500 1000 650

 

 

Wavenumber (cm'l)
Figure 6.3e: The difference spectra between 5 minute sulfonated polypropylene and

unsulfonated polypropylene.

65

occur within the stated envelopes it could be that besides sulfonic acids there are several
other species containing S=O linkage also being formed.

The other region which shows a change is 1600-1750 cm". This peak position
is characteristic of C ==C vibrations (1600-1680 cm") and C=C linkage (1720 cm"). The
intensity of this composite band also increases as time of sulfonation is increased. The
peak position however remains almost unchanged as time of sulfonation is changed.
Between 3000-3600 cm'1 three peaks appear. These peaks are clearly separated which
can be seen from the subtracted spectra for higher times of sulfonation. The peaks are
positioned in 3100 cm‘1 region, 3200 cm’1 region and 3450 cm'1 region. The 3450 cm’1
band is a broad and is attributed to -OH species. 3200 cm‘1 is attributed to -NH linkage

and the peak in 3100 cm‘1 is attributed to =C-H linkage [31].

6.3 FTIR RESULTS.

The present section discusses the probable chemical species responsible for the
bands observed above. Each region has been discussed separately.

6. 3.] Band Region 1170 cm" and 1450 cm"

Two band regions are attributed to S = O vibrations and contributions from -NH '
of the NH4“. Within this envelope the main sulfonic acid peaks ﬁxed at 1167, cm'1 and
1045 cm’1 are also located. A careful inspection of the peaks in this region shows that
as the time of sulfonation increases not only does the intensity increase but there is an
apparent change in the peak growth pattern. It appears that additional peaks are growing
within the same enve10pe in 1170 cm‘1 region. This leads us to believe that possibly

there is the formation of additional chemical species besides sulfonic acids.

66

The literature shows that besides formation of sulfonic acids there is a possibility
of formation of sultones, alkene disulfonic acids, alkene sulfonic acids and ketones in the
system [30, 32, 33]. The contribution in 1390-1430 cm’1 region can also be from -NH
of NH,+ [31] besides S =0 vibrations. Due to a lack of pertinent literature, it is difﬁcult
to make a conclusion as to the exact nature of this band or which compounds will yield
the speciﬁc bands. Sufﬁce it to say that it does support the conclusion that species other
than sulfonic acids which contain S =0 kind of bonds. Various vibrations in the 800-.
1000 cm'1 can be attributed to S-O-C kind of linkage [31]. Thus, it can be concluded
that as a result of sulfonation the system contains the following species:

- s=o kind of linkages. ’

- S-O—C kind of linkages.

- NH kind of linkages.

6. 3.2 Band Region 1600-1750 cm"

The band region 1600-1680 cm‘1 is attributed to alkene kind of structures (C=C .
linkages ) and the band at 1720 cm‘1 is linked to the C=C structure which can be a
ketonic, aldehyde, ester or a carboxylic acid species [ 34 ]. A review of the literature
shows that as a result of sulfonation, the formation. of a conjugated system of double
bonds is probable. Literature values ﬁx bands due to conjugation near 1600 cm’l and for
unconjugated system the region has been identiﬁed at 1680-1620 cm‘1 [34]. Observation
of the subtracted spectra shows the appearance of a broad peak in this region as a result
of sulfonation which implies that there is an arrangement of conjugated as well as
unconjugated carbons in the system under investigation. The band at 1720 cm‘1 attributed

to C=C grows along with the band attributed to C=C linkage.

67
6.3.3 Band region 3000-3500 cm"

An inspection of the spectra shows the appearance of three bands in this region
as sulfonation is carried out. The three bands can approximately be ﬁxed at 3030 cm‘1 ,
3220 cm'1 and 3450 cm". .Each of these is attributed to speciﬁc chemical species. The
band at 3030 cm‘1 can be attributed to -CH =CH- stretching [34]. 3220 cm'1 is attributed
to -NH stretching vibration from NH,“ [34] and the broad band located at 3450 cm'1
is attributed to -OH linkage. Also, it can be concluded that this -OH linkage is not free -
OH kind because the band attributed to free -OH appears in 3650-3590 cm" region [34,
short course]. From the preceding discussion it can be concluded that the following
kinds of chemical linkages are also present as a result of sulfonation:

- S =0 linkages.

- C =C linkages.

- -OH, -NH linkages.

- C=O linkages.

6.4 DISCUSSION OF RESULTS.

Sulfonation of polymer surfaces has been investigated by several workers and the
chemical changes have been documented. The general consensus has been the formation
of sulfonic acid groups on the surface. For e.g Ihata et. al [30] sulfonated polyethylene
and proposed that the ﬁnal product contains sulfonic acids having highly conjugated C =C
' unsaturated bonds. Cameron et. al. [29] have proposed that the most signiﬁcant chemical

changes on the surface is oleﬁnic conjugation leading ultimately to carbonisation.

68

In both the instances the investigators proposed that desulfonation via expulsion
'of Sulfurous acid (or SO; + H20) is a dominating step in the whole ”reaction scheme.
This is in agreement with the present work. However, Ihata’s as well as Cameron’s
work does not focus on the fact that once the double bond is formed then this itself
becomes a potential site for attack by 803 due to the high electron density at this site.
This is an important focus of the present work. Cameron et. al. have proposed that
formation of B-sultone is a possibility in the scheme of reaction but they concluded that
B-sultone would desulfonate to yield diene as a ﬁnal product. However, this is not
always the case as it is well-documented that besides desulfonation there are other
reaction products which can be formed with B-sultone as the starting point [ 10,32,33].
These include alkene sulfonic acid, C—sultone, D-sultone, alkene disulfonic acid, ketones
etc. Also, there is a probability of formation of pyrosultone which can rearrange to give
several other products. 0
6. 4. 1 Proposed Reaction Scheme
As soon as Polypropylene is subjected to reaction with SO3 the tertiary carbons
are the most probable to be attacked due to their high electrongdensity. However, it has
been well-established in the literature that the resulting sulfonic acid tends to desulfonate
by extracting hydrogen from the neighbouring carbon and expelling sulfurous acid by the

following mechanism.

69

 

 

 

CH CH
! 3 $03 I 3 " H2803 I 3
H303 ( A)
CH3 CH CH H H
. so3 3 3 — H so (I 3 3
—c=CH— —>—c=CH—é|-CH2—‘ 2 =3» C=CH =CH—

Figure 6.4: The figure illustrates the formation of conjugated system of double
bonds as a result of sulfonation.

This reaction mechanism continues until there is a system of conjugated carbons which
are formed due to desulfonation. Now, these double bonds in turn become potential sites
for attack by 80;, resulting in formation of B-sultones in the ﬁrst step followed by several
other products.

It is worthwhile to take a closer look at the subtracted spectra of 1 minute treated
sample. The main change in all the other samples has been depicted by the sulfur species
band at 1170 cm“. But, in 1 minute sample the main band is stationed at 1110 cm“.
This is the band location for secondary alcohol. That is, the main effect of sulfonation
in the ﬁrst minute is formation of an alcohol and not a sulfonated species. An indication
of a similar effect is seen in the XPS and contact angle data also which will be discussed

shortly. There is a possibility of formation of alcohol species with alkene as the starting

70
point as in Figure 6.5 [40]. The H* can come from the residual acid in the system,
while Water is available from the aqueous ammonium hydroxide solution used to

neutralise the sulfonated polymer.

CH3 + CH3
I H , Hzo ' I
—c=--CH— 3. —CH2—CH—(|IH—

OH

 

Figure 6.5: The ﬁgure shows the formation of secondary alcohol species in the ﬁrst

minute of treatment. The incorporation of the sulfonic acids starts after the second

minute of sulfonation.

6. 4. la Formation of C,D-sultones.

The mechanism of formation of C,D-Sultones has been delineated in Figure 6.6. B-
Sultone is an unstable chemical species thus the reaction is driven towards the formation
of C and D-sultones. Work carried out by Herman de Groot shows the formation of
these species [ 32 ]. Further there is a possibility that the B-Sultone may desulfonate to

yield a double bond structure as proposed by Cameron et. al. [ 29 ].

CH3 ' CH3

new“...
' _J.

025

B Sultone

71

CH3 CH3

 

 

028—0
C Sultone

» —-<l:—CH2—éH—CH—

 

 

D Sultone

Figure 6.6: The ﬁgure illustrates the rearrangement of B-Sultones to C,D-Sultone.

72

6. 4. 1b Formation of Alkene Sulfonic Acids.
Alkene sulfonic acids are compounds which contain sulfonic acid group as well as a
doubly bonded. carbon [ 10, 32 ]. The reaction scheme for formation of alkene sulfonic

acids has been delineated below.

CH3 CH3 CH3 CH3
028——0 Hso3
B Sultone Alkene Sulfonic Acid
CH3 CH3
l—C “"
NHjso;

Figure 6.7: The ﬁgure illustrates the rearrangement of B-Sultone to alkene sulfonic

acids followed by neutralisation with ammonium hydroxide.

73
6. 4. 1c Formation of Hydroxysulfonic Acids.

In presence of water molecules the B-Sultone can make hydroxy sulfonic acids; The .

reaction path is delineated in Figure 6.8.

 

CH3 CH3 CH3 CH3
_I _I

|-——(|IH H— a —C H-—-
B Sultone Hydroxy sulfonic acid

NH4'OH

CH3 CH3
——&|—-C|H——éH—-
NH4?SO3' OH

. Figure 6.8: The ﬁgure shows the rearrangement of B-Sultone to hydroxysulfonic

acid.

6. 4. 1d Reaction of B-Sultone with $03.

It has been shown above that B-Sultone can rearrange to yield a variety of
different products. However, besides rearrangement there is a possibility that it may
further react with more SO3 to give pyrosultone. But, eventually pyrosultone

decomposes or rearranges to give C-Sultone, D-Sultone, alkene sulfonic acid or hydroxy

74

sulfonic acid. The conversion of B-Sultone to pyrosultone and then to hydroxysulfonic
acid is illustrated in the ﬁgure below. As can be seen, in the last step of the reaction

sulfuric acid is I formed which can further sulfonate a potential site in the chain.

 

 

CH3 CH3 CH3 CH3
$03 |
—C H—— » —C H—
"W" °zf’ ‘I’
B Sultone . 0——802 Pyrosultone
H20
CH CH
CH3 CH3 H20 3 3
—CH—-(|.‘H~— 4 —é—(|3 H——
HSO3 OH HSO3 (l)
Hydroxysulfonic acid HSO3
NH,,OH .
CH3 CH3

 

 

Nufso,‘ OH

Figure 6.9: The ﬁgure shows the reaction of SO3 with B-Sultone to make pyro
sultone. This can rearrange to give several products. Rearrangement to hydroxy

sulfonic acid has been shown above.

75

6. 4.12 Formation of Ketone

The peak position at 1720 cm‘1 is. characteristic of the C =0 of structure. There
is a possibility of the formation of such a structure in the system under investigation.
This has been shown by Ihata et. al [ 36 ]. The reaction sequence proposed in that work
was based on the cleavage of C-S bond due to VIS light. However, the following

reaction sequence is also possible and proposes the product based on chemisz only.

|H3 C|2H

C EH3 3 (I333
‘iH—“i i”? _’ ‘iHl ‘
02$ 0 0— 02 Hso3 02

1...

 

 

EH3 as . H 5.: 3*"
—(iH—CH—¢i—(f— _‘i —f $-
H503 0 30311 Hso3 0H H803

Figure 6. 10: The ﬁgure shows the possible route of formation of ketone species.
The formation of ketone was explained by Ihata et.al. on the basis degradation due

to VIS light.

76
6.5 XPS OBSERVATIONS AND RESULTS

X-Ray Photoelectron Spectroscopic Analysis (XPS) or Electron Spectroscopy for
Chemical Analysis (ESCA) is a surface sensitive technique, analysing the top 50-60 A
of the surface. In the previous section IR results were analysed. IR is not a purely
surface sensitive technique as its depth of analysis is 1-2 um( lum = 10‘ A ). In this
section the results obtained from ESCA shall be analysed and conclusions about the
chemical changes occuring on the top few layers of the surface hall be made.-
Quantitaitive changes occuring on the surface can also be made using this technique.
Thus, it will also aid in quantifying the "amount of change" which occurs on the surface
and at the same time if there is a pattern of change on the surface. Besides quantitative
changes ESCA can also yield qualitative results about the nature of chemical bonding on
the surface.
6. 5 . 1 Quantitative Analysis of ESCA Results.

The following table gives the atomic concentrations of the different elements in
percentage terms. In some of the samples contaminants were detected which have been

mentioned.

77

Table 6.2: The table shows the data for atomic concentration for polypropylene
sulfonated ' for 1 thru 5 minutes as obtained from XPSA.

 

Sample Carbon Oxygen Sulfur Nitrogen Others

 

Ominute '95.7:t0.53 3.55:0.72 O.57:l:0.06 - Silicon
lminute 93.4106 5.4109 1.01:0.1 - Fluorine

 

2 minute 69.8i3.6 22.4:L-3.0 4.6i0.3 2.8:t0.7 Calcium
3 minute 64.8:t0.5 25010.2 5.7:l:0.4 4.21:0.4 Calcium
4 minute 59311.8 28.2i0.9 710.5 5.4:l:0.4 -

5 minute 59.0iO.6 29i0.4 6.810.01 5.11:0.2 -

 

 

 

 

 

 

 

 

 

 

 

 

The following conclusions can be made from the data in the table.

(i) The percentage composition of the untreated sample and the 1 minute sample is
not very different. Also, nitrogen is absent in the 1 minute sample although this was
neutralised in NH4OH. However, the adhesion results show that the adhesion of the 1
minute treated sample did improve over the untreated sample [ 25 ]. This means that
some change has occured on the surface in the ﬁrst minute of treatment albeit not of the
nature of sulfonation. IR data also shows that the net impact of the ﬁrst minute of
sulfonation is formation of an alcoholic species rather than induction of sulfonic acids in
the system.

(ii) There is a marked change in atomic compositions from 1 minute to 2 minute
. sample and subsequent treatment times. It can be concluded that an effective chemical

reaction starts after the ﬁrst minute of treatment.

78 "

(iii) The magnitude step of atomic concentration change of the various elements is

very small after 2 minute through 5 minute treatment. All the values seem to be '

reaching a saturation value. This implies that there is a maximum amount of sulfonation

that can be carried out without extensively damaging the surface.

 

E] Nitrogen

I Sulfur

I Oxygen

 

 

 

 

 

 

 

 

 

 

 

5:33:00 39:85“;

All

Time of Sulfonation (min)

Figure 6.11: The plot shows the contribution of different elements as sulfonation

progresses. The contribution of carbon decreases as sulfonation time increases.

79
In order to visualise the chemical changes that have occured on the surface it is

helpful to determine the changes in terms of ratios of the various elements which has

been tabulated below.

Table 6.3: The data presented above shows the respective atomic ratios using the

data from table 6.2. Notice the saturation levels reached at 4 minutes.

 

 

 

 

 

 

Sample 0/c S/C S/O N/S
0 minute 00410007 000510.001 011510.014 -
.1 minute 0.0610009 0.0110001 0.1810003 -
2 minute 0.3410058 0.0810020 02210035 0.61 10.130
3 minute 0.3810007 0.0910006 0.2310014 07310.117
4 minute 0.4810032 0.1210011 0.2510008 0.7810008
=5 minute 0.49_1=0012 0.1210001 0.2410005 0.7610035

 

 

 

 

 

 

 

 

 

 

Theoretically, the impact of sulfonation is the formation of sulfonic acid species (-HSOa)
This means that the SIG ratio should be around 0.33 (or, O/S should be almost 3).
However, the data above shows that S/O rests around 0.25-0.16 This implies that
besides the formation of sulfonic acids there is a probability of other oxygen compounds
(not containing sulfur) being formed. The probable species are ketones, carboxylic acids,
aldehydes, alcohols etc. There was an indication of these species in the IR data too.
Another important observation to be made from the above data set is that in the 1 minute
sample no nitrogen was detected although these samples were neutralised in ammonium

hydroxide. The following reaction sequence should have occured.

80
R'SOgH + NH4OH ----- > R_(SOB-l )(NH4+1)

However, the data shows absence of nitrogen. Also, for 1 minute sample there is a very
small increase in the atomic concentrations (and the respective ratios) of other elements.
The reaCtion above also shows that S/ N ratio should be theoretically one if complete
neutralisation occurs. It can be observed that this is not the case in any of the samples.
This leads to the belief that besides steric factors probably some of the sulfur containing
species is not available for neutralisation. The important requirement for neutralisation
with ammonium Hydroxide is the availability of hydrogen which can be replaced with
ammonium ion (NHf'). As was proposed in the IR results there is a possibility of C,D-
sultones in the system. In the structure of C,D-sultones the hydrogen which can
exchange with ammonium is not available which implies that these centers cannot be
neutralised. This reinforces the proposed presence of sultones species in the system.

Another argument which reinforces the proposition that there is a possibility of other
compounds containing oxygen besides sulfonic acids is the fact that in the idealised case
when sulfonic acids are the only products 0/ N ratio should be around 3. However, the
data presented‘indicates that the ratio is much higher than 3 (between 5.0-6.0). Thus,
there is a possibility of oxygen compounds which neither contain sulfur nor nitrogen.
IR data clearly indicates that presence of alcohols, ketones groups etc is a viable option

which lead to higher numbers for O/ N and O/ S ratio than proposed theoretically.

81

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.8 1 a
; —e— SIC
0.6?H _.__ S/O
0.5-}7 —ea— N/S —--'EI
.. . / r
g 0 4 . j/
(5 ' r
05 1
0.3; P/ .
0.2 . _
: M
0.1 9 _———§"
:14
0.1
0 1 2 3 4 5

Time of sulfonation (min)

Figure 6.12: The ratios achieve a saturation value at almost 3 minutes treatment.

6. 5. 2 Qualitative Analysis of XPS Results.

Qualitative analysis of XPS data to determine the chemical information like nature
of bonding etc. is based on the principle that the peak obtamd from the narrow scan
(also called multiplexing) can be mathematically resolved into its constituents. Each peak
individually represents a speciﬁc chemical species. This process is called deconvolution
of the peak. In the present discussion deconvolution of the peaks obtained from

sulfonated polypropylene shall be carried out.

82
The data clearly indicates that C-C/C=C/C-H linkages are the most abundant

chemical linkages in the system. Thus, the carbon peak was shifted to 284.6 eV from
its original position. The peaks for the other elements in the system ( Ols, S2s, le )
were also shifted by the same amount respectively. The new peak positions assumed by
each of the elements for different treatment times turned out to be the same. The oxygen
peak assumed a position of 531.8 1 0.16, sulfur at 167.6 1 1.63 and nitrogen at 401.8
1 0.2 (no nitrogen detected in 1 minute treated sample ). For 1 minute treated sample
the sulfur peak did not always show a change. It was closer to sulfur seen in untreated
signal. This indicates that in the ﬁrst minute probably the chemical change is not being
effeCted signiﬁcantly. The peak position of oxygen is consistent with the literature
values. Briggs et. al. [39] obtained their oxygen peak at 532.0 eV after chromic acid
etching of polymers. Most of the oxygen in the system under investigation is bonded to
sulfur as in ammonium salt of sulfonic acid (NH,+1 803"). The other linkages are O-C,
O=C, and -COOH. The peak position of sulfur at 167.6 is consistent with the work
done by Seigbahn et. al. where the peak position for R-SOZOR kind of compounds was
ﬁxed at 167.5 eV. In the system under investigation also all the sulfur is bonded to
carbon on one side and oxygen on the rest of the sites. The nitrogen peak position at
401.8 eV is in consonance with the value of 401.3 eV for .(NH4)2SO4 [41].

Since carbon is the main constituent of the system and undergoes maximum
chemical changes this shall be analysed ﬁrst. Before sulfonation essentially there are
two kinds of carbon in the system. They are C-H and C-C. Due to the presence of
oxygen there is a possibility of some C-O/C=O kind of linkage also. For purposes of

XPS C-H linkage is same as C-C linkage. So, it can be concluded that there are two

83

kinds of carbon in the system. However, after sulfonation chemical changes occur on
’some of the carbon atoms. The new kinds of carbon based on IR results which appear
are as follows.

(i) C-S linkage from sulfonic acid, sultones.

(ii) C-OH linkage from hydroxysulfonic acids.

(iii) C=O linkage from ketones and -COOH from acids.

The energy position for each of these types of carbon has been ﬁxed empirically.
However, the value for OS in the system under investigation is not well documented
(C-S as in sulfonic acids, sultones etc ).

The deconvolution of the peak shows the presence of four peaks after
sulfonation. In the present work the ’ goodness of fit’ of the data had to be sacriﬁced
because of extensive sample charging problems. The position of these peaks is highly
mobile. The region of movement is as high as 0.8eV. Approximately, the peak
positions can be ﬁxed in 284.3-284.6, 2850—2860, 2860-2870, 2880-2890 region.
All the peakswere manually shifted to 284.6 eV (Cls). Due to the nature of the sample
under investigation a high degree of sample charging was observed as is normally
associated with polymers. Probably this is an important factor contributing to the peak
mobility. Thus, deﬁnitive conclusions about exact nature of the peaks or their growth
pattern cannot be made. However, it is clear that the contribution due Cls peak ﬁxed
at 284.6 eV decreases progressively as treatment time increases. The data has been

presented in the following table.

34
Table 6.4: The table shows the percentage contribution of the different peak regions

after deconvolution of the Cls peak. Notice that the contribution from the peak in

284.3-284.7 region decreases as treatment time increases.

 

 

 

 

 

 

0 minute 99.0 :1: 0.28 0.62 :l: 0.9 - 0.42 :1: 0.60
1 minute 98.5 i 1.09 1.14 :1: 1.61 - 0.36 :l: 0.52
2 minute 86.0 :1: 86.0 9.9 i 8.0 2.13:1: 1.54 1.9 :l: 0.48
3 minute 77.4 :1: 7.88 10.5 :1: 5.1 9.6 :l: 2.48 2.5 :l: 0.28
4 minute 72.0 :t 3.50 13.3 :1: 3.13 11.9 :1: 0.72 2.8 :t 0.34
5 mimlte 57.0 :1: 1.72 28.7 i 2.8 _ 11.5 :1: 0.58 2.8 :l: 0.49

 

 

 

 

 

The position of C-C/C=C linkage is empirically ﬁxed at 284.6 eV, C-O at ~ +1.5eV,
C=O at ~ +3.2eV and -COOH at ~ +4.2eV. As can be observed from the table
above, it is not possible to ﬁx the position of the peaks based on the above empirically
ﬁxed positions and conclude about the nature of peak growth and patterns. The problem
is complicated by the fact that there is no documented evidence about the position of OS
bond as present in the system under investigation. Earlier work by Briggs et.al. on
chromic acid etching of polymers where sulfonic acids are introduced [ 37 ] shows that
the shift due to C-S03H linkage should be ~1-2 eV. Their investigation also reports the
introduction of the different kinds of carbon listed above. In the presented data it is not
possible to assign ﬁxed positions to the respective linkages conclusively due to the
absence of past literature values. The major conclusion which can be made is that the
contribution due to C-C/C =C/C-H linkage goes down progressively. In other words,
the contribution due to C-8, C-0, C=O, -COOH goes up. This means that as

sulfonation time is increased more sulfur and oxygen compounds are introduced in the

 

85
system. It is also possible that the mobility of the peaks is not an experimental artifact

but indicates that there actually is no strict control over the kinds of chemical species that
are introduced into the system. This is possible because the surface undergoes
degradation as it is sulfonated. There is no control over the nature of substance degraded

as it is dependent upon several factors beyond control.

 

    

 

 

 

 

 

 

 

. 5 \\\\\\\\\\\\\\\\\\\\\ , --;»:~ 2843-2847
333% \\\\\\\\\\\\\‘\\\\\\\\\\\\\\\\\\\\8 .3 g 2884,39
§2 W513

g1 WW1

 

I:

on“

0 10 20 30 40 50 60 70 80 90100
Percentage Contribution

 

Figure 6.13: The ﬁgure shows the contribution of different peaks in the Cls-

envelope. The contribution of C-C linkage decreases with increasing time.

Oxygen is the next most abundant element in the system after carbon. The IR
data shows that it will be available as oxygen doubly to sulfur (sulfonic acids, sultones)
oxygen singly bonded to sulfur (sultones), singly and doubly bonded to carbon

(hydroxyls, ketones, carboxylic acids). The values for sulfur species are not available.

86

But, on the basis of ﬁrst principles, it can be concluded that the values will be in the same
range as carbon since the Pauling electronegativity of carbon and sulfur is the same.
Deconvolution of the oxygen peak yields the following results. The percentage

contribution of the different peaks has been shown in Table 6.5.

Table 6.5: The table illustrates the peaks resulting due to deconvolution of oxygen
peak. The contribution of the 53l.8-532.5 eV region is maximum. This is also the

region where most of the O = S signal falls.

 

530.5-531.5 eV 531.8-532.5 eV 533.0-533.6

 

0 minute 17.4 i 2.45 53.7 :t 2.94 28.9 :l: 5.40
lminute 23.3 i 17.74 55.9 :1; 14.8 20.8 :i: 2.92
2 minute 15.2 :l: 0.57 62.0 :1: 3.8 22.8 :1: 3.23
3 minute 17.3 i 3.6 62.2 :l: 6.66 20.4 :l: 3.07
4 minute 19.6 :1: 0.92 63.9 :1: 0.47 16.5 :l: 1.34
5 minute 12.9 :1: 2.02 69.4 i 7.42 17.6 :1: 5.40

 

 

 

 

 

 

L J

As the above data shows, the peak region 531.8-532.5 eV contributes maximum towards
the peak. Most of the oxygen in the system is linked to sulfur ( sulfonic acids, sultones
etc). Thus, this region can be assigned to this kind of oxygen. This is in
consonancewith work done by Clark et. al. [ 38 ]. They estimated that the oxygen of
polysulfone would lie around 532.5 eV. The other carbons present in the system are of
the kind O-C, O=C, -COOH. In the same work Clark et. al. ﬁxed the value for singly

and doubly bonded oxygen in 533.5 eV region. The envelope 533.0—533.6 eV can be

87
assigned to these kinds of linkages. The lowest energy envelope ( 530.5-531.5 ) is

difﬁcult to assign due to a lack of relevant literature. Probably, this is a result of

charging as is generally associated with polymers.

 

NR8 I 5300-5315
531.8-532.5

533.0-533-6

 

 

 

 

 

 

 

 

 

 

 

 

Time (min)

 

 

 

 

 

 

 

 

I: rv II i. ii]

"‘l "'l""l' 'l' 'l' ‘l""l""l’ ' "I
0 10 20 30 40 5O 60 70 80 90100

Percentage contribution

Figure 6.14: The ﬁgure illustrates the contribution of the different deconvoluted

peaks in the Ols peak.

88

6.6 CONTACT ANGLE RESULTS

The contact angle results are important to study changes occuring on the top most
surface. For the present work, contact angles were measured directly using a Rame-Hart
Goniometer Model 10000 with deionised water as the probe liquid. 5 11L drops were
manually placed on the surface and the contact angle measured through a calibrated eye-

piece. 15-20 drops were measured for each sample. The results are presented below.

Table 6.6: The table shows the contact angle obtained on sulfonated polypropylene
using water as a probe liquid. Note that at 1 minute treatment the contact angle

does not change signiﬁcantly.

 

   
   
 
 
   
    

Contact Angle (0)
(degrees)

 

90.7 :l: 5.4 .
1 minute 82.8 i 5.9 ' 0.125

- 2 minute 14.5 :1: 5.7 0.968
3 minute 20.8 :1: 5.3 0.935
4 minute 18.6 ;|: 3.9 ‘

 

 

 

5 minute 18.3 i 4.9

 

There are certain important observations which were made in the process of measuring
these results. Some drops tended to spread as soon as they were placed on the surface.
They tended to asSume a random shape and spread unevenly. At times this spreading

was time dependent. That is, the drop would stay static for sometime and then start to

89 '
spread (within 30-45 seconds). This made the data collection difﬁcult. If the drops
were not sufﬁciently far apart then these drops would tend to merge with each other.
This points to the. chemically uneven surface that may have resulted due to sulfonation
and uneven degradation. The data clearly indicates that the ﬁrst minute of treatment does
not change the surface appreciably. This is also supported by the ESCA data where the
percentage change in atomic concentration of the respective elements is insigniﬁcant and
IR data ‘where the nature of chemical change is formation of additional oxygen
compounds and very small sulfonation. The surface tends to reach a saturation state

which is indicated by the constant contact angle achieved» (18.5“).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1.1
‘ W‘ " :3 L]
0.9 /
0.7
E /
8
7,; 05
E
m
8 /
0.3
01/
'0.1 I I T 1 v 1 v x v v f r j t I if: I 1 I I
0 l 2 3 4 5

Time of sulfonation (min)
Figure 6.15: The ﬁgure shows the change in contact angle of water on

polypropylene with increasing time of sulfonation. Note the saturation reached on

the surface at about 2 minutes.

90
6.7 DEGRADATION OF POLYPROPYLENE.

It Was observed that due to sulfonation the surface of polypropylene would
discolour to brown. This layer would slough off during neutralisation in ammonium
hydroxide. This was especially true for higher times of sulfonation. The phenomenon
of discolouration has been reported in earlier works by other investigators also. But,
there is no documented evidence about the nature of the degraded material. For e. g.
Car'neron et. al. concluded that it may be a completely carbonised layer of polypropylene.
To investigate the nature of this brown degraded material, it was collected from the
surface of the degraded polypropylene by scratching the surface with a razor blade prior
to neutralisation. This was then air-dried and a KBr disk was made with it to carry out
an IR analysis. The results are shown in Figure 6.16. The ﬁgure clearly indicates that
the degraded material is not a plain carbonised layer. It has several species the most
dominant of which is the broad sulfur compounds peak located at 1176 cm“. The peak
at 1630 cm'1 shows the presence of -C=C- kind of linkage. Also, there is a shoulder in
this composite peak at 1700 cm’1 which indicates the C=O kinds of species from
ketones, carboxylic acids etc. The broad peak at 3400 cm" is attributed to -OH kind of
linkages. Notice the absence of the -NH peak which has been seen in all the samples that
have been neutralised with ammonium hydroxide. All the above information shows that
the degraded material consists of a mixture of several kinds of sulfur compounds along

with oleﬁnic carbons, ketones etc.

91

 

60‘

50%—
1
40{e
j

30{-

Transmittance

20{—
10{»

0 .

l-OH

I l L l

 

 

4000

Figure 6.16: The ﬁgure illustrates the IR spectra of degraded polypropylene. The

spectra was collected by preparing a KBr disk with degraded material scraped off

I 7 I I I l I I 1 I V

I r

q=s=o .

. I . .4 . I 4 .-., . . ..
3500 3000 2500 2000 1500 1000

Wavenumber (cm' 1)

the surface of sulfonated polypropylene.

 

92

6.8 DISCUSSION

The data presented above clearly shows that major changes occur on the surface
of the polymer as a result of sulfonation leading to a more polar surface. A comparison
of the atomic concentrations of untreated polymer with 1 minute polymer shows that
chemical change on the surface has not been signiﬁcant. The contact angle also shows
that the drop in contact angle values is insigniﬁcant and the IR data indicates that
sulfonation is not the major impact of treatment in the ﬁrst minute. The deconvoluted
ESCA peaks of the untreated and lminute treated sample indicates that the nature of
carbon has essentially remained same. But, the adhesion values clearly show a marked
improvement [ 25 ]. This means that some other effect besides chemical modiﬁcation
contributes towards adhesion. In the case under investigation it seems that the impact
of 1 minute treatment has been the removal of the Weak Boundary Layer on the surface
of untreated polymer besides addition of some oxygen species. probably in form of
alcohols (as shown in IR data). The removal of this layer leaves behind a chemically
pure surface which leads to improved adhesion.

To test the efﬁcacy of sulfonation over other available techniques, polypropylene
was ﬂame treated and acid etched as these are the most popular techniques. The ESCA
results were compared with untreated and 1 minute sulfonated sample. The atomic

concentration values are tabulated below.

 

 

93

Table 6.7: The data shows the atomic concentration comparison of flame treated,
acid etched, 1 minute sulfonated and untreated polypropylene. Note that the major

impact of ﬂame treated and acid etching has been oxidation of the surface.

L

 

    

 

 

 

 

 

 

Untreated 95.7 3.55 0.57 Si
1 minute 93.4 5.4 1.0 F
Flame etch 82.7 16.4 - Cl, P

Acid etch 91.0 8.6 0.5 -

 

 

The data shows that the net impact of ﬂame treatment and acid etching has been
oxidation of the process. The adhesion results point that it has also helped in removal
of the Weak Boundary Layer. These two factors together contribute towards increased
adhesion [25 ]. 1
However, after the ﬁrst minute of treatment there is a distinct change in the
chemical nature of the surface. This is mainly characterised by the increase in oxygen
and sulfur levels as detected by XPS. As was discussed in earlier sections, the nature
of data points to the formation of species other than sulfonic acids. They include alkene
sulfonic acids, sultones, ketones, carboxylic acids, hydroxy sulfonic acids etc. It cannot
be determined at present as to the nature of growth of each of these individual species.
Extensive careful and controlled work is required in this area due to the highly reactive
nature of 803. The deconvoluted XPS data also shows that oxygen containing species
increase on the surface as sulfonation time is increased until a state of saturation is

reached when no more sulfur - oxygen species can be placed on the surface Without

94

extensively damaging the surface. This proposed state of saturation is also reinforced
by the contact angle data. After 2 minute treatment the contact angle assumes almost a
constant value of about 18 °. As the surface is being chemically saturated a competitive
phenomenon is also occuring simultaneously. That is of degradation. Sulfonation is an
extremely exothermic reaction. The energy released can contribute towards disrupting
the surface. Also, as was shown in earlier discussion, sulfonation of the different sites
leads to a complex mixture of rearranged products the exact nature of which cannot be
commented upon. This requires chain movement which coupled with the fact that the
new chemical groups (1 -HSO3, NH,“SO;“, -OH, sultones ) are bulky leads to
concentrated strains on the backbone chain of polypropylene contributing towards
disruption. The IR data of the degraded material clearly shows that carbonisation is not
the dominating effect as was proposed by Cameron et.al. Various chemical species are
being formed dominated by the S =O kind of linkages. These include sultones, alkene

sulfonic acids etc. In the process S02 and C0; are released.

6.9 CONCLUSIONS.

It has been shown above that sulfonation of polypropylene leads to a mixture of
various kinds of compounds besides sulfonic acids. The surface is a complex
arrangement of conjugated and unconjugated carbons, various kinds of sultones, hydroxy
sulfonic acids, carboxylic acids and ketone moieties. There is a ceiling on the amount
of sulfonation that can be carried out on the surface as indicated by contact angle and
XPSA data. At higher times of sulfonation the polymer surface tends to scission leading

to degradation.

Sulfonation of Polystyrene

 

Chapter 7

 

In the previous. chapter the effect of sulfonation on polyprOpylene was discussed.
Polypropylene is an aliphatic polymer with no phenyl rings in its structure. In order to
study the effect of sulfonation on a polymer system containing phenyl rings in its
backbone, polystyrene was chosen. Polystyrene is used in a wide range of applications
varying from industrial to daily life. In applications where adhesion is important, the
surface properties of polystyrene are critical. Sulfonation is a surface modiﬁcation
technique which enhances the adhesion capabilities by incorporating polar groups on the
surface, thereby resulting in improved wetting. Investigations into adhesion properties
of (sulfonated) polystyrene have been studied by other investigatiors [25]. The present
discussion will focus on the chemical changes which occur on the surface of the

polystyrene as treatment time is varied.

7 .1 NATURE OF POLYSTYRENE

Polystyrene contains phenyl rings in its backbone. The repeating unit of
polystyrene has been shown in Figure 7.1 . As can be seen in the ﬁgure, polystyrene
is characterised by a phenyl ring bonded to a secondary carbon. The repeating unit does
not have a primary carbon as in the case of polypropylene discussed in the previous
chapter. The reactivity of this carbon should be similar for all polymers which contain

phenyl rings in their. backbone.

95

96

{on—on}

Figure 7.1: The figure illustrates the repeating unit for polystyrene.

The probability of sulfonation at the phenyl ring as compared to the aliphatic
carbons is much greater due to the fact that sulfonation is a highly electrophilic reaction.
In addition the concentration of carbons in the phenyl ring compared to aliphatic carbons
is also much higher. The reaction at aliphatic carbons does take place however. Phenyl
ring has a resonance structure. It can be seen from the structure of polystyrene that the
phenyl ring is bonded to -CH. Due to the ortho and para directing inﬂuence of -CH (-
CH is electron donating) these are the probable sites for sulfonation [40]. Also, since
sulfonic acid is a bulky group, steric hinderance will make the para position more
susceptible to attack as compared to ortho position. Thus, it would be expected that
during sulfonation, ortho and para positions will be the most probable sites for attack
with para position as the most favoured site. It is well-documented in the literature that
benzene ring has an extremely stable structure due to the extra stability provided by the
delocalised electrons. Due to this reason the phenyl ring (paralleled from benzene) stays
intact in the presence of powerful oxidants like nitric acid and sulfuric acid [43].
Aliphatic compounds do not have this ability to stay intact due to absence of delocalised

electrons which makes them more susceptible to degradation in presence of such a harsh _

97

chemical environment. This was ‘ discussed in the previous chapter while focussing on
the sulfonation of polypropylene. In order to investigate the nature of chemical changes
effected due to sulfonation of polystyrene, several analytical techniques were used. The
observations and details from each of them will be discussed in the following sections.
The following results are for polystyrene micro-washed and treated with 1% (10.2)
SO3-air mixture for 1 thru 5 minutes followed by neutralisation with ammonium

hydroxide.

7 .2 FTIR-SPECTROSCOPY RESULTS
‘ Several changes occur on polystyrene as a result of sulfonation. The main peaks
and bands have been tabulated in the Table 7.1. Figure 7 .2 illustrates the IR data of
sulfonated polystyrene (in transmission mode) and Figure 7.3 illustrates the subtracted
spectra (in absorbance mode, sulfonated minus unsulfonated) which highlights the

changes due to sulfonation [31,34,35].

98

Table 7.1: The table illustrates the relevant peak positions and the probable
chemical species associated with them.

 

Peak Position (cm") Possible Chemical Species
696,748 Monosubstituted benzene 1

1008,1034 Monosubstituted benzene

1450 C =C skeletal in-plane vibration
1620-1680 C=C, water

1040 O = S = 0 str.

1168 O = S =0 str.

3200 -NH_ of ammonium

3400 -OH of alcohol, phenol, water and
3000-3100 -CH vibrations of phenyl ring

 

 

 

The major bands that appear after sulfonation are as follows.
- A sharp band at 1040 cm".
- A broad band at 1170 cm“.
- A broad band in 1400-1500 cm‘1 region which encompasses two peaks.

- A broad band at 3200 cm".

The bands at 1040 em" and 1170 cm‘1 can be attributed to O=S=O str. vibrations
[31]. These are associated with sulfonic acid groups. The exact nature of chemical
species in the system under investigation will be discussed in the following sections. The
band region around 1430 cm‘1 can be a contribution from -NH of ammonium and also
S :0 vibrations [31]. This band although broad does not extend into the 1700-1800 cm"
region showing the absence of C=O kind of species. The broad band in 1620-1680 cm’1

is attributed to water (absorbed by the hygroscopic sulfonic acids) and/or C=C kind of

99
linkages. There are two notable bands to the left of 3000 cm". The band at 3200 cm'1

is attributed to -NH of NH.+l [34]. Note the extremely low intensity of the peculiar
band at around 3030 cm’1 which was found in polypropylene. This was attributed to -
C=C- kind of linkages. There is also a broad band at 3500 cm" which can be attributed
to -OH resulting from water and phenol/alcohol. The exact nature of this band will be
decided in the following discussion. Thus, from the above it can be concluded that the
following ‘chemical species are present in the system under investigation.
- O=S=O kind of chemical linkages.
- -OH kind of chemical linkages. I
- -NH kind of chemical linkages.
- -C=C- kind of chemical linkages.
7 .3 DISCUSSION OF RESULTS

The results listed above agree with the literature available on sulfonation of
polystyrene [35]. The formation of sulfonic acid species is the main effect of sulfonation
on polystyrene. Also, note that unlike polypropylene, the nature of 'spectrum the is
almost same throughout for 1 minute through 5 minute sulfonation. This indicates that
most probably all the chemical changes occur on the surface in the ﬁrst minute thereby
leading to a saturated surface. This proposition is also supported by the XPS and contact

angle data which will be discussed later.

100

 

 

. Transmittance
\)
LII

 

 

 

 

7 l l J l J l
50 I l I I l I I I I l I I I I I I I I I T I I I I I I f

. r . .
3000 2500 2000 1500 1000 650
Wavenumber (cm- 1)

Figure 7.2a: The ﬁgure illustrates the IR spectrum of unmrlfonated polystyrene.

 

 

957W I
I

1L4

Transmittance

85-
75-3

65-{

-OH

-N

55;-

 

l l l l l

45

. o=s=o

 

J

 

TI‘l""l""l"'rl"71'
2500 2000 1500

Wavenumber (m“)

Figure 7 .2b: The ﬁgure illustrates the IR spectrum of polystyrene sulfonated for

lminute followed by neutralisation with ammonium hydroxide.

I77 f l I

1000

650

101

 

 

Transmittance

 

 

 

 

1 1 l l l l l 1
50 I I V T I I I I T I I I T f I I Tﬁ T I I I I I W l l -

I r ' 7 .
4000 3500 3000 2500 2000 1500 1000 650

 

Wavenumber (cm- 1)

Figure 7.2e: The ﬁgure illustrates the IR spectrum of polystyrene sulfonated for

2minutes followed by neutralisation with amonimn hydroxide.
95

 

90
85
80
75

o=s=o

7O
65

Transmittance

 

 

 

l l l l l l
55 l I I I I I I I I I l I I I r I r T T I I I I I r I j I I I j

. , .
4000 3500 3000 2500 2000 1500 1000 650

 

Wavenumber (cm- 1)
Figure 7.2d: The ﬁgure illustrates the IR spectrum of polystyrene sulfonated for 3

minutes followed by neutralisation with ammonium hydroxide.

102

 

80:

I
d
-
q
u:

Transmittance
\l
LII
l

 

 

 

“ I l l l l l
55 I I I ‘l I I I f I T I I I I I I I T I T I I I I I I I I f

, 1 ,
4000 3500 3000 2500 2000 1500 1000 650

 

Wavenumber (cm-1)

Figure 7.2e: The figure illustrates the IR spectrum of polystyrene sulfonated for

4minutes followed by neutralisation with ammonium hydroxide.

 

 

 

 

951—
. 5 z:
85':— Z.
0 I
o d
5 75i- 9
'§ : %
‘ o
g 651'
{- .
55%-
‘ 1 1 1 L 1 1
45 T I I I I ﬁI I I l I I I I ‘ I I I I Iﬁ I I I I I T 1

 

r , . . ,

4000 3500 3000 2500 2000 1500 1000 650
Wavenumber (cm- 1)

Figure 7.2f: The figure illustrates the IR spectrum of polystyrene sulfonated for 5

minutes followed by neutralisation with ammonium hydroxide.

103

0.06

 

ﬂ
4

0.05 {-

.J

S=O

O:

0.04 {-

-NH

0.03 {—

4

-OH

 

Absorbance
C / H20

l

002—:

C

 

l

0.01-

1
1
.I
01.1 I11. 1 1 111 L

[’Y‘ TIITrTIIWfI

I I I I I r .
4000 3500 3000 2500 .2000 1500 1000 650

 

 

 

I r

Wavenumber (cm- 1)
Figure 7.3a: The difference spectra between 1 minute sulfonated polystyrene and

unsulfonated polystyrene.
0.07 .

.1

 

0.06 {—

ﬁ

0.05 {-

o=s=o

 

0.04%

 

Absorbance

0.03~j
0.025

0.01 -5

.I
l l l l l l
O I 7 Y I T Y I T T F ﬁr 1 T v 1 1 r 1 v I I I I

7 Y I ‘ I 1 l ' ' '
4000 1 3 500 3 000 2500 2000 l 500 1 000 650
Wavenumber (cm'l)

 

 

 

 

Figure 7.3b: The difference spectra between 2 minute sulfonated polystyrene and

unsulfonated polystyrene.

Absorbance

104

 

(109
(108
(107
(106
(105
(104
(103
(102
(101

0 1

4000

Ii

t

-OH

1

I 1 T I I I

3500

-NH

1

1'
3000

o=s=o

 

C\H20

C:

 

 

 

l l 4 l J

IIIIT III1II—Tﬁll

. I I .
2500 2000 1500 1000 650
Wavenumber (cm'l)

Figure 7.3c: The difference spectra between 3 minute sulfonated polystyrene and

unsulfonated polystyrene.

Absorbance

 

0.09 .
0.08-3
0.07—i

53

O

O‘
1111 L11

(105 1

53
c>
.n

I [114111

0.03-f
002-3
001-:

q

 

0 .

 

O=S=O

 

 

 

 

l

 

4000

rf

j

.I I
3500

I 1'
3000

III # f I I TT I fj Iﬁ I

2500 2000 1500

I I . .
1000 650
Wavemmbedcm’l)

Figure 7.3d: The difference spectra between 4 minute sulfonated polystyrene and

unsulfonated polystyrene.

105

014‘

 

012{—

01-}

(red)

008§L

 

Absorbance

006:
1

004{

 

002§

 

 

 

‘ 4L 1 1 1 1
O Y T I I . I I f I I I I I W I I I I I I I I I

v v 1 ' I 1 1
4000 3500 3000 2500 2000 1500 1000 650

 

I T —I‘

Wavenumber (cm'l)
Figure 7.3c: The difference spectra between 5 minute sulfonated polystyrene and

unsulfonated polystyrene.

106
7. 3. 1 Proposed Reaction Scheme

Unlike polypropylene, there is no documented evidence of degradation of
polystyrene under the inﬂuence of SO3 /HZSO4 or other strong oxidants. In the present
work also no degradation was observed. There was no degraded/ low molecular weight
material which sloughed off on neutralising. This inertness against strong oxidants has
been attributed to the stability imparted to the structure due to the delocalised electrons
of the phenyl ring. Desulfonation of sulfonated species is a low probability event in
polystyrene because the probability of formation of H280, by the extraction of 1 hydrogen
from the neighbouring carbon is extremely low because of the aromatic nature of the
ring. In the case of polypropylene, all the carbons were aliphatic which makes the
extraction of hydrogen easier and thus formation of H280, easier.

The largest fraction of carbons in the polystyrene polymer are contained in its
phenyl rings. As the data shows, the ortho and para positons are the main sites for
attack by 803 due to its high electron density and steric availability resulting in formation
of o,p-sulfoni_c acids. However, there is a probability of the aliphatic carbons also being
sulfonated albeit very less. However based on these IR results it can be concluded that
sulfonic acids are the most probable species in the system linked to the phenyl ring
primarily at the para position and less at the ortho position. The proposed scheme of
reaction has been shown in Figure 7.4 and 7.5. Para substituted sulfonic acids are the
main product along with ortho substituted sulfonic acid. However, as was indicated
earlier due to steric hinderance the probability of formation of ortho sulfonic acids is

less. This is in consonance with the past literature available on sulfonation of polystyrene

[35].

 

107,

 

303, NH,0H

N114803 para sullbnic acid

 

$03, NH,0H

 

. CH—CHZ—
NH.ISO3

ortho sulfonic acid

Figure 7 .4: The figure illustrates the formation of aromatic sulfonic acids on
reaction with $0,. The para position is the most probable site of attack due to ease

of steric reasons.

108

 

 

03
so3
—CH-CH2— > —CH-—CH—
(A)
so3
’ 803
—CH—CH-—— > —CH=CH-

0 ©

Figure 7 .5: The figure illustrates the formation of alkene species during sulfonation

of polystyrene.

109
7.4 XPS OBSERVATIONS AND RESULTS

As was pointed out in the previous chapter, X-ray Photoelectron Spectroscopy
(XPS) is an surface sensitive technique analysing the top 2-5 nm of the surface. Similar
analysis was carried out for polypropylene. In this section XPS results for polystyrene
have been discussed.
7. 4. 1 Quantitative Analysis of XPS Results.

The following table gives the atomic concentrations of the different elements in
percentage terms.

Table 7.2: The table shows the data for atomic concentration for polystyrene
sulfonated for 1 thru 5 minutes as obtained from XPS.

 

 

     

0 minute 96.7 i197 3.2 i184 - -
II 1 minute 61.4 :1: 0.20 24.5 i 0.32 6.9 :l: 0.03 6.84 i 0.60
.l 2 minute 61.1 :1: 0.70 24.6 :1: 0.42 6.9 i 0.08 7.3 :l: 0.2

 

 

 

3 minute 60.8 i 0.66 24.4 i 0.55 7.0 i 0.05 7.5 i 0.45
4minute 61.7 :1; 1.35 23.7 :1: 1.81 7.1 :t 0.16 7.1 i 0.07

5 minute 61.2 :1: 2.44 24.1 1: 2.18 7.1j; 0.14 7.5 :t 0.11 ll

 

 

 

 

 

 

 

 

The following observations can be made from data in the above table.

(i) The percentage composition of the untreated sample is changed in the very ﬁrst
minute of treatment and stays constant with additional treatment.

(ii) The percentage concentration of the various elements are a c0nstant within the

experimental errors. Unlike polypropylene where there were marked changes with

110

treatment time in polystyrene there is no detectable change. The surface saturates in the
ﬁrst minute.

(iii) No visible signs of degradation (e.g. darkening) were seen on the surface.

 

I Nitrogen
I Sulfur

I Oxygen
I Carbon

 

 

 

Percentage Contribution

   

 

 

 

 

 

 

0 1 2 3 4. 5
Time of sulfonation (min)

Figure 7.6: The figure illustrates the atomic concentrations of various elements
present in polystyrene as a result of sulfonation. Notice the constancy achieved in

. all the samples.

111

The changes in terms of atomic ratios has been shown below.

Table 7 .3: The data presented in the table shows the reSpective atomic ratios using
the data from Table 7 .2. Notice the constant saturation levels reached in the first
minute of treatment itself. The ratios stay constant throughout.

 

 

0 minute 0.03 :l: 0.02 - - -

 

1 minute 0.39 i- 0.007 0.112 0.000 3.54 :t 0.063 0.919 :1: 0.084

 

2 minute 0.40 :1: 0.011 0.113 :1: 0.002 3.54 :1: 0.013 1.053 :1: 0.016

 

3 minute 0.40 i 0.013 0.115 :t 0.002 3.48 :i: 0.054 1.063 :1: 0.056

 

 

4 minute 0.38 :1: 0.038 0.115 :1: 0.000 3.32 :l: 0.336 0.997 :l: 0.032

 

 

 

 

 

 

5 minute 0.39 :1: 0.052 0.116 :1: 0.007 ‘ 3.39 :1: 0.239 1.051 d: 0.006
E—E—

It is clear from the above table that a state of saturation is cached on the surface of
polystyrene in the ﬁrst minute of treatment itself. Observation of the above data shows
that the 0/s. ratio rests around 3.3 while ideally it should be almost 3.0. This
descrepancy can be attributed to the presence of moisture due to the hygroscopic nature
of sulfonic acids. This is supported by the IR spectra also where peak region 1600-1700
cm’1 is a contribution from moisture. However, an interesting result is that N/ S ratio is ,
ahnost unity throughout. This indicates that there has been almost complete
neutralisation of the sulfonated species. In other words, all the sulfur containing species
have the hydrogen which can be replaced by NH.+ from ammonium hydroxide. This
further supports the conclusion that the formation of sultones species is almost negligible

in sulfonation of polystyrene when the hydrogen for replacement with NHﬁ is not

 

 

1 12
available.

7. 4. 2 Qualitative Analysis of XPS Results

The process of analysis of XPS data for polystyrene was similar to that followed
for polypropylene. Since the main linkages are of C-C/C-H type, the Cls peak was
chosen as the reference peak and all the Cls peaks obtained were shifted to 284.6 eV.
Correspondingly, the 015, 82p and le peaks were also shifted by the same amount
respectively. The ﬁnal peak positions assumed by the respective elements with Cls at
284.6 eV are 531.8 :1; 0.08 eV for 015, 167.9 i 0.1 for' 82p, 401.8 :1: 0.1 for le.
These values are in good agreement with the values obtained for Polypropylene (531.8
j; 0.16 for Ols, 167.6 :1: 1.63 for 82p and 401.8 1 0.2 for N15). Thus, it can be
concluded that essentially the nature of species being formed in both polymers is the
same.

Since carbon is the main constituent of the system and undergoes maximum
chemical changes, Cls peak was deconvoluted to determine the nature of chemical
changes occuring on it. The data shows that only one peak can be fitted in the main
peak located at 284.6 eV. Now, from IR data and previous literature it is known that
formation of sulfonic acid at ortho and para position is the main chemical change
occuring in the system. Thus, it implies that C-S linkage is located very near the main
-C-C/C-H/C=C peak. This is in agreement with the work done by Clark et. al.[38].
They proposed that the substituent effect of sulfur on Cls is very small (~ 0.4 eV). The
other interesting point which emerges from the deconvolution of Cls peak is the absence

of peaks related to C-O/C =O/-COOH. This supports the propositions that almost all the

113
oxygen present in the system is bonded to sulfur ( 4180,) as illustrated in the reaction

scheme presented in Figures 7.4 and 7.5.

7.5 CONTACT ANGLE RESULTS

In order to characterise the topmost layer of the surface contact angle analysis was
carried out with water as the probe liquid. A Rame-Hart Goniometer Model 100-00 was
used. 5 pl drops were placed manually on the surface with a pipette and the contact
angle measured through a calibrated eye-piece. Almost 10-15 drops were measured for
each sample. The results are presented in Table 7 .4 below.
Table 7 .4: The table shows the contact angles obtained on sulfonated polystyrene

using water as a probe liquid. Note the saturation achieved in the first minute of
treatment itself.

[ 'l Contact Angle (0) cosme 0
--(degreos)

 

      
 

 

 

As in the case of polypropylene, the drops tended to spread unevenly on the surface in
this case also. The drops would collapse when placed on the surface and had to be
placed sufﬁciently far apart so that they do no merge into each other. The data however,
clearly supports the proposition that the surface is chemically altered in the ﬁrst minute

of treatment.

114

 

 

 

 

 

o o .
Ll! \)
\

 

cosme(theta)
p . .
U

\

\

 

0.1

 

 

 

 

 

 

 

0 1 2 3 4 5
Time of sulfonation (min)

Figure 7.7: The plot illustrates the variation of contact angle of water on sulfonated

polystyrene as time of sulfomuion is varied.

7 .6 DISCUSSION _

The data presented in the preceding sections shows that the surface of polystyrene
is saturated with sulfonic acid species in the ﬁrst mimite of treatment itself. The high
rate of reaction is attributable to the highly electrophilic nature of the sulfonation
reaction. The high density of electrons on the phenyl ring drives the reactive S03

towards the ring. This is also aided by the greater concentration of phenyl ring carbons

 

115
as compared to the aliphatic carbons. In polypropylene, desulfonation occurs by

extraction of a hydrogen from a neighbouring carbon. This is not the case in polystyrene
because extraction of hydrogen from the phenyl ring is extremely difﬁcult because of the
resonance stabilisation of the phenyl ring. The XPS data along with the IR data shows
the absence of any kind of oxygen carbon linkage. This indicates that the formation of
B-sultone, pyrosultone etc. species is a highly unlikely process in polystyrene. ' Also, as
pointed out earlier the carbons in the phenyl ring are in a greater concentratin than
aliphatic carbons in this polymer which increases the probability of SO3 reacting with

the ring.

7 .7 CONCLUSIONS

The sulfonation of polystyrene results in an extremely high energy surface in the
ﬁrst minute of treatment itself. The IR data alongwith XPS data shows that sulfonic acid
species at ortho and para positions is the most dominant chemical species which results
due to sulfonation. Also, the surface is saturated within the ﬁrst minute of treatment

itself. There are no signs of degradation or desulfonation in polystyrene as was seen

in polypropylene.

Conclusions

 

Chapter 8

 

The following conclusions can be drawn from the presented work.

1. The sulfonation of polymer surfaces is a highly electrophilic reaction as has been
shown by the results on sulfonation of polypropylene and polystyrene. 80;, tends to react
at centers of high electron density. In polypropylene this center is at tertiary carbon and

in polystyrene it is the aromatic ring .

2. Under the given conditions of reaction it is observed that desulfonation (which
results in formation of H2803) occurs easily in case of polymers containing only aliphatic
carbons as compared to a polymer containing aromatic carbons. This can be explained
on the basis of ease of hydrogen extraction from the neighbouring carbons which is
required to form H2803. Hydrogen extraction is easier in aliphatic systems because it
results in the formation of a stable system of doubly bonded carbons while an aromatic

ring is already stable due to the resonance structure.

3. The surface of polymers cannot be sulfonated beyond a limit. The main reason
for this behaviour can be attributed to the large volume occupied by the -NH,SO3 group.
As this gets incorporated in the system, there is chain movement in the polymer to
accomodate it. After a limit spatial restrictions do not permit additional -NH4SO3 groups

which leads to degradation.

116

117

In polypropylene the limit is reached in about 3 minutes of treatment time ‘while
in polystyrene the surface is saturated in the ﬁrst minute itself. Also, it was observed
that in polypropylene during the ﬁrst minute the main chemical effect is oxidation of the

surface rather than incorporation of sulfonic acids.

4. Degradation of the surface due to desulfonation occurs conspicously in
polypropylene while in polystyrene it is not visible. Although, there may be chain
scission at higher times of sulfonation in polystyrene too. An important reason for this
besides differences in chemical nature of the polymers is crystallinity. Polypropylene is
an ordered crystalline polymer while polystyrene is amorphous in nature. When
sulfonation occurs the most important chemical species being formed (after neutralisation)
is -NH.," 803'. Being a large molecule it would occupy substantial volume. The free
volume is larger in polystyrene due to its amorphous nature as compared to
polypropylene. This results in -NI-I,+ 80; being transported easily in polystyrene. On
the other hand in polypropylene the chain movement is restricted due 1 to crystallinity.
As sulfonation proceeds there is extensive chain movement due to incorporation of -

NH4+ SO; which leads to chain scission and ultimately degradation.

5. The sulfonation of polypropylene leads to formation of several kinds of chemical
species. The ﬁnal product not only contains a conjugated system of doubly bonded
carbons but it has several chemical species which include alkene sulfonic acids, hydroxy
sulfonic acids, ketones and aldehydes. On the other hand in polystyrene it is seen that

the product is mainly para substituted sulfonic acid and lesser amounts of ortho sulfonic

118

acid. The formation of ortho sulfonic acid is less as compared to para because of steric
reasons.

6. Adhesion results show that sulfonation is a viable technique and competitive with
other techniques currently available. The polypropylene results show that for a 1
minute treated sample the failure is interfacial and as treatment time increases the failure
tends to be cohesive in the polymer. For polystyrene the failure is cohesive for all the

treatment times [25].

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