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Irradiation effects on the rheological behavior of composite polymer systems

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Irradiation effects on the rheological behavior of composite polymer systems

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Content Ir r a d i a t i o n E f f e c t s o n t h e R h e o l o g ic a l B e h a v io r o f
C o m p o s it e P o l y m e r S y s t e m s
by
Tao Miao
A Thesis Presented to the
FACULTY OF THE SCHOOL OF ENGINEERING
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree of
MASTER OF SCIENCE
(Chemical Engineering)
May 1995
Copyright 1995 Tao Miao
This thesis, 'written by
Tao Miao
under the guidance of hi& aculty Committee
and approved by all its members, has been
presented to and accepted by the School of
Engineering in partial fulfillment of the re
quirements for the degree of
Master of Science
April 20, 1995
Date..............................................
Faculty Committee
Dedication
To m y parents, m y brother,
and Pei.
Acknowledgments
I would like to express my sincere gratitude to my advisor, Dr. Ronald Salovey,
who patiently reviewed drafts of my thesis. His knowledgeable advice, support, and
encouragement have guided me through my graduate studies and research. I would like
to thank Dr. Wenji V. Chang and Dr. Scott Shaffer for many helpful discussions. It is
my pleasure to thank Karen and Min for the happiness that they have brought to the
department. I am greatly indebted to Qingsong Li, who taught me how to run the
Weissenberg and has given me numerous suggestions in research. I am also greatly
indebted to Mr. Wallace Hall in StyreGenics International Inc. for his support in sample
irradiation. I am truly grateful to Huashi Zhang and Braden in Department of Chemistry
for their kind help in preparing vacuum. I am also very grateful to Dr. Fu-Wen Shen for
his suggestions in crosslinking, to Yifan Zhu and Bill Onstot for sharing sports
enthusiasm, to Jianfen Cai and Xiaoshan Li for helpful discussions. My sincere thanks
extend to faculty and my fellow students in the chemical engineering department, who
have made my stay at USC a memorable experience.
iv
Contents
Dedication ii
Acknowledgments iii
List of Figures v
Abstract ix
1. Introduction............................................................................................................ 1
1.1 Rheology of Filled Polymer Composites......................................................... 1
1.2 Radiation Effect on Polymeric Materials.........................................................12
2. Synthesis of PMMA Particles............................................................................ 31
2.1 Introduction.......................................................................................................31
2.2 Experimental Details...................................................................................... 36
2.3 Results...............................................................................................................37
2.4 Discussion.........................................................................................................42
2.5 Conclusion........................................................................................................ 43
3. Radiation Effects................................................................................................... 46
3.1 Introduction.......................................................................................................46
3.2 Experimental Details...................................................................................... 36
3.3 Results...............................................................................................................37
3.4 Discussion.........................................................................................................42
3.5 Conclusion........................................................................................................43
List of Figures
Fig. 1-1. Viscosity vs. shear rate plot in the shear thinning phenomenon................ 2
Fig. 1-2. The spring-dashpot model of Maxwell model............................................ 4
Fig. 1-3. A cone-plate configuration ..................................................................11
Fig. 1-4. Two types of free radicals generated by irradiation of polystyrene............. 15
Fig. 1-5. Mechanism of PMMA chain scission........................................................... 22
Fig. 2-1. SEM picture of monodisperse PMMA particles. The particle is
10% mole crosslinked with EGDMA in Run 0824 at temperature
of 80 °C............................................................................................................. 38
Fig. 2-2. SEM picture of monodisperse PMMA particles. The particle is
5% mole crosslinked with EGDMA in Run 0829 at temperature
of 80 °C............................................................................................................. 39
Fig. 2-3. SEM picture of monodisperse PMMA particles. The particle is
2% mole crosslinked with EGDMA in Run 0912 at temperature
of 80 °C............................................................................................................. 40
Fig. 2-4. SEM picture of monodisperse PMMA particles. The particle is
10% mole crosslinked with EGDMA in Run 1003 at temperature
of 80 °C.............................................................................................................41
Fig. 3-1. Storage modulus G’ vs. frequency in dynamic mechanical analysis
of PS and irradiated PS.................................................................................... 51
Fig. 3-2. Loss modulus G’ vs. frequency in dynamic mechanical analysis
of PS and irradiated PS.....................................................................................52
Fig. 3-3. Dynamic viscosity T|' vs. frequency of PS and irradiated PS.......................53
Fig. 3-4. Viscosity vs. shear rate from steady shear measurement of PS
and irradiated PS...............................................................................................54
Fig. 3-5. Storage modulus vs. frequency of 10% wt PMMA (10% crosslinked)
in PS matrix of unirradiated composite and composite irradiated
in vacuum............................................................................................................55
Fig. 3-6. Loss modulus vs. frequency of 10% wt PMMA (10% crosslinked)
in PS matrix of unirradiated composite and composite irradiated
in vacuum............................................................................................................56
Fig. 3-7. Dynamic viscosity vs. frequency of 10% wt PMMA (10% crosslinked)
in PS matrix of unirradiated composite and composite irradiated
in vacuum............................................................................................................57
Fig. 3-8. Steady shear viscosity vs. shear rate of 10% wt PMMA (10% crosslinked)
in PS matrix of unirradiated composite and composite irradiated
in vacuum............................................................................................................58
Fig. 3-9. Storage modulus vs. frequency of 10% wt PMMA with different degree of
crosslinking in PS matrix...................................................................................59
Fig. 3-10. Loss modulus vs. frequency of 10% wt PMMA with different degree
of crosslinking in PS matrix.............................................................................. 60
Fig. 3-11. Loss factor vs. frequency of 10% wt PMMA with different degree of
crosslinking in PS matrix...................................................................................61
Fig. 3-12. Steady shear viscosity vs. shear rate of 10% wt PMMA with
different degree of crosslinking in PS matrix...................................................62
Fig. 3-13. Storage modulus vs. frequency of 10% wt PMMA (irradiated at
3.21 Mrad) with different degree of crosslinking in PS m atrix..................... 63
Fig. 3-14. Loss modulus vs. frequency of 10% wt PMMA (irradiated at
3.21 Mrad) with different degree of crosslinking in PS matrix.................... 64
Fig. 3-15. Loss Factor vs. frequency of 10% wt PMMA (irradiated at 3.21 Mrad)
with different degree of crosslinking in PS matrix.......................................... 65
Fig. 3-16. Steady shear viscosity vs. shear rate of 10% wt PMMA
(irradiated at 3.21 Mrad) with different degree of crosslinking in PS
matrix...................................................................................................................66
Fig. 3-17. Storage modulus vs. frequency of 10% wt PMMA with different
degree of crosslinking in PS matrix (irradiated together)...............................67
Fig. 3-18. Loss modulus vs. frequency of 10% wt PMMA with different
degree of crosslinking in PS matrix (irradiated together).............................. 68
Fig. 3-19. Dynamic viscosity vs. frequency of 10% wt PMMA with different
degree of crosslinking in PS matrix (irradiated together)..............................69
Fig. 3-20. Steady shear viscosity vs. shear rate of 10% wt PMMA with different
degree of crosslinking in PS matrix (irradiated together)...............................70
Fig. 3-21. Storage modulus vs. frequency of 10% wt PMMA (irradiated at
3.21 Mrad) in PS matrix in vacuum and in a ir.............................. 71
Fig. 3-22. Loss modulus vs. frequency of 10% wt PMMA (irradiated at
3.21 Mrad) in PS matrix in vacuum and in a ir...............................................72
Fig. 3-23. Dynamic viscosity vs. frequency of 10% wt PMMA (irradiated at 3.21
Mrad) in PS matrix in vacuum and in air........................................................ 73
Fig. 3-24. Steady shear viscosity vs. shear rate of 10% wt PMMA
(irradiated at 3.21 Mrad) in PS matrix in vacuum and in a ir.........................74
Fig. 3-25. Storage modulus vs. frequency of 10% wt PMMA with different
degree of crosslinking in PS matrix (irradiated together in air and in
vacuum)........................................................................................................... 75
Fig. 3-26. Loss modulus vs. frequency of 10% wt PMMA with different
degree of crosslinking in PS matrix (irradiated together in air and in . . .
vacuum).......................................................................................................... 76
Fig. 3-27. Dynamic viscosity vs. frequency of 10% wt PMMA with different
degree of crosslinking in PS matrix (irradiated together in air and in
vacuum)........................................................................................................... 77
Fig. 3-28. Steady shear viscosity vs. shear rate of 10% wt PMMA with different
degree of crosslinking in PS matrix (irradiated together in air and in
vacuum)........................................................................................................... 78
Fig. 3-29. Fracture surface of PS matrix filled with 10% wt 10% crosslinked
PMMA without being irradiated..................................................................... 79
Fig. 3-30. Fracture surface of unirradiated PS matrix filled with 10% wt
10% crosslinked irradiated PMMA................................................................ 80
Fig. 3-31. Storage modulus vs. frequency of 10% wt PMMA (10% crosslinked)
in PS with composite irradiated in vacuum and only PMMA particles
irradiated in vacuum......................................................................................... 87
Fig. 3-32. Loss modulus vs. frequency of 10% wt PMMA (10% crosslinked)
in PS with composite irradiated in vacuum and only PMMA particles
irradiated in vacuum......................................................................................... 88
Fig. 3-33. Dynamic viscosity vs. frequency of 10% wt PMMA (10% crosslinked)
in PS with composite irradiated in vacuum and only PMMA particles
irradiated in vacuum.........................................................................................89
Fig. 3-34. Steady shear viscosity vs. shear rate of 10% wt PMMA (10% crosslinked)
in PS with composite irradiated in vacuum and only PMMA particles
irradiated in vacuum.........................................................................................90
Fig. 3-35. Storage modulus vs. frequency of 10% wt PMMA (10% crosslinked)
in PS with composite irradiated in air and only PMMA particles
irradiated in air.................................................................................................92
Fig. 3-36. Loss modulus vs. frequency of 10% wt PMMA (10% crosslinked)
in PS with composite irradiated in air and only PMMA particles
irradiated in air................................................................................................. 93
Fig. 3-37. Dynamic viscosity vs. frequency of 10% wt PMMA (10% crosslinked)
in PS with composite irradiated in air and only PMMA particles
irradiated in air................................................................................................. 94
Fig. 3-38. Steady shear viscosity vs. shear rate of 10% wt PMMA (10% crosslinked)
in PS with composite irradiated in air and only PMMA particles
irradiated in air................................................................................................. 95
Fig. 3-39. Comparison of unirradiated filled composites (PS filled with 10% wt
10% crosslinked PMMA) and irradiated PS matrix (in vacuum).................. 96
Fig. 3-40. Summary plot of steady shear viscosity (not including the
viscosity at the highest T |)................................................................................ 98
ABSTRACT
Irradiation at a dose of 3.21 Mrad caused polystyrene matrix to crosslink, which
enhanced rheological properties. Irradiation of PS matrix filled with synthesized
monodisperse crosslinked poly(methyl methacrylate) microparticles caused these
properties to decrease. At low frequency, in dynamic mechanical analysis and at small
shear rate, in steady shear measurement, a higher degree of crosslinking of PMMA
increased elasticity and led to higher values of rheological properties. Two cases were
compared: (A) unirradiated PS matrix filled with pre-irradiated PMMA microparticle
powders; (B) filled composite of PMMA and PS irradiated together. Effects of oxygen
were examined. For case A, irradiation in air caused rheological properties to decrease
less due to more efficient oxidation and generation of 'peroxy radicals; for case B,
irradiation in air caused rheological properties to decrease more due to less peroxide
generated, cage recombination of PMMA scission fragments, and reaction of oxygen
with radical precursors of crosslinking in PS.
1
Chapter 1
Introduction
1.1 RHEOLOGY OF FILLED POLYMER COMPOSITES
Rheology deals with the flow and deformation of matter.1 Flow behavior of
polymer composites have been extensively studied and numerous papers have been
published annually. An excellent review can be found in Ref. 2. This section will be
focused on an introduction to basic rheology theory and its application to filled polymer
composites.
There are basically five kinds of rheological behavior exhibited by polymer
systems, namely, Newtonian behavior, Bingham behavior, pseudoplastic behavior,
dilatancy, and thixotropy.3 The most important rheological characteristic of filled
polymer composites is a non-Newtonian behavior.4 As shown in Fig. 1-1, viscosity
decreases with increasing shear rate. This so called “shear thinning” facilitates the
processing of polymeric materials, such as in injection molding and extruding.
However, die swell may occur due to increasing elasticity in the system with shear rate4.
2
c
Fig. 1-1. Viscosity vs. shear rate plot in the shear thinning phenomenon.
It has been reported that the incorporation of filler can greatly affect the
rheological behavior of polymer composites3. Ertong reported that the addition of
carbon black to rubber elastomer increased apparent viscosity and developed
thixotropy.5
1.1.1 Polymer Melts
Most polymeric melts of industrial importance exhibit viscoelaticity. By
definition, a viscoelastic fluid is a non-Newtonian fluid which satisfies the following
equations1:
t = tj ( y ) f (1.1)
=Wi(7)72 (1.2)
*22- 'r 3 3 =,¥'a(y)y2 (L3>
where Tj2 = shear stress (1 - direction of flow; 2 - direction normal to the flow); '
T;/, % 2 2 , t 33 = normal stresses (3 - neutral direction);
y/j, iff2 = material functions defining first and second normal stress function;
T) = viscosity, a function of shear rate y .
It can be seen when 77 (7 ) = r }0 = const, we have a Newtonian fluid. When y/j, y/2 - 0,
the equation describes the behavior of non-Newtonian but inelastic fluid. 1 Normally two
kinds of measurements are performed to obtain information on fluid rheology, one of
which is steady shear viscosity, the other being dynamic viscosity.
The velocity field for steady shear flow between two parallel infinitely long
planes separated by distance h is1
Vz =f(y),Vx =Vy = 0. (1.4)
If the upper plane moves at a constant velocity V , Vz (y = h) = V , then the shear rate is
given as
dV,
dy
= const. (1.5)
The rate of deformation tensor d of shearing flow is given by 1
0 7 / 2 °|
1 1 4 =
7 / 2 0 0
0 0 o|
(1.6)
If the relative velocity of the shearing plate is oscillatory, a complex viscosity 77* can be
defined by measuring the storage modulus G ' and loss modulus G"
= - in' (1.7)
where r j 's dynamic viscosity: 77' = G'Vco, 0) = oscillatory frequency;
77"= imaginary viscosity: n "= G'/ax
77'"measures the stored energy while 77" measures the energy dissipated.
1.2.2 Viscoelastic Theories for Polymer Melts
Various theories have been developed to describe the rheology of polymer melts.
Two approaches are adopted. One is to treat the polymer as a continuum and another is
to use a molecular theory to describe rheological behavior.1 However, in either of these
two approaches, one has to establish a relationship between the deformation of a fluid
and the stress tensor.1
Continuum Theories
Continuum theories are divided into differential and integral models. The most
common differential model, which can be found in nearly every textbook about polymer
viscoelasticity, is the Maxwell model, which uses a spring-dashpot mechanical model to
simulate a viscoelastic model as shown in Fig. 1-2.
Dashpot
• Spring
t
Fig. 1-2. The spring-dashpot model of Maxwell model.
5
The Maxwell equation is1
r + Xt -^- = 2 rj0d
where |I t|I =
*1 1 *12
0
*12 * 2 2
0
0 0
*33
( 1.8)
(1.9)
Xs = time constant, Xi-T]o/ G, rj0 = viscosity, G = elastic modulus;
d = rate of deformation tensor.
Because of its simplicity, the Maxwell model has served as the basis for the relation
between t and rj. However, it is limited only to infinitesimally small deformation
because it cannot describe nonlinear viscoelastic phenomena, such as shear-dependent
viscosity.1
Various differential models have been developed to improve the Maxwell
model. Ref. 1 gives the viscometric material functions for some of them.
The integral models use Boltzmann’s superposition principle.1 Han gave the
viscometric functions for some integral type rheological models.1 They differ from
differential models in that there are memory functions presented in the models. As many
flow problems of industrial importance require solution of equations of motion to
predict the flow patterns or to determine an optimal die geometry, integral type models
are preferred to investigate the deformation history.1
Molecular Theories
Understanding the relationship between molecular parameters and rheological
properties is essential for polymer synthesis and processing. For polymer melts, Han
where 0) = frequency of oscillation;
c = density;
k = Boltzmann constant;
T = absolute temperature;
Xp = relaxation time of the p the segment defined by
proposed a model for oscillatory shearing flow:1
( 1.10)
(1.11)
Xp = [6i)o M]/7i?p2 pRT (1.12)
where T }o = zero-shear viscosity;
M = molecular weight;
R = gas constant.
For steady shear flow, Graessley proposed a theoretical expression:7
(1.13)
where rjo = zero-shear viscosity;
6 =(X>T / 2)(V /7Jo)
(1.14)
where Xq = relaxation time.
Other molecular models have also been developed.
7
1.1.3. Rheological Behavior of Model-Filled Polymer Melts
Fillers are widely used in industry to reduce cost and/or to improve physical
properties, particularly hardness, stiffness, and impact strength.4 For example, wood
flour is added to reduce the brittleness of phenol formaldehyde materials; carbon black
can improve resistance to stress-cracking and UV degradation of polyolefins. A
knowledge of flow behavior is critical for processing and for controlling the properties
of filled polymer melts. Because the fillers used in industry are complicated in structure,
the use of model-filled system for rheology study, which, under normal circumstances,
incorporates monodisperse spherical particles into the matrix of known properties, such
as molecular weight and glass transition temperature, Tg, is of great importance both in
academic research and in industrial application.
Factors That Affect Rheology
The rheological behavior of polymer filled with particles is extremely
complicated due to particle-particle interactions and particle migration.6 Particle-matrix
interaction is also important.8 The effect of particle-particle interaction is obvious
because rheological properties change with the filler concentration. At low shear stress,
three scenarios can result from particle-particle interaction: (i) yield behavior as
additional stress does not induce further strain; (ii) shear thickening (opposite to shear
thinning) which is a phenomenon in which the viscosity increases with shear stress; (ili)
shear thinning as described before.6 At very high concentration of filler (>40% by
volume) discontinuous viscosity curves were obtained by Hoffman.8
Besides concentration, particle size also may affect rheology. Freundlish and
Jones noted that suspensions of small particles were different from larger particles.9
Small particles tended to form aggregates which at low shear rates resulted in yield
behavior and thixotropy.9 When studying glass spheres of diameter 10 - 60 Jim, White et
al, found a constant viscosity at low shear rate which decreased with increasing shear
rates.8 The viscosity vs. shear rate behavior was similar to that of pure matrix. When the
diameter of the filler was small, say, less than 1 Jim, which was the case in our research,
the rj-y behavior became very complicated and also depended on particle-matrix
interaction. Agawarl has used crosslinked PS (d =260 nm and 315 nm) and crosslinked
PMMA (d = 240 nm and 200 nm) in PMMA matrix with different molecular weight and
found that chemical interaction between filler and matrix dominated rheological
properties.1 0 For particles that are “compatible” with matrix, i.e., have the same or
similar structure to the matrix, the system behaves like a Newtonian fluid.1 0 Highly non-
Newtonian behaviors are observed when “incompatible” particles are added to the
matrix. It was also reported that a matrix with a higher molecular weight results in a
higher particle-matrix interaction and therefore better dispersion of incompatible fillers
and poorer dispersion in “compatible” fillers at low shear rate.1 0
Theory
It is surprising that filled polymers are not well studied despite their wide
applications. The viscosity of a filled system is often obtained by empirical equations or
direct experimental measurement.1 1 However, there is effort to develop theories which
have achieved a certain degree of recognition.
When the concentration of a filler is very low, the polymer melt can be treated as
a suspension system. Einstein studied the particulate suspension system and proposed
the following equation:1 1
ri/rjo = 1 + 2.5 < j > (1.15)
where T ]o = viscosity of suspension medium;
< f > s volume fraction of the particulates.
This equation assumed that the suspension acted as a Newtonian fluid. However, in
practice, there is hardly any system which fits this assumption.
Shear thinning behavior at low shear rate can be approximated by a power-law.1 1
t = k y m (1.16)
or t = rjaf (1.17)
where m = 1 for a Newtonian fluid;
m < 1 for a pseudoplastic fluid.
Rao et al. have developed analytical expressions for flow in dilute suspensions of
spheroidal particles.1 2 Expressions for simple shear flows are found to be very accurate
for dilute suspensions and rheological properties are shown to depend on components of
a modified Finger strain tensor.1 2 For composites with higher filler concentration,
10
particle-particle interaction must be taken into account. Tanaka and White have shown
that a Herschel-Bulkley model with a yield value of T o can approximate the viscosity of
concentrated suspensions of swollen particles in a power law fluid (1.18).1 3
tj = ~~ + k y m~l (1.18)
7
Although various empirical mathematical expressions have been developed to
describe the rheological behavior of filled polymers, researchers often use experimental
techniques to directly measure rheological properties and draw conclusions from the
results.1 ’I0 ’1 2 ’1 4 -1 6
1.2.4. Rheology Measurement
Various instruments are available to measure the viscosity and other rheological
properties of molten polymers. They can be divided into two categories: steady state
measurements and complex viscosity and normal stress measurements. The former
includes simple shear viscometers, capillary rheometers, and parallel plate viscometers.4
The latter includes cone-plate rheogoniometer, dynamic rheometers, and orthogonal
rheometers 4 The Weissenberg cone-plate rheogoniometer is introduced as follows and
has been the major instrument used in our research.
Weissenberg pioneered a method to measure shear and the first normal stress
11
difference during shear flow of an elastic fluid, 48 years ago. Nowadays, a cone-plate
rheogoniometer following Weissenberg’s idea has been one of the most popular
instruments for rheological measurement.
The Weissenberg rheogoniometer consists of a flat plat and a coaxial with the
apex of the cone resting on the plate (Fig. 1-3). The cone angle is very small and
sometimes the cone is called flat cone.1 8
Fig. 1-3. A cone-plate configuration.
Recently modifications have been made to allow the measurement of two normal
stress differences during the shear flow of polymer melts below 250 °C. Meissner et al.
have developed a new heating system that keeps a temperature variation of less than
0.01 °C.1 9 They have also partitioned the plate into an inner disk connected to the
torque/normal force transducer and an outer ring fixed to the frame of the device.1 9 Two
normal stress differences and their time response can be separated by testing samples
with different radii.1 9 Eggers and Schummer recently exhibited a new modified
Weissenberg rheogomiometer.2 0 Their method eliminates wall slippage and rim fracture
— the major difficulties in steady shear viscosity measurement at large shear rate. Only
the force that acts on the central part is measured. A “screening ring” is added to the
12
upper cone or disk, which ensures that the region of ideal viscometric flow undisturbed
by edge effects extends beyond the radius of the measuring plate.2 0
Theory
1 B
The steady shear viscosity can be measured by
3 a M
' - (1 1 9 )
where T ] = steady state viscosity;
a = the angle in radians which the come makes with the flat plate;
M = torque required to rotate the cone relative to the plate at frequency co;
R = radius of the plate.
The complex viscosity can be obtained by measuring the storage modulus G' and the
loss modulus G" 21. From equation 1.7, rj* = T]'- if]"
1.2 EFFECTS OF RADIATION ON POLYMERIC MATERIALS
The effects of radiation on polymeric materials have rapidly attracted attention
due to their practical significance. The importance of radiation effects are widely
recognized in the electronics industry as photolithographic techniques utilize 365 - 436
nm UV radiation in the fabrication of today’s commercial silicon chip integrated
• * O ')
circuits. Radiation effects are evident in equipment for superconducting magnets and
in equipment used in earth-orbiting satellites, which would be exposed to a total dose of
104 Mrad of ionizing radiation after 30 years in geosynchronous orbit.2 3 High-energy
radiation is also used in the pharmaceutical industry for sterilizing medical instruments.
Irradiation of polymers causes modification of properties which is currently the basis of
industries manufacturing heat shrinkable film and tubing, crosslinked polymers and
grafted copolymers.2 4 Radiation crosslinked polyethylenes, for example, are used in
mining, reinforcement of concrete, manufacture of light weight high strength ropes and
high performance fabrics.2 2 Another example is radiation induced chemical grafting,
which may enhance adhesion of a polymeric coating to a substrate.
1.2.1 Types of Radiation
High-energy radiation may be divided into two categories: photon radiation and
particulate radiation. Gamma radiation is one kind of photon radiation, which is utilized
for fundamental studies and for low dose irradiation with deep penetration. The
radioactive isotope Co60 (Cobalt-60) is the main source of gamma radiation, another
being Cesium-137. Electron radiation is particulate radiation, which is normally
obtained from electron accelerators to give beams with energy in the MeV range and
penetration in the mm range.
14
1.2.2 Effects of Radiation on Polymers
The effects of radiation on polymer include crosslinking and side group or main
chain scission.
Crosslinking
Among radiation-induced reactions in polymers, crosslinking has the most
important industrial significance. For example, radiation-induced crosslinking of
engineering plastics improves heat resistivity without using glass fibers25. Due to ease of
process, reasonable cost, and excellent performance, radiation crosslinking is widely
recognized and increases at a 15% annual growth rate in many countries around the
world.2 5 The crosslinking effect of ionizing radiation was discovered by A. Charlesby in
1952 after the irradiation of low density polyethylene.26 M. Dole had also reported that
the tensile strength of PE films was increased by irradiation under vacuum but
decreased when irradiation was performed in air.23 Nowadays radiation crosslinking is
applied not only to PE, but also to many and a total world irradiation capacity to treat
polymers of 1 million tons has been installed.2 7 Despite large industrial application,
however, many aspects of the mechanism are not fully understood.
Chain crosslinking results in an increase in molecular weight and formation of a
macroscopic network. It is commonly known that aromatic groups have radiation
resistance. Early work showed that radiolytic hydrogen yields from cyclohexane were of
15
125 times greater than from benzene in the liquid phase. A mixture of these two
materials showed a pronounced protective effect by the aromatic components.2 3
Because of its simplicity, polystyrene has been extensively studied. Polystyrene
is used as a radiation-stable, transparent, scintillator matrix. A knowledge of the
radiation chemistry of polystyrene is necessary to understand our research.
Energetic electrons excite and ionize molecules of the materials through which
they pass 2 8 The ionized molecules are neutralized by electrons or oppositely charged
species within their Columbic fields and quickly lose the excess vibrational energy.
Normally they revert to their lowest vibrational state. Without radiation the upper
electronic states can transit to the lowest singlet or triplet state in times short compared
to the lifetime of the excited singlet or triplet.2 8 It was estimated that approximately
equal amounts of singlet and triplet states of naphthalene dissolved in polystyrene are
formed by pulsed x-ray irradiation.2 8 Basile noted that residual styrene monomer, at a
concentration less than that in commercial grade polystyrene, dominated fluorescence
emission for excitation by UV irradiation. The polymer contributed excitation energy
to the monomer. Two different free radicals are formed as a result of irradiation:
disubstituted benzyl radical formed by the loss of a hydrogen atom bound to the carbon
atom a to the benzene ring (Fig. l-4a) and a cyclohexadieny 1-type radical (Fig. l-4b).
ESR studies prove the existence of the disubstituted radical.2 8 Ohnishi et al. assigned
the polystyrene ESR spectrum after irradiation to cyclohexadienyl-type radical.3 0 It was
16
~ c h 2 — c —c h 2 ~ CHz— CH— CH2
©
a. disubstituted benzyl radical b. cyclohexadienyl-type radical
Fig. 1-4. Two types of free radicals generated by irradiation of polystyrene.
estimated that the concentration of radical (a) was 25 % of the total radicals in PS
irradiated at -196 °C.2 8
When linear polystyrene is irradiated in vacuum, its molecular weight is
observed to increase. Charlesby observed that at suitably large radiation dose an
insoluble gel was produced, which indicated the formation of a three dimensional
structure.3 1 Slovokhtova et al. irradiated PS with energetic electrons and proposed that
cyclohexadiene rings formed by the addition of H atom and crosslinking to the phenyl
ring.2 8 A shift in the UV absorption edge at about 270 nm to a long wave length of 380
nm was observed and attributed to the formation of conjugate unsaturated groups.2 8
Parkinson explained that the radiation-induced spectral shift was due to the following
reaction (1) through (4). Reaction (1) is the rupture of main chain bonds to the phenyl
ring. H atoms are abstracted to form hydrogen molecules (reaction (2)). Addition of H to
the ring occurs in reaction (3) and stabilization of the cyclohexadienyl radicals formed
by abstracting an H atom from a neighboring chain to form a benzyl radical (reaction (3)
CH2 — C H ~ y-irradiation ( ~ CHr—C— CH=CH ~ + C6H6
(1)
H H
-I-
CH2 — CH— CH — C~
[Q] [Ol
-> ~ CH2 — CH—CH =C ~ + H2 (2)
C o l [o]
H + ~C H 2 — C H ~ ~ CH2 — CH ~
( S L
H \ H
H
CH: ; h ~
H
CH2 — CH— CH — CH-
[O] [Ol
- * > ~ CH2— c — c h 2 —• c ~
[O] {01
(3)
(4)
- (4)). Hydrogen, benzene, and aliphatic unsaturation are thus produced. Reaction (5)
through (10) are widely accepted mechanism for irradiation of PS.32
CH2 — < j 2 H— CH2 ~ 7-irradiation ( ~ CH2— CH— CH2 ~ + H (atom) (5)
H (atom) + ~ CH2 — CH— CH2 ~ - ► ~ CH2— C— CH2 ~ + H 2 (6)
18
H + ~ CH2—CH—CH2 ------------------- ► ~CH 2—CH—CH2 ~ + H 2 (7)
(A) + (B) ^ crosslinks (8 )
(A) + (A)
(A) + (B) ► 2 - CH2—CH—CH2 ~ (9)
2 H ---------------- ► H2 (10)
While this mechanism is simple, it is unable to explain the effects of linear energy
transfer on radiation yields of crosslinking, G(X) and of hydrogen, G(H2). Thus,
Hensinger and Rosenberg proposed a more detailed mechanism to account for the
effects of radiation on polystyrene ( 1 1 to 19).3 3
< RH* (11)
RH+ + e* ( 1 2 )
where RH denotes polystyrene, RH is excited polystyrene.
RH+ + RH-----------» RH2 + + R- (13)
RH2 + + e"-----------» R- + H2 (14)
2 R. > R-R (crosslink) (15)
and/or a hot atom mechanism of the type
RH* > R- + H hot (16)
Hhot+ RH-----------> R + H 2 (17)
19
RH'
H th e rm a l + RH-
~ ^ H th e rm a l + R‘
— > r h 2-
(18)
(19)
For the post-irradiation effect, Hensinger and Rosenberg proposed the following
reactions ((20) - (22))
33
~ CH2 — CH ~ ~ CH2—CH - - * > ~ CH2 — C - + ~CH2—CH~ (20)
H2
2 ~ CHz—CH ~
[ol
- * ■ crosslinks
S 3
X H2
H H
(21)
2 ~ CH2 —CH ~
&
- CH2 —CH ~ + ~C=CII ~ (22)
This mechanism is able to explain a number of major effects of irradiation on PS.
However, it predicts that G(X) = G(H2 ), which a number of investigators have found
untrue. 32'34 This means reactions (5) - (8 ) cannot be totally ignored. 28
Chain Scission
Another radiation effect is side group or main chain scission. Chemical bonds
and groups particularly sensitive to this effect include — COOH, — C—X, where X =
halogen, — S 0 2— , —NH2, and — C=C. Chain scission results in a decrease in
molecular weight and substantially changes the properties of a polymeric materials.
Also small molecule products are produced by bond scission followed by abstraction or
changes in PMMA attract most attention in research because of its wide application in
practice. PMMA is a resist for advanced microlithography.35 A good image can be
developed if the PMMA resist is irradiated properly, which depends on the intrinsic
radiation sensitivity of the methacrylate resist.35
The degradation of PMMA subject to high energy irradiation has been
extensively studied, which includes x-ray, y-ray, and E beam irradiation.35'39 Most of the
reactions lead to loss of ester groups in atactic PMMA. Hydrogen is abstracted from the
a-methyl and the ethylene group. High energy radiation-induced degradation processes
in PMMA is shown in equation (23) to (27).3 5
combination reactions. 22 Among this kind of radiation induced degradation, chemical
CH3 c h 3
(CH2—C ) ~ hv (~ CH2 —C ~)© (23)
c = o c= o
o c h 3 o c h 3
c h 3 c h 3
(~ CH2—c - )© ~C—CH2 ~+ C0 2CH3 (24)
©
c = o
OCH3
•co 2c h 3 ♦ CH4, CH3OH, C02, CO (25)
21
CH3 c h 3
-C —CH2 ~ -------> ~C—CH2 ~ (26)
©
c h 3
I main chain scission
~C—CH2 ~ ^ or c o o c h 3
c o o c h 3
(27)
It can be seen that when PMMA is irradiated, it undergoes chain scission and
methyl formate is produced, which decomposes to form a large number of gases such as
CO, C 02, CH4 , and methanol. Gupta et al. measured quantum yields of principal
photoprocesses on direct excitation of PMMA at 303 K in vacuum and proposed a
mechanism of photodegradation. They suggested that the ester side chain undergoes
chain scission and most chain radicals are terminated by recombination with small
radicals. 38
It is concluded that the photolysis of PMMA results in random scission of the
polymer backbone by a radical process. Three major reaction might occur at the same
time.40
( i ) random homolytic scission of main-chain carbon-carbon bonds;
22
(ii) photolysis of the ester side group;
(iii) photodissociation of the methyl side groups.
The main products of the irradiation of PMMA in vacuum are methyl formate,
methanol, and methyl methacrylate with quantum yields of 0.14,0.48, 0.20,
( i ) unzipping (end chain depolymerization)
(ii) trapped residual monomer from polymerization
(iii) random thermal degradation.
A maximum zip length of about five monomer units per scission was reported.41 Based
on product analysis study, Genskns et al. proposed the following mechanism(Fig. 1-5) 42
PMMA y PMMA ® + e ‘
! —y
respectively.40 In air the products are the above plus methane, hydrogen, carbon
monoxide, and carbon dioxide 40 Monomer can evolve from three sources.
~ CHr— C—CH2 ~ + *COOCH3 -------- ► CO + CHO- or C 0 2 + CH3
e
c h 3 c h 3
-CH 2—c h = c h 2 + c —c h 3
c o o c h 3
Fig. 1-5. Mechanism of PMMA chain scission.
Recently, however, Ichikawa et al denied main chain scission by direct action of
ionizing reaction.3 9 ,4 2 ,43 They irradiated PMMA powders in vacuum with 6 0 Co y-ray at
77K to a dose of 5.0 Kgy (0.5 Mrad) and used ESR to analyze the radicals formed
during irradiation.42 They found the major radical species generated and stabilized at
77 K are — CH— and — COOCH2— cation radicals and — COOH3' anion radical.
More recently Ichika and Yoshida irradiated PMMA in vacuum in a high purity quartz
tube with 6 0 Co y-ray at room temperature at a dose rate of 3.9 Kgy/h (0.39 Mrad/h) and
determined radicals from conventional ESR spectra. After measuring the intrapair
distance of radical pairs they concluded that the —COOCH2 — and the — CH—
radicals are initially generated by the following reactions:43
PMMA hv ( PMMA-+ + PMMA ' + H- H COOCH2
o r— CH—
(28)
H- + PMMA ► H2 + — COOCH2 or — CH— (29)
H. + — COOCH2 or — CH— ► PMMA (30)
PM MA+ ► H* + —COOCH2 or — CH— (31)
PMMA-' --------► PMMA + e" (32)
e' + lT -------* H- (33)
Generation of unsaturation in PMMA is another important issue. Usually
prepared PMMA contains a small number of unsaturated groups mainly in the form of
24
unreacted monomer and a much smaller amount of chain ends. A pronounced influence
of unsaturation on properties of PMMA was reported.37 As shown previously irradiation
increases unsaturation. Conjugated unsaturation in PMMA is generated. Unsaturation
is due to main chain scission and hydrogen abstraction. 35 After irradiation, residual
monomer may react with unterminated polymer free radicals. Kalachandra et al. showed
that a sample with 1.43% unsaturation before irradiation decreased to 0.70%
unsaturation after storage for 34 days.37 Therefore storage of irradiated PMMA at room
temperature results in a decrease of unsaturation. The reaction scheme is
M + P n ------------- ► Pn (34)
Pn + Pn ------------- ► nonradical product. (35)
1.2.3 Irradiation Effects on PMMA in An Aromatic Matrix or with Additives
Because PMMA is easily degraded by high energy radiation, a protective effect
may be desirable for improving the properties of PMMA during and after irradiation.
Incorporation of phenyl groups into a polymer can enhance the stability of PMMA by
protecting it via energy transfer processes. The phenyl group in a polymer demonstrates
a substantial intramolecular protective effect. G values for the evolution of H2 are low.44
More important, the phenyl group protects other molecules in the same mixture by
reducing molecular scission produced by irradiation. Basheer and Dole investigated G
values of H2 yield in block and random copolymers of polybutadiene and polystyrene.44
They claimed that there was no protective effect in block copolymer. However, in
random copolymer a significant drop in G values was found because phenyl groups
were much closer to the polybutadiene group. This caused a reduction in the G value of
hydrogen from the polybutadiene segments. The phenyl group acted as a sink for the
transmitted energy. Wundrich reported that G values for main-chain scission in PMMA
were decreased by aromatic addictives.36 The decrease is proportional to the
concentration.3 6 The relation is
G/Gp = Pf= 1 + PmC (1.20)
where Gp = the G value of a mixture containing C% additive;
Pm = defined as the proportionality factor for the molar inhibition coefficient.
Pf= the inhibition factor as defined by G/Gp.
The interaction between PMMA and these additives was investigated by studying the
efficiency o f additives in preventing chain scission as a function of the ease of excitation
and the radical and electron affinity,
1.2.4 Effect of Oxygen and Vacuum
The irradiation environment is one of the most important factors determining
reactions. Irradiation effects differ in air and in vacuum. For example, when ultra-high-
molecular-weight polyethylene is irradiated in nitrogen, the crystallinity decreases with
radiation dose; when it is irradiated in air (in the presence of oxygen), the crystallinity
increases with dose. 4 5 For polymer irradiated in air, oxygen penetrates into the
26
amorphous domains.46 Oxygen reacts with carbon radicals to form peroxy radicals as in
reaction (36) 46
— C- + O2
* —c —0 2 '
(36)
This peroxy radical can seize a hydrogen from polymer (reaction (37)) to form a
hydroperoxide group and a carbon radical.
The cycles of reaction (36) and (37) result in oxidation of the polymer. The
hydroperoxide formed during radiation may decompose after irradiation, causing
irradiated polymer to degrade during storage. Therefore at the same dose, the effects of
irradiation on a sample irradiated in air may differ from the sample irradiated in
vacuum. Wilski observed that the outer layer of a sample irradiated in air degraded
while crosslinking took place throughout47 Degradation may be prevented by irradiation
and storage under nitrogen or in vacuum or by introducing antioxidants. Wilski also
proposed that increasing molecular weight or adding PH3, SF4, or S 0 2 may act against
degradation.47 Antioxidants can also effectively reduce or eliminate the oxidization
effect in irradiation processes 47
(37)
27
REFERENCES
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1981.
2. L. Sun, Ph.D. Thesis, University of Southern California, Los Angeles, 1992.
3. S. S. Schwartz and S. H. Goodman, Plastics Materials & Processes, Van Nostrand
Reinhold, New York, 1982.
4. L. E. Nielsen, Polymer Rheology, Marcel Dekker, New York, 1977.
5. S. Ertong, H. Eggers, and P. Schummer, “Steady-State Shear Flow Properties of
Carbon-Black Filled Rubber Compounds,” Rubber Chemistry and Technology, 67,207
(1994).
6 . H. V. Oene, “Rheology of Polymer Blends and Dispersions,” in Polymer Blends, D.
R. Paul and S. Newman, ed., 1. Academic Press, New York, 1990.
7. W. W. Graessley, J. Chem. Phys., 47, 1942 (1967).
8 . J. L. White, Principles o f Polymer Engineering Rheology, Wiley, New York, 1990.
9. H. Frendlich and A. D. Jones, J. Phys. Chem,, 40, 1217 (1936).
10. S. Agawarl, M. S. Thesis, University of Southern California, Los Angeles, 1992.
11. R. Jin and Y. Hua, Polymer Physics (Chinese edition), Chemical Industrial Press,
1990 (ISBN 7-5025-0879-1/G.247).
12. B. N. Rao e ta l, “Rheological Properties of Non-Brownian Spherical Particle
Suspensions,” J. RheoL, 38, 1335 (1994).
13. H. Tanaka and J. L. White, J. Non-Newtonian Fluid Mechanics, 7, 333 (1980).
14. L. Sun et al., “Model Filled Polymers. XDT. Mixing and Time-Dependent
Rheological Behaviors of Polymer Melts Containing Crosslinked Polymer Particles,”
Polym, Eng. and Sci., 32, 1418 (1992).
28
15. L. Sun, J. J. Aklonis, and R. Salovey, “Model Filled Polymer. XIV. Effect of
Modifications of Filler Composition on Rheology,” Polym, Eng, and Sci.t 33, 1310
(1993).
16. S. Agarwal and R. Salovey, “Model Filled Polymer. XV The Effect of Chemical
Interactions and matrix Molecular Weight on Rheology,” in Press, 1994.
17. K. Weissenberg, Nature, 159, 310 (1947).
18. H. Janeschitz-Kriegl, Polymer Melt Rheology and Flow Birefringence, Springer-
Verlag, Berlin, 1983.
19. J. Meissener, R. W. Garbella, and J. Hostettler, “Measuring Normal Stress
Differences in Polymer Melt Shear Flow,” J. Rheol., 33, 843 (1989).
20. H. Eggers and P. Schummer, “A New Method for Determination of Normal-Stress
Differences in Highly Viscoelastic Substances Using a Modified Weissenberg
Rheometer,” J. Rheol., 38, 1169 (1994).
21. N. G. McCrum, C. P. Buckley, C. B. Bucknall, Principles o f Polymer Engineering,
Oxford Science, New York, 1988.
22. E. Reichmanis et al„ “Radiation Effects on Polymeric Materials,” in Irradiation o f
Polymeric Materials, American Chemical Society, Washington, DC, 1993.
23. M. Dole, “ The Radiation Chemistry of Polymer Composite,” Radiat. Phys. Chem.,
37, 65 (1991).
24. R. L. Clough and S. W. Shalaby, Radiation Effects on Polymers, American
Chemical Society, Washington, DC, 1991.
25. K. Ueno, “The Radiation Crosslinking Process and New Products,” Radiat. Phys.
Chem., 35, 126 (1990).
26. A. Charlesby, Proc. R. Soc. Land., A215, 187 (1953).
27. L. Wiesner, Radiat. Phys. Chem., 37 (1991).
28. W. W. Parkinson and R. M. Keyser, “Polystyrene and Related Polymers,” in The
Radiation Chemistry o f Macromolecules, Vol. 2, M. Dole, Ed., Academic, New York,
1973.
29. L. J. Basile, Trans. Faraday Soc., 60, 1702 (1955).
29
30. S. Ohnishi, T. Tanei, and I. Nitta, J. Chem, Phys,, 32,2402 (1962).
31. A. Charlesby, “Swelling Properties of Polystyrene Crosslinked by High Energy
Radiation,” J, Polym. Sci,, XI, 521 (1953).
33. H. Hensinger and A. Rosenberg, in “ Symposium on Large Radiation Sources
Industrial Processes,” E. R. A. Beck ed., 151, IAEA, Vienna, 1969.
34. L. M. Alberino and W. W. Graess, J. Phys. Chem., 72 (1968).
35. J. A. Moore and J. O. Choi, “Degradation of Poly (methyl methacrylate),” in
Radiation Effects on Polymers, R. L. Clough and S. W. Shalaby, ed., American
Chemical Society, Washington, DC, 1991.
36. K. Wundrich, “Effect of Temperature and Physical State on the Inhibition by
Additives of Radiation-Induced Degradation of Poly(methyl methacrylate),” J. Polym.
Set: Polym. Phys., 12, 201 (1974).
37. S. Kalachandra et al, “Postirradiation Reactions of Monomers in Poly (methyl
methacrylate): Analysis by CP/MAS 1 3 C NMR,” Macrom., 27, 5948 (1994).
38. A. Gupta, R. Liang, F. D. Tsay, and J. Moacanin, “Characterization of a Dissociative
Excited State in the Solid State: Photochemistry of Poly (methyl methacrylate).
Photochemical Processes in Polymeric System. 5,” Macrom., 13, 1696 (1980).
39. T. Ichikawa, K. Oyama, T. Kondoh, and H. Yoshida, “Efficiency of Radiation-
Induced Main-Chain Scission of Poly (methyl methacrylate) depends on the irradiated
Temperature Because of Coexisting Monomer," J. Polym. Sci.: Part A: Polym. Chem.,
32. 2487 (1994).
40. B. Randy, Photodegradation, Photo-Oxidation and Photostabilization of Polymers,
Wiley, London, 1975.
41. R. B. Fox, L. G. Isaacs, and S. Stokes, J. Polym. Sci. A l, 1,1079 (1963).
42. T. Ichikawa and H. Yoshida, “Mechanism of Radiation-Induced Degradation of
PMMA as Studied by ESR and Electron Spin Echo Methods,” J. Polym. Sci.: Polym.
Chem., 28, 1185(1990).
43. T. Ichikawa and H. Yoshida, “Spatial Distribution of Radicals in Gamma-Irradiated
Poly (methyl methacrylate) Studied by Electron Spin Echo Method,” Radiat. Phys.
Chem., 37, 367(1991).
30
44. R. Basheer and M. Dole, “Effect of Copolymer Composition on the Formation of
Ionic Species, Hydrogen Evolution, and Fee-Radical Reaction in y-Irradiated Styrene-
Butadiene Random and Block Copolymers,” J. Polym. Sci.: Polym. Phys., 22, 1313
(1984).
45. A. Shinde and R. Salovey, “Irradiation of Ultra-High-Molecular-Weight
Polyethylene ” J. Polym. Sci.: Polym. Phys., 23,1681 (1985).
46. D. J. Carlsson and S. Chmela, “Polymer and High-Energy Irradiation: Degradation
and Stabilization,” in Mechanisms of Polymer Degradation and Stabilization, G. Scott,
ed., Elsevier Applied Science, London, 1990.
47. H. Wilski, “Radiation Stability of Polymers,” Radiat. Phys. Chem., 35, 186 (1990).
Chapter 2
Synthesis of PMMA Particles
31
In order to investigate the rheological behavior of filled polymers, we have synthesized
a series of PMMA particles with different degrees of crosslinking. Monodisperse
crosslinked particles have been obtained and observed by scanning electron microscopy
(SEM).
2.1 INTRODUCTION
2.1.1 Synthesis of Crosslinked Monodisperse Polymer Particles by Emulsifier-Free
Emulsion Polymerization
Before investigating the effect of radiation on the rheology of filled polymers, an
understanding of the synthesis of fillers is necessary. Well-characterized, crosslinked,
monodisperse polymer microspheres will be used on fillers to explore rheological
32
changes of dispersions. The synthesis of polymer beads by emulsifier-free emulsion has
received increasing attention in recent years. We examine two aspects:
( 1) the kinetics of emulsion polymerization; 1'7
(2 ) well-characterized fillers for studying the properties of filled systems. 8' 17
The mechanism of emulsion polymerization has been investigated for more than
40 years. Yet the theory is not completely understood. Although various mechanisms
have been proposed, the three stage model proposed by Harkins and Smith is still the
basis for explaining phenomena in emulsion. 18' 19 The mechanism include “entry,”
“exit,” and termination of free radicals. 20 Free radicals which initiate emulsion
polymerization are produced from water-soluble initiators, such as potassium persulfate.
Large amounts of emulsifier are used to form oriented clusters known as micelles,
which have a diameter of 50 A and consist of about 1 0 0 molecules each. 19 A
hydrophobic core is formed inside the micelle by hydrocarbon chains pointing
1 9
inwards. Free radicals can react with monomer dissolved in the aqueous phase and
then the oligomeric radical species enters the micelles. 18 At the same time monomer
droplets about 1 |im in diameter ( 1 0 0 0 times larger than micelles) are formed and
stabilized by emulsifier in the solution. Because the number of micelles is significantly
larger than the number of monomer droplets, the collision of a free radical with a
swollen micelle is strongly favored. 19 The monomer droplets serve primarily as a
reservoir for monomer to maintain the monomer’s concentration in the micelles.
Polymerization takes place within micelles and micelles are consumed. The end of the
first stage of emulsion polymerization arrives after the micelles are completely used up,
which normally occurs by a conversion of 10% . 18 Seymour reports that all micelles
disappear when the conversion reached 20% . 19 In a second stage of emulsion
polymerization, monomer droplets continue serving as the source of monomer and the
rate of propagation remains constant throughout the stage. A third stage arrives when all
the monomer droplets are depleted and only the monomer left inside the particles is
polymerized. 18 Smith and Ewart proposed the following equation for particle number,
NS f in Stage I :21
Ns = K p a4SP6 (2.1)
where K = constant;
p = initiation rate;
S = emulsifier concentration.
1 S
The rate of propagation per latex particle, Rp, in stage II is
R„ = kp[M](n/Nt )Np (2.2)
where kp = rate constant for propagation;
[M] = monomer concentration;
n = average number of radicals per latex particle;
Na = Avogadro constant;
Np = number of latex particles per unit volume of latex.
Particle growth and the rate of polymerization are major areas of emulsion
polymerization research. One particle growth mechanism has been proposed by Grancio
and Williams.22 They suggest that the particle grows in core-shell structure, with
monomer growing on a core of polymer 22 Recently, Chen and Lee performed a series of
experiments on seeded styrene latex emulsion and proved that if the particle size
exceeded a critical value (~0.15-0.20 |i.m), subsequent particle growth occurs mainly in
the shell region, a shell growth mechanism.23 However, if the particle radius is less than
about 100 nm, polymerization occurs uniformly in the latex.23 Last year, Casey et al
detailed “free radical exit” theoretical models in emulsion polymerization and proposed
a simple mechanism for particle growth and free radical exit. 24 They assumed that
emulsion polymerization took place homogeneously in the latex particles, which were
monodisperse and no new nucleation occurs. Furthermore they assume only one free
radical within each latex particle and any entry of another free radical would terminate
the reaction instantaneously. Just like Smith and Ewart it was proposed that only
monomeric free radicals could enter the latex particles.24 From experimental data
presented on the exit rate coefficient as a function of particle size, monomer
concentration, and aqueous-phase free radical concentration for polystyrene at 50 °C, it
was found that n never exceeds O.5.20 Measuring the rate coefficient of propagation,
Verdurmen et al experimentally calculated kp from a plot of kpn vs. particle diameter.25
Emulsifier-free emulsion polymerization, however, differs from ordinary
emulsion polymerization in that no surfactant is added in the solution. In 1988, Song
and Poehlein proposed a two stage model for particle nucleation in emulsifier-free
emulsion polymerization of styrene using potassium persulfate as initiator. 1 A so-called
in-situ micellization mechanism divided the particle nucleation period into two stages.
In the first stage, which was less than 10 minutes, a large number of stable oligomeric
micellar nuclei are formed. In the second stage, these nuclei coagulate and the particle
size increases. Coagulation and polymerization prevail during the second stage. A
constant particle number is achieved in the second stage. Song and Poehlein1 extended
the model to predict the variations of conversion, particle size and molecular weight
with time and concluded that the two stage model could describe the kinetics of styrene
polymerization when K2S2O8 was used as the initiator. Our research group at the
University of Southern California has presented a series of papers on the synthesis of
monodisperse particles.8 '1 5 It was suggested that before particle nucleation, surface
active oligomers of low molecular weight polystyrene exist in the latex and the
mechanism of emulsifier-free emulsion polymerization depended on the solubility of
monomer in water.9,26'2 7 For monomers of low aqueous solubility, such as styrene, the
orientation of surface active oligomers produced in the first stage of emulsion results in
particle nucleation within micelles.9’2 6 For monomer of relatively high aqueous
solubility, such as methyl methacrylate (MMA), growing polymer chains precipate from
the water solution unto the particles and homogeneous nucleation occurs.9,26,2 8 Under
the same conditions, more particles may form with monomer of larger solubility such as
MMA, and, thus, polymerization of styrene results in a larger particle size than MMA.9
Alternative reaction routes have been tried in order to increase the size of particles
formed. Momomer was added onto pre-existing polymer seeds and the system
polymerized without emulsifier, yielding styrene particles of size greater than 1 |xm,
compared to 400 - 500 nm obtained through normal emulsifier-free polymerization.1 1
36
Crosslinked polystyrene and poly(methyl methacrylate) particles of maximum sizes
about 800 nm and 300 nm, respectively, were reported.9,1 2
2.2 EXPERIMENTAL DETAILS
2.2.1 Materials and Instrument
Methyl methacrylate monomer with assay 98% inhibited with 25+-ppm
hydroquinene (HQ) was from Fisher Scientific Co. Ethyleneglycol dimethacrylate
(EGDMA) the crosslinking agent, with 98% purity and inhibited with 100 ppm HQ was
from Aldrich Chemical Co. Potassium persulfate, the inhibitor, was a certified Fisher
Scientific product. Deionized water used for emulsion and cleaning was from
Sparklettes. Dry nitrogen used to remove oxygen from the reaction system was from SG
Industries Gas Products. Methanol used to wash the synthesized beads was from
Mallinckrodt Inc. The scanning electron microscope (SEM) was Cambridge 360 from
Cambridge Instruments.
2.2.2 Synthesis
The experimental approach closely followed one reported.9 The reaction kettle
was washed with tap water and deionized water three times each. 650 ml deionized
water was added to the kettle and heated to 80 °C. 74 ml MMA and 13.5 ml (for 10%
37
mole crosslinking) EGDMA were added to the kettle. After 20-30 minutes, K2S2O8
(0.16g) was added to the system and reaction started. After 4 to 5 hours when
agglomerates formed on the surface of the solution, the reaction was shut down. The
latex was stored in PE bottles and when cool, dry ice was used to separate water from
the polymer. The approach was to put the bottles in dry ice for 12 hours in a refrigerator
and then allow them to melt at room temperature. The water was on the top and the
polymer on the bottom. Water was then discarded and the polymeric beads washed with
water and methanol, separately. The product was dried in an air oven for a day or two
and, then, dried in a vacuum oven to constant weight.
2.2.3 SEM
A drop of latex was placed onto a cover glass which was attached to a sample
holder. Dried samples were coated with 200 A of gold. Sample were examined in SEM
at magnifications up to 20,000. Polaroid photos were taken of typical fields.
2.3 RESULTS
Crosslinked PMMA particles were largely monodisperse. Five runs with 3
concentrations of crosslinking agent, EGDMA, yield particle sizes ranging from 315 nm
to 422 nm (Table 2-1). SEM pictures taken of latex particles are shown in Fig. 2-1 to
Fig. 2-4.
38
Table 2-1. Particle size of synthesized PMMA beads.
Run 0824 0829 0908 0912 1003
EGDMA
(% mole)
10 5
2 2
10
Particle Diameter
(nm)
422 346 344 362 315
L- SE1 EH T- 10.0 K U
5.00um I —---
RunQ824 PI8W ulth lO Z E G O n fl
Fig. 2-1. SEM of PMMA particles. The particle is 10 mo!e% crosslinked with EGDMA at SO °C.
39
^poook
Fig. 2-2. SEM o f PMMA particles. The particle is 5 mole% crosslinked with EGDM A at SO °C,
40
L- SE I E H T * 10.0 K U U 0- 10 n r n
2.00uin I -------------■ -------- 1
Rnu0312 Ptfflft u ttn 22EGTO____
Fig. 2-3. SEM PMMA particles. The panicle is 2 mole% crosslinked with EGDMA at 80 °C.
41
L* SE1 E H T * 10.0 K U UD- 10 inn
2.00|im 1 ------------ ----------- 1
Bunl003 Ptfflfl ulth lO Z E G O IT ft
Fig. 2-4. SEM of PMMA particles. The panicle is 10 mole% crosslinked with EGDMA at 80 °C in a
different run.
42
2.4 DISCUSSION
Poly(methyl methacrylate) particles were synthesized with different degrees of
crosslinking at 80 °C. 80 °C was selected as the reaction temperature because at this
temperature only 4-5 hours were required to achieve conversions of 90%. Moreover, the
sizes of particles achieved was reasonable for rheological study. It can be seen that
emulsifier-free emulsion polymerization resulted in largely monodisperse microspheres,
ranging in diameter from 315 nm to 422 nm.
There were no clear relation between the degree of crosslinking and particle size
and, therefore, the two might not be closely related. For 2 % crosslinked PMMA particle
sizes for two different runs are very close. However, syntheses of 10 % crosslinked
PMMA resulted in different particle sizes o f422 nm and 315 nm. We found that in run
0824, the input nitrogen was turned off accidentally one hour after initiation and
interrupted for an hour before being switched on again. This might result in back
diffusion of oxygen into the reaction system from the cooling column. Since oxygen
radicals react with initiator at a rate larger than that of monomer reacting with initiator,
the reaction was retarded to some degree. However, since the amount of oxygen
backflowing into the system was small, the retardation was not substantial. Therefore,
the reaction still went on with rate being slowed down. The nucleation rate decreased
and more monomer diffused into the latex and formed bigger particles (422 nm).
43
Another phenomenon observed was that when filtering PMMA particles, it was
more difficult to filter PMMA with only 2 % degree of crosslinking. Particles with 2 %
crosslinking were not as rigid as 5% or 10%, which resulted in a more difficult
separation.
2.5 CONCLUSION
Monodisperse PMMA particles were synthesized for rheological studies.
Emulsifier-free emulsion polymerization at 80 °C yielded particle size ranging from 315
nm to 422 nm. There was no apparent relation between particle size and crosslink
density. Because the particle size was directly measured from SEM images, it is
suggested that more accurate measurement might need to be adopted if particle size and
particle size distribution become important in future study.
44
REFERENCES
1. Z. Song and G. W. Poehlein, “Particle Formation in Emulsifier-Free Aqueous-Phase
Polymerization of Styrene,” J. Colloidal Interface Sci., 128,501 (1989).
2. J. L. Guillaume, C. Pichot, and J. Guillot, “Emulsifier-Free Emulsion
Copolymerization of Styrene and Butyl Acrylate, m . Kinetic Studies in the Absence of
Surfactant,” 7. Polym. Sci.: Part A: Polym. Chem., 28,119 (1990).
3. J. L. Guillaume, C. Pichot, and J. Guillot, “Emulsifier-Free Emulsion
Copolymerization of Styrene and Butyl Acrylate. IH. Kinetic Studies in the Presence of
a Surface Active Comonomer, the Sodium Acrylamido Undecanoate,” J. Polym. Sci.:
Part A: Polym. Chem., 28, 137 (1990).
4. S. Chen and H. Chang, “Kinetics and Mechanism of Emulsifier-Free Emulsion
Polymerization. m.Styrene/nonionic Comonomer (2-hydroxyethyl Methacrylate)
System,” J. Polym. Sci.: Part A: Polym. Chem., 28, 2547 (1990).
5. Z. Song and G. W. Poehlein, “Kinetics of Emulsifier-Free Emulsion Polymerization
of Styrene,” J. Polym. Sci.: Part A: Polym. Chem., 28, 2359 (1990)
6. B. W. Brooks and J. Wang, “Kinetics of Seeded Emulsion Polymerization of Vinyl
Acetate with No Added Emulsifier,”
7. H. Schluter, “Theory of Colloid Stability and Particle Nucleation Kinetics in
Emulsion Polymerization,” Colloid Polym. Sci., 271, 246 (1993).
8. D. Zou et al., “Model Filled Polymers. I. Synthesis of Crosslinked Monodisperse
Polystyrene Beads,” J. Polym. Sci.: Part A: Polym. Chem., 28, 1909 (1990).
9. D. Zou, S. Ma, R. Guan, M. Park, L. Sun, J. J. Aklonis, and R. Salovey, “Model
Filled Polymers. V. Synthesis of Crosslinked Monodisperse Polymethacrylate Beads,"
J. Polym. Sci. Part A: Polym. Chem., 30,137 (1992).
10. Z. Y. Ding, J. J. Aklonis, R. Salovey, “Model Filled Polymers. VI. Determination of
the Crosslink Density of Polymeric Beads by Swelling,” J. Polym. Sci.: Part B: Polym.
Phys., 2 9 ,1035, (1991).
45
11. D. Zou, L. Sun, L. L. Aklonis, and R. Salovey, “Model Filled Polymers. VHL
Synthesis of Crosslinked Polymeric Beads by Seed Polymerization, ” J. Polym. Sci.:
Part A: Polym. Chem., 30, 1463, (1992).
12. Z. Ding, S. Ma, D. Kriz, J. J. Aklonis, and R. Salovey, “Model Filled Polymers. IX.
Synthesis of Uniformly Crosslinked Polystyrene Microbeads," J. Polym. Sci.: Part B:
Polym. Phys., 30, 1189, (1992).
13. D. Zou, J. J. Aklonis, and R. Salovey, “Model Filled Polymers. XI. Synthesis of
Monodisperse Crosslinked Polymethacrylonitrile Beads, ” J. Polym. Sci.: Polym. Chem.,
30, 2443(1992).
14. Q. Li, M. S. Thesis, University of Southern California, Los Angeles, 1994.
15. L. Sun, Ph.D. Thesis, University of Southern California, Los Angeles, 1992.
16. J. Lee and M. Senna, “Preparation of Monodisperse Polystyrene Microspheres
Uniformly Coated by Magnetite via Heterogeneous Polymerization,” Colloid Polym.
Sci., 273,76 (1995).
17. M. Okubo and T. Nakagawa, “Formation of Multihollow Structures in Crosslinked
Composite Polymer Particles,” Colloid Polym. Sci., 272, 530 (1994).
18. R. J. Young and P. A. Lovell, Introduction to Polymers, 2nd ed., Chapman & Hall,
London, 1991.
19. R. B. Seymour, Introduction to Polymer Chemistry, McGraw-Hill, New York, 1971.
20. B. R. Morrison etal., “Free Radical Exit in Emulsion Polymerization. II. Model
Discrimination via Experiment,” ," J. Polym. Sci.: Polym. Chem., 32, 631 (1994).
21. W. V. Smith and R. W. Ewart, J. Chem. Phys., 16, 592 (1948).
22. M. R. Grancio and D. J. Williams, J. Polym. Sci. A l , 8, 2617 (1970).
23. S. Chen and S. Lee, “Seeded Latex Polymerizations: Studies on the Particle Growth
Mechanism of Latex Particles,” Polymer, 33, 1437 (1992).
24. B. S. Casey et al., “Free Radical Exit in Emulsion Polymerization. I. Theoretical
Model,” J. Polym. Sci. Part A: Polym. Chem. 32, 605 (1994).
25. E. M. Verdurmen et al., “Seeded Emulsion Polymerization of Butadiene. 1. The
Propagation Rate Coefficient,” Macromolecules, 26, 268 (1993).
46
26. Z. Song and G. W. Poehlein, “Particle Nucleation in Emulsifier-Free Aqueous-
Phase Polymerization: Stage 1,” J. Colloid Interface Sci., 128, 486 (1989).
27. A. R. Goodall, M. C. Wilkinson, and J. Hearn, “Mechanism of Emulsion
Polymerization of Styrene in Soap-Free Systems,” J. Polym. Sci., Polym. Chem., Ed.,
15, 2193 (1977).
28. R. M. Fitch and C. H. Tsai, in Polymer Colloids, R. M. Fitch, Ed., Plenum, New
York, 1971.
Chapter 3
Radiation Effects
47
3.1 INTRODUCTION
As previously introduced, the irradiation of polymers has been extensively
studied.1 ' 1 0 Research has often been focused on radiolytic mechanisms.1 1 '1 2 Little has
been done on rheology. The objective of this research is to investigate the effect of
radiation on the rheological behavior of filled polymer systems. We performed
experiments on the following aspects:
(1) the effect of radiation on a pure matrix;
(2) the effect of radiation on a filled polymer composite;
(3) the influence of degree of crosslinking of fillers on radiation effects;
(4) the effect of oxygen atmosphere during irradiation;
(5) comparison between a matrix filled with pre-irradiated particles and irradiation
of a filled composite.
48
3.2 EXPERIMENTAL DETAILS
3.2.1 Materials and Instruments
The filler, poly(methyl methacrylate) particle, was synthesized as described in
Chapter 2. The matrix, polystyrene (PS), was from Dow Chemical (Dow Styron 685)
with weight-average molecular weight of 270 000 and polydispersity of 3.37, as
determined by gel permeation chromatography in tetrahydrofuran calibrated with
polystyrene standards. The antioxidant was 2, 6-di-terf-butyl-4-methy 1-phenol, 99+ %
purity from Aldrich Chemical Co.
A plasti-corder of type PL-V3AA from C. W. Brabender Instruments, Inc.
(South Hackensack, NJ) was used for mixing the matrix and filler. A Dake press (Grand
Haven, MI) model 44-226 was used to compression mold the sample into a disc. The gel
permeation chromatograph was from Water Associates. The key instrument was the
Weissenberg rheonogiometer (serial No. AD 358510) from Sangamo Western Controls,
Ltd. Scanning electron microscopy (SEM) was Cambridge 360 from Cambridge
Instruments.
3.2.2 Composite Preparation
50 g PS was added to the Brabender plasti-corder along with 0.3% wt
antioxidant and heated to 175 °C. Then 5 g of PMMA particles was added in and mixed
49
at 50 rpm for 7.5 minutes and 100 rpm for 7.5 minutes. Approximately 3 g of the
mixture was molded in the press into a disc 1 mm thick and 5 cm in diameter. The
mixture was heated to 350 °F and maintained at that temperature for more than 1 hour
before applying pressure. The pressure was 1.5 x 107 Pa (10 tons on 3 5/8” diameter
ram).
3.2.3 Fracture Surface
The molded composite was immersed into liquid nitrogen, broken, mounted on a
sample holder, and coated with gold. Scanning electron microscope was used to
examine the surface of the composites.
3.2.4 Irradiation
Samples were taken to StyreGenics International Inc. (Tustin, CA) for
irradiation. Irradiation was done by y-rays from 6 0 Co. The average dose was 3.21 Mrad.
Samples were stored in plastic bags for irradiation in the air and in pyrex flasks, which
were vacuumized, for irradiation in vacuum.
3.2.5 Rheology Measurement
Rheological measurement of polymer melts was carried out on a Weissenberg
rheogoniometer (Model 19) at 200 °C. After gap setting at 200 °C, the disc-shaped
50
sample was loaded onto the lower cone and heated to 200 °C. The excess sample
flowing out of the rim of the cone was cleared and the system was kept at 200 °C for at
least one hour. Dynamic mechanical analysis was carried out at different frequencies
from 0.001896 to 18.96 Hz. Then, steady shear measurement was performed from
0.013859 s'1 to 4.373 s'1 .
3.3. RESULTS
Data collected from the experiment are processed and the results are plotted in
Fig. 3-1 through Fig. 3-28. The units of G’ and G” are Pa and of T|' and T|, Pa-s. The
unit of shear rate is sec'1 and of frequency, Hz.
Fig. 3-1 to 3-4 show irradiation effects on pure polystyrene matrix. Fig. 3-5. to
3-8 show irradiation effects on filled polymer composite. Fig. 3-9 to 3-12 show the
effect of degree of crosslinking of filler on rheology of composites. Fig. 3-13 to 3-20
show the effect of degree of crosslinking on the irradiation effect on filled in air. Fig. 3-
21 to 3-28 compares the difference between the two cases, one of which is to irradiated
PMMA particles first and then mix them with PS matrix, another being to irradiate the
whole composite. We examined these cases in vacuum and in air.
Fracture surfaces of PS matrix containing 10%wt 10% crosslinked PMMA
particles were examined. We also examined the PS matrix containing 10% wt 10%
crosslinked PMMA particles which had been previously irradiated. The SEM pictures
are shown in Fig. 3-29 and Fig. 3-30.
6
5.5
5
4
3.5
3
2.5
2
■ 2 -I 0 2 1
Fig. 3-1. Storage modulus G’ vs. frequency in dynamic mechanical analysis.
+ PS;
PS irradiated in vacuum to a dose of 3.21 Mrad.
lo g G
5.5 --
4.5 --
4 --
3.5
• 2 1 0 1
lo g F
Fig. 3-2. Loss modulus G" vs. frequency in dynamic mechanical analysis.
+ PS;
PS irradiated in vacuum to a dose of 3.21 Mrad.
5.2
5
4.8
4.6
4.4
V
u
_o
” 4.2
4
3.8
3.6
3.4
-2 -1.5 -1 -0.5 0 0.5 I 1.5
tog F
Fig. 3-3. Dynamic viscosity t\ ' v s . frequency.
+ PS;
PS irradiated in vacuum to a dose of 3.21 Mrad.
.4— ,— ,— 1 — ,— I — ,— ,— ,— 1 — j— 1 — .— ,— , — |— .— ,— ,— ,— i— , — ,— ,— ,—
la g n (p aj)
4.6
4.2 ■■
4 --
3.8
3.6 -
3.4 -
3.2 --
2.8 ■■
2.6
■ 2 -1.5 -0.5 0.5 1 0 1
tog shear rate (I/sec)
Fig. 3-4. Viscosity vs. shear rate from steady shear measurement.
+ PS;
PS irradiated in vacuum to a dose of 3.21 Mrad.
6.5
6 ••
5.5 ■ ■
5 - ■
4.5 *
4 -
3.5 -•
•2 -1.5 1 -0.5 0 0.5 1
log F
Fig. 3-5. Storage modulus vs. frequency of 10% wt PMMA (10% crosslinked) in PS matrix.
o unirradiated composite;
* composite irradiated in vacuum.
lo g G
56
5 .8
5 ,6
5 .4
5 .2
4 .8
4 .6
4 .4
4 .2
■ 2 1 0 -1 ,5 -0 .5 1 0 .5 1.5
log F
Fig. 3-6. Loss modulus vs. frequency of 10% wt PMMA (10% crosslinked) in PS matrix.
o unirradiated composite;
* composite irradiated in vacuum.
log G*
5.5
5 .3 -;
5.1 - 1
4.9
4.7 -1
4.5
4.3 --
4.1
3.9 - 1
3.7 - :
3.5
-2 -0 J 0 1,5
log F
Fig. 3-7. Dynamic viscosity vs. frequency of 10% wt PMMA (10% crosslinked) in PS matrix.
o unirradiated composite;
* composite irradiated in vacuum.
lo g T i (pa.s)
4.4--
4 -
3 .8 -
3,6 -
3.4 •
• 2 -15 0 1
log shear rate (M sec)
Fig. 3-8. Steady shear viscosity vs. shear rate of 10% wt PMMA (10% crosslinked) in PS matrix.
o unirradiated composite;
* composite irradiated in vacuum.
59
6 -
4 - :
-15 05 - 2 • 1 -05 0 15 1
lcgF
Fig. 3-9. Storage modulus vs. frequency of 10% wt PMMA with different degrees of crosslinking in PS
matrix.
+ 10% crosslinked;
A 5% crosslinked;
2% crosslinked.
60
O
tn
o
4 -
1 ■05 -2 -L5 Q5 1.5 0 1
logF
Fig. 3-10, Loss modulus vs. frequency of 10% wt PMMA with different degrees of crosslinking in PS
matrix.
+ 10% crosslinked;
A 5% crosslinked;
2% crosslinked.
Loss Factor
30
25 --
20 - -
15 --
10 --
5 --
—*
-3 -2.5 ■ 2 0 0.5 1 -1.5 1 -0.5 1.5
log F
Fig. 3-11. Loss factor vs. frequency of 10% wt PMMA with different degrees of crosslinking in PS
matrix.
+ 10% crosslinked;
A 5% crosslinked;
2% crosslinked.
4.5
4.3 --
4.1 --
O ) 3.9 -■
3.7 --
3.5 --
3.3
-2 0 0.5 -0.5 1 -1.5 -1
l o g y
Fig. 3-12. Steady shear viscosity vs. shear rate of 10% wt PMMA with different degrees of crosslinking in
PS matrix.
+ 10% crosslinked;
A 5% crosslinked;
2% crosslinked.
63
6 -
5.5-
O
O )
o
4.5-
4 -
-05 -1.5 Q5 ■ 2 0 1 1 .5 1
logF
Fig. 3-13. Storage modulus vs. frequency o f 10% wt PMMA (irradiated at 3.21 Mrad in air) with different
degrees of crosslinking in PS matrix.
+ 10% crosslinked;
A 5% crosslinked;
2% crosslinked.
lo g G
64
5.8 --
5.4
4.6 - 1
4.4
1.5 0 0.5 •0.5 1.5
lo g f
Fig. 3-14. Loss modulus vs. frequency of 10% wt PMMA (irradiated at 3.21 Mrad in air) with different
degrees o f crossiinking in PS matrix.
+ 10% crosslinked;
A 5% crosslinked;
2% crosslinked.
Loss Factor
65
3.5
2.5 --
1.5 --
1 - -
0.5
-2.5 -1.5 -I -0.5 0 0.5 1
Log F
1.5
Fig. 3-15. Loss factor vs. frequency o f 10% wt PMMA (irradiated at 3.21 Mrad in air) with different
degrees of crosslinking in PS matrix.
10% crosslinked;
5% crosslinked;
2% cross I inked.
66
4.4 --
4.2 -
4 -
3.6 --
3.4 --
3.2 --
■ 2 -1.5 0.5 -1 -0.5 0
logy
Fig. 3-16. Steady shear viscosity vs. shear rate of 10% wt PMMA (irradiated at 3.21 Mrad in air) with
different degrees of crosslinking in PS matrix.
+ 10% crosslinked;
A 5% crosslinked;
2% crosslinked.
logG’
67
6.5
5.5 -
4.5 -
4 --
3.5
-2 0.5 -0.5 0 1.5 1.5 - 1
log F
Fig. 3-17. Storage modulus vs. frequency of 10% wt PMMA with different degrees of crosslinking in PS
matrix (composite of PS and PMMA irradiated at 3.21 Mrad in air).
+ 10% crosslinked;
A 5% crosslinked;
2 W o crosslinked.
loeG"
68
5.5 --
4.5 --
■ 2 1.5 -0.5 0.5 0 1.5
log F
Fig. 3-18. Loss modulus vs. frequency of 10% wt PMMA with different degrees o f crosslinking in PS
matrix (composite of PS and PMMA irradiated at 3.21 Mrad in air).
+ 10% crosslinked;
A 5% crosslinked;
2% crosslinked.
69
5.3
4.9
4.7 --
4.5 --
4.3 --
3.9 --
3.7 --
3.5
-2 -0.5 1.5 0 0.5 1.5
log F
Fig. 3-19. Dynamic viscosity vs. frequency o f 10% wt PMMA with different degrees o f crosslinking in PS
matrix (composite o f PS and PMMA irradiated at 3.21 Mrad in air).
+ 10% crosslinked;
A 5% crosslinked;
2% crosslinked.
70
4.4
4.2
3.6
3.4
3.2
0 0.5 - 0.5 ■ 2 1.5 - 1
lo g y
Fig. 3-20. Steady shear viscosity vs. shear rate of 10% wt PMMA with different degrees of crosslinking in
PS matrix (composite of PS and PMMA irradiated at 3.21 Mrad in air).
+ 10% crosslinked;
A 5% crosslinked;
2% crosslinked.
log G'
71
6.2
5.8
5.6 --
5.4 -•
5.2 -■
5
4.8 --
4.6 ■ ■
4.4 ~
4.2 -•
4 -■
3.0 - 1
3.6 -•
3.4
-1.5 0.5 1.5 - 1 -0.5 0
log F
Fig. 3-21. Storage modulus vs. frequency of irradiated 10% wt PMMA (10% crosslinked) in unirradiated
PS.
o unirradiated composite;
* PMMA irradiated in vacuum (3.21 Mrad);
x PMMA irradiated in air (3.21 Mrad).
log G
72
5.5 -•
4.5 --
-1.5 - 2 -0.5 0 0.5 1.5
log F
Fig. 3-22. Loss modulus vs. frequency of 10% wt PMMA (10% crosslinked) in PS.
unirradiated composite;
PMMA irradiated in vacuum (3.21 Mrad);
PMMA irradiated in air (3.21 Mrad).
log n'
73
5.5
3.5 --
0.5 1.5 •2 1.5 -0.5 0
log F
Fig. 3-23. Dynamic viscosity vs. frequency o f 10% wt PMMA (10% crosslinked) in PS.
unirradiated composite;
PMMA irradiated in vacuum (3.21 Mrad);
PMMA irradiated in air (3.21 Mrad).
lo g n (pu.s)
4.4 --
4.2 --
4 --
3.8 --
3.6 --
3.4 --
3.2
-2 -1.5 -I -0.5 0 0.5 I
log shear rate (1/sec)
Fig. 3-24. Steady shear viscosity vs. shear rate of 10% wt PMMA (10% crosslinked) in PS.
o unirradiated composite;
* PMMA irradiated in vacuum (3.21 Mrad);
+ PMMA irradiated in air (3.21 Mrad).
lo g G‘
6.5
5.5 -•
5 --
4.5 -■
4 -■
3.5 -■
■ 2 0.5 1.5 1.5 ■0.5 0
log F
Fig. 3-25. Storage modulus of 10% wt PMMA (10% crosslinked) in PS irradiated in vacuum and
o unirradiated composite;
* composite irradiated in vacuum;
x composite irradiated in air.
76
5.5 -
4.5 --
4 ■ ■
3.5
■ 2 1.5 43.5 0 0.5 1.5
log F
Fig. 3-26. Loss modulus vs. frequency of 10% wt PMMA (10% crosslinked) in PS irradiated in vacuum
and air.
o unirradiated composite;
* composite irradiated in vacuum;
x composite irradiated in air.
lo g t|*
77
5.5
5.3 --
4.9 ••
4.7 -
4.5 ■ ■
4.1
3.9 --
3.7 -
3.5
0.5 ■ 2 0 -0.5 1.5 1.5
log F
Fig. 3-27. Dynamic viscosity vs. frequency of 10% wt PMMA (10% crosslinked) in PS irradiated in
vacuum and air.
o unirradiated composite;
* com posite irradiated in vacuum;
x com posite irradiated in air.
(mil) L 3o|
78
4 ,4 • ■
4.2
3.8
3.6
3.4 -■
3.2
■ 2 1.5 0.5 - 0.5 0
lag shear rate (1/sec)
Fig. 3-28. Steady shear viscosity vs. shear rate of 10% wt PMMA (10% crosslinked) in PS irradiated in
vacuum and air.
o unirradiated composite;
* composite irradiated in vacuum;
x composite irradiated in air.
79
Fig. 3-29. Fracture surface of PS matrix filled with 10% wt 10% crosslinked PMMA without being
irradiated.
80
Fig. 3-30. Fracture surface of unirradiated PS matrix filled with 10% wt 10% crosslinked irradiated
PMMA.
81
3.4 DISCUSSION
The effects of radiation on the rheological behavior of pure polystyrene matrix
and of polystyrene filled with crosslinked PMMA particles were investigated. The
influence of degree of chemical crosslinking either on rheological behavior of
unirradiated filled polymer or polymer with irradiated filler or irradiated filled polymer
composites was also studied. The effect of oxygen was examined. The rheology
behavior of irradiated composites and composites filled with irradiated PMMA particles
were compared.
For pure PS matrix, irradiation in vacuum caused PS to crosslink and therefore
the viscosity increased. It was surprising that the change was so large at this low dose
(Fig. 3-1 to Fig. 3-4). As introduced in section 1.2, irradiation resulted in two different
free radicals: cyclohexadienyl-type radical and disubstituted benzyl radical.1 1 Addition
of H to the former could cause crosslinking to the phenyl ring. It was also possible that
the disubstituted benzyl radicals reacted and crosslinked. An insoluble gel was formed
above a critical dose. We could not dissolve the irradiated PS into THF. From Fig. 3-4,
it could be seen that the pure PS matrix had Newtonian flow behavior over a
considerable range from log shear rate of -2 to 0.5. A flat curve over a large range was
observed for PS with lower molecular weight.1 3 However, after irradiation, the viscosity
curve tended to decrease with increasing shear rate, exhibiting Non-Newtonian
behavior. It could be seen that the viscosity decreased drastically when log shear rate
82
was greater than -0.5, corresponding to 0.3 s'1 . The increase in dynamic mechanical
moduli and viscosity was not a simple shift of the curve as could be seen in Fig. 3-1, 3-
2, and 3-3. The curves differed most at low frequency. At higher frequency, the two
curves approached each other, indicating similar rheological behavior at high frequency.
At high frequency small units in both of the matrices could oscillate more freely.
However the crosslinking effect was not eliminated by high frequency as the lower
curve never crossed the upper curve in these three figures.
For the PS matrix filled with PMMA particles, various factors had to be taken
into account to consider irradiation effects on rheology. Fig. 3-5 showed the storage
modulus vs. frequency of PS filled with 10 % wt PMMA (10% crosslinked with
EGDMA). Irradiation of the composite in vacuum caused a slight decrease in G ’. The
same phenomenon happened to G” and r[\ which equals G’Vfrequency. The steady shear
viscosity vs. shear rate plotted in Fig. 3-8 also showed the same trend. These will be
discussed later in this section.
Effects of different degree of crosslinking were investigated. It was previously
reported that the effect of degree of crosslinking d i. not affect the rheological behavior
of the filled composite. We observed the same phenomenon at intermediate frequency in
DMA. However at low frequency 5% crosslinked particles tended to have the largest G’
while 2% crosslinked PMMA had the smallest G \ There was a 50% difference in G’ of
5% crosslinked PMMA between the two. Since G’ indicates energy stored in a system,
crosslinking enhanced the elastic response and therefore 5 % crosslinked PMMA
exhibited less energy dissipation (Fig. 3-9 and 3-10) and more energy storage when
oscillation frequency was low, resulting in considerably large difference in G’ and G”.
From loss factor plot we also observed a large difference at low frequency. The steady
shear viscosity also exhibited the same trend, i.e., large difference at low shear rate and
little difference at intermediate shear rate. At large shear rate, especially at the largest
shear rate of our measurement, data obtained were not reliable since we observed that
the sample was sheared out of the rim of the Weissenberg rheogoniometer.
To investigate the influence of irradiation, rheological measurements were
carried out for two cases: (A) matrix filled with preirradiated PMMA particles and (B)
irradiated composite filled with PMMA. For case A, no difference was observed for G’
at intermediate and high frequency for different particle crosslink density (Fig. 3-13).
What seemed interesting was that G’ of 2% crosslinked PMMA was greater than that of
10 % crosslinked PMMA while 5% crosslinked PMMA was situated between the two at
low frequency. The steady shear viscosity (Fig. 3-16) showed a different and clear
pattern with 10 % crosslinked PMMA on top and 2 % on the bottom of the curves.
Irradiation to 3.21 Mrad effectively decomposed PMMA particles, causing chain
scission depolymerization, and yielding gaseous products. Here, the degree of chemical
crosslinking was much more important than for the unirradiated sample. The more the
PMMA microparticle was crosslinked, the less irradiation damage was possible on the
particles. The network formed by crosslinking served to protect PMMA from
depolymerizing.
The same trend was observed in case B as shown in Fig. 3-20. However the
decrease in viscosity from 10% crosslinked particles to 2 % was not as big as in case A.
In case A, the decrease of rj at low shear rate ranged from 36.9% to 25.9% from 10%
crosslinked to 2% crosslinked PMMA on basis of the former. In case B, the drop was
only approximately 20% at low shear rate. The reason was that when composites of
particles and matrix are irradiated, rheological properties were not only affected by the
degree of chemical crosslinking of the particles but also by radiation-induced
crosslinking of the matrix, as will be discussed later in this section. Therefore, although
the same trend has been observed, the effect of degree of crosslinking of particles
became less important compared to case A (Fig. 3-16). However, crosslinking did
influence the irradiation effect and results obtained have been consistently reproduced.
The effects of oxygen are shown in Fig. 3-21 to Fig. 3-28. We carried out similar
experimental measurements as those showing the effect of the degree of crosslinking.
10% wt 10% crosslinked PMMA particle powders in PS matrix was chosen for this
research. 10% chemical crosslinking could effectively retain properties of the particles
when they were mixed in the Brabender. Two experimental cases previously described
as case A and case B were investigated. Samples were sealed in glass flasks which were
connected to a vacuum pump to a pressure of lO^psi to remove oxygen from the system
before irradiation. They were measured within one week after irradiation to minimize
postirradiation effects.1 4
Two different phenomena were observed in case A and case B. In case A, the
larger degradation was observed for PMMA particles irradiated in vacuum (Fig. 3-21 to
3-24). Rheological properties of PMMA irradiated in air were greater than those in
vacuum. Since irradiation of PMMA produced mainly chain scission, reaction with 0 2
formed peroxy radicals. When mixing with PS matrix, these radicals might react with
PS, causing slight crosslinking or stronger particle-matrix interaction, which offset the
degradation caused by irradiation. Therefore G’ and G” of PMMA irradiated in air was
greater than that in vacuum. It is noticed that at lowest frequency, it was even greater
than that of unirradiated composites. The steady shear viscosity vs. shear rate plot (Fig.
3-24) also clearly showed this trend.
In case B, however, the trend was different. The composite irradiated in vacuum
had greater moduli, dynamic viscosity, and steady shear viscosity than those of the
composite irradiated in air. Although changes from original unirradiated composite were
rather small, the trends were all the same with unirradiated composite on top, followed
by composite irradiated in vacuum and composite irradiated in air on the bottom of the
curves. In steady shear measurement, at higher shear rate the composite irradiated in
vacuum reached the same values as the composite irradiated in air. The reason for case
B to differ from case A resulted from several considerations. As discussed in 1.2,
oxygen could penetrate the surface of the irradiated composite when it was being
irradiated in oxygen atmosphere. Oxidation enhanced degradation of the PMMA
particles. Compared to case A, however, where oxidation of PMMA microparticle
powder was much more efficient due to large amount of oxygen presented, which
generated peroxides, less peroxides were generated in case B. Moreover, reaction of
oxygen with radical precursors of crosslinking in PS reduced the amount of crosslinking
in PS. Also oxygen enhanced chain scission in PMMA by reducing cage recombination
of scission fragments. In vacuum radicals from scission of PMMA could react with
polystyrene. Thus, the presence of oxygen resulted in more degradation than in the
absence of oxygen, causing a decrease in rheological properties. Another possible
reason was that PS protected PMMA from depolymerizing to some degree in vacuum
while in air penetration of oxygen decrease this protective effect.
One of the major objectives in this research project was to compare PS matrix
filled with pre-irradiated PMMA particles (case A) to the irradiated composite of PS
filled with PMMA particles (case B). We carried out experiments both in vacuum and in
air. We have replotted the data previously shown in order to facilitate a direct
comparison.
When irradiated in vacuum, both irradiated composite and composite filled with
irradiated PMMA particles showed degradation from the original unirradiated
composite (Fig. 3-31 to 3-34). The dynamic moduli and viscosities of irradiated
composite were greater than those of the composite filled with pre-irradiated PMMA.
When PMMA was irradiated alone, only chain scission took place. This resulted in a
decrease in rheological properties of PS composite containing degraded PMMA particle,
here, G \ T |\ and r\. When the whole composite containing PMMA was irradiated, the
lo g G'
PMMA particles degraded, but the PS matrix was crosslinked, which could increase
viscosity and moduli.
6.5
6 --
5.5 -
5 ■ ■
4.5 -
4 -
3 .5 ••
■ 2 1 0.5 1 -1.5 -0.5 0 1.5
lo g F
Fig. 3-31. Storage modulus vs. frequency of 10% wt PMMA (10% crosslinked) in PS.
o unirradiated composite;
* composite irradiated in vacuum (3.21 Mrad);
x Only PMMA particles irradiated in vacuum (3.21 Mrad).
*
o
a >
o
4 - ■
3 J
-2 - 1 J 1 1 1 J 0
log f
Fig. 3-32. Loss modulus vs. frequency o f 10% wt PMMA (10% crosslinked) in PS.
o unirradiated composite;
* com posite irradiated in vacuum (3.21 Mrad);
x Only PM M A particles irradiated in vacuum (3.21 Mrad).
log
5.5 i
5.3 - ■
5.1 -•
4.9 --
4.7 --
4.5 -
4.3 ■ ■
4.1 -■
3.9 --
3.7 --
3.5
0.5 1 • 2 -1.5 1 0 1.5
to g F j
• f
j
Fig. 3-33. Dynamic viscosity vs. frequency of 10% wt PMMA (10% crosslinked) in |’S.
o unirradiated composite; /
* composite irradiated in vacuum (3.21 Mrad); v
x Only PMMA particles irradiated in vacuum (3.21 Mrad). j
t o g I I ( p a j )
4.4 -•
4.2 -■
4 --
3.S --
3.6 -•
3.4 --
3.2
- 2 1 0 0 . S t
log shear rate (1/sec)
Fig. 3-34. Steady shear viscosity vs. shear rate of 10% wt PMMA (10% crosslinked) in PS.
o unirradiated composite;
* composite irradiated in vacuum (3.21 Mrad);
x Only PMMA particles irradiated in vacuum (3.21 Mrad).
When irradiated in air, the rheological responses (Fig. 3-35 to 3-38) differed
from those for irradiation in vacuum. Composites containing irradiated PMMA showed
larger rheological properties than those of the irradiated composite. As explained before,
the presence of oxygen affected irradiation in air. In the presence of oxygen, radiation-
induced crosslinking of PS was reduced and so the degradation of PMMA produced
reduced rheological properties. On the other hand, when PMMA microparticle powders
were irradiated alone in presence of oxygen, oxidation of these powders was much more
efficient and generated more peroxides than that in composites. Peroxide decomposition
during mixing and molding led to linking of PMMA and PS, increasing dynamic moduli
and viscosities.
As it can be seen from Fig. 3-39, incorporation of 10% wt PMMA particles into
PS matrix significantly increased the viscosity. It was greater than that of the
crosslinked (by irradiation) PS matrix at the same shear rate. Therefore irradiation of the
whole composite did not increase the values of rheological properties. Even if the PS
matrix in the composite were crosslinked, the resulting viscosity did not surpass the
value for the unirradiated composite. Nevertheless, due to the low dose rate and small
dosage in irradiation, we did not observe significant change in every case. For example,
in fracture surface observation, we found no difference between unirradiated and
irradiate samples (Fig. 3-29 and 3-30). Larger dose of irradiation would be highly
desirable for further research.
lo g G'
6.5
6 ■ ■
5.5 --
5 - -
4.5 -
4 ■ -
3.5 -•
1.5 •2 0.5 1 -0.5 0 I
log F
Fig, 3-35. Storage modulus vs. frequency of 10% wt PMMA (10% crosslinked) in PS.
o unirradiated composite;
* composite irradiated in air (3.21 Mrad);
x Only PMMA particles irradiated in air (3.21 Mrad).
to g G'
6
55
5 ••
45 -
4 ■■
3 5 ■ + -
-2 -15 - 1 -05 0
log F
05 15
Fig. 3-36. Loss modulus vs. frequency o f 10% wt PMMA (10% crosslinked) in PS.
o
*
unirradiated composite;
composite irradiated in air (3.21 Mrad);
Only PMMA particles irradiated in air (3.21 Mrad).
lo g i f
5.5
5 • ■
4 .5 -■
4 - ■
3 .5 ■ ■
-2 -0.5 1 - 1 0 0.5 1.5
lo g F
Fig. 3-37. Dynamic viscosity vs. frequency o f 10% wt PMMA (10% crosslinked) in PS.
o unirradiated composite;
* com posite irradiated in air (3.21 Mrad);
x Only PM M A particles irradiated in air (3.21 Mrad).
logT|(pJU)
4.4 -•
4.2 --
4 - •
3.8 --
3.6 -•
3.4 --
3.2
-2 -1.5 ■ 1 -OJ 0.5 0 1
log shear rate (1/sec)
Fig. 3-38. Steady shear viscosity vs. shear rate of 10% wt PMMA (10% crosslinked) in PS.
o unirradiated composite;
* composite irradiated in air (3.21 Mrad);
x Only PMMA particles irradiated in air (3.21 Mrad).
96
4,4
4 --
3.8 --
O )
o
3.6 ■ ■
3.4 --
- 2 -0.5 0 0.5 -1.5 1 1
log shear rate
Fig. 3-39. Comparison of unirradiated filled composite (PS filled with 10% wt 10% crosslinked PMMA)
and irradiated PS matrix (in vacuum).
+ Filled composite;
Irradiated PS matrix.
97
3.5 CONCLUSION
Irradiation at a dose of 3.21 Mrad caused polystyrene matrix to crosslink, which
increased values both of dynamic rheological properties, i.e., G', G", andT|', and of
steady shear viscosity, i.e., t\. Irradiation of PS matrix filled with PMMA particles
decreased values of these properties due to radiation-induced decomposition of PMMA
particles. The degree of PMMA particle crosslinking affected the rheological behavior
of unirradiated filled composite at low frequency and small shear rate as higher degrees
of crosslinking increased elasticity and led to higher values of rheological properties.
Two cases were compared: (A) matrix filled with pre-irradiated PMMA particles; (B)
filler particles and matrix irradiated together. In all cases rheological properties declined
after irradiation. The presence and absence of oxygen were compared. For case A, the
larger degradation occurred in vacuum while for case B it occurred in air. The rheology
behavior of irradiated composites and the composites filled with irradiated PMMA
particles have also been examined. In vacuum, the larger degradation occurred in case A
while in air it occurred in case B.
From the above information, it can be concluded that irradiation has a profound
effect on the rheological behavior of polymer composites. Even at such a low dose, the
behavior of polymer composites was changed For pure PS crosslinking occurred on
irradiation and for filled composites degradation took place. Degree of crosslinking of
98
PMMA microparticles also affected rheology. A summary of the results is shown in Fig.
40.
4.4
4 -
V
as-
o 0.5 - 1 •0.5 -2 -1.5
log shear rats
Fig. 3-40. Summary plot of steady shear viscosity (not including the viscosity at the highest y ).
+ PS;
PS irradiated in vacuum (dotted line);
• PS matrix filled with 10 wt% PMMA (10 mole% crosslinked with EGDMA);
x Unirradiated PS matrix filled with preirradiated PMMA in air (dashed line);
* Composite irradiated in vacuum;
o Composite irradiated in air;
A Unirradiated PS matrix filled with preiiradiated PMMA in vacuum (dashed line).
99
REFERENCES
1. A. Charlesby, Atomic Radiation and Polymers, Pergamon, New York, 1960.
2. J. H. Bowen, Jr. and D. V. Rosato, “Radiation,” in Environmental Effects on
Polymeric Materials, D. V. Rosato and R. T. Schwartz, ed., Vol. 1, Interscience
Publishers, New York, 1968.
3. J. B. Birk, Photophysics o f Aromatic Molecules, Wiley, New York, 1970.
4. N. Grassie, Development in Polymer Degradation — 1, Applied Science Publishers,
London, 1977.
5 .1. Y. Petrov and V. L. Karpov, Radiation Chemistry o f Polymeric System, A. Chapiro
ed., Wiley, New York, 1962.
6. C. David, D. Fuld, and G. Genskens, “Radiolysis of PMMA in the Presence of
Ethylmercaptan,” Die makromoleculare Chemie, 39, 269 (1970).
7.D. Phillips, Polymer Photophysics, Chapman and Hall, London, 1985.
8. D. W. Clegg and A. A. Collyer, Irradiation Effects on Polymers, Elsevier Applied
Science, London, 1991.
9. A. Holmes-Siedle and L. Adams, Handbook o f Radiation Effects, Oxford University
Press, Oxford, 1993.
10. E. E. Schneider, Discuss. Faraday Soc., 19, 1995.
11. J. A. Moore and J. O. Choi, “Degradation of Poly (methyl methacrylate),” in
Radiation Effects on Polymers, R. L. Clough and S. W. Shalaby, ed., American
Chemical Society, Washington, DC, 1991.
12. T. Ichikawa and H. Yoshida, “Mechanism of Radiation-Induced Degradation of
PMMA as Studied by ESR and Electron Spin Echo Methods,” J. Polym. SciPolym.
Chem.,2%, 1185 (1990).
13. L. Sun, Ph.D. Thesis, University of Southern California, Los Angeles, 1992.
14. S. Kalachandra et a l, “Postirradiation Reactions of Monomers in Poly(methyl
methacrylate): Analysis by CP/MAS 1 3 CNMR,” Macrom., 13, 1696 (1980).
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Asset Metadata

Creator Miao, Tao (author)

Core Title Irradiation effects on the rheological behavior of composite polymer systems

School School of Engineering

Degree Master of Science

Degree Program Chemical Engineering

Degree Conferral Date 1995-05

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag chemistry, polymer,chemistry, radiation,engineering, chemical,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Salovey, Ronald (committee chair), Chang, Wenji V. (committee member), Shaffer, John Scott (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c18-3157

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Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

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