Chapter - 1

INTRODUCTION

Abstract

Polymer W s are mixtures of at least two

polymers andlor copolymers comprising

more than two macmmolecular species.

Most bknds are immiscible and need to be

compatibilised. Compatibilisation not only

ascertains the improvement in performance

but also makes the blend reproducible,

insenswe to forming and repeated

processing. This chapter covers the need

for blending of polymers, classification of

polymer blends, thermodynamics of

miscibility, various aspects related to

physical blending. The techniques for

techndoglcal compchbilisation and theories

of compatibilisa~n are also discussed. A

brief anaiysis comparing physical bknding

to reactive Mending is given with special

emphasis on the advantages and

disadvantages of one technique over the

other. Morphology development and conbol

are discussed. A rev& of the latest

developments in the field of

ampatibilisabbn is also given.

1.1 Polymer Blends

Blending of polymers is one of the easiest and most flexible methods

to generate new polymeric materials. The alternative to blending is

synthesising new polymers, which involves exorbitant costs and

cannot often fulfill all industrial requirements. The additivity of the main

properties of two polymers results only when these form a multiphase

system, whereas only an average value of properties is obtained by

homogeneous blends. When two or more polymers are mixed, the

phase structure of the resulting material can be either miscible or

immiscible. Due to their high molar mass, the entropy of mixing of

polymers is relatively low and consequently specific interactions are

needed to obtain blends, which are miscible or homogeneous on a

molecular scale'. In the case of immiscible systems the overall

physicomechanical behaviour depends critically on two demanding

structural parameters2: a proper interfacial tension leading to a phase

size small enough to allow the material to be considered as

macroscopically 'homogeneous' and an interphase 'adhesion' strong

enough to assimilate stresses and strains without disruption of the

established morphology.

1.2. Classification and Preparation o f Polymer Blends

On the basis of the method of development, polymer blends can be

classified into the following categories such as mechanical polyblends,

mechanochemical polyblends, chemical polyblends, solution-cast

polyblends, latex polyblends etc. Various blending techniques have been

extensively reviewed in the literature3. Each method has its merits and

demerits. Mechanical polyblends are generally prepared by melt mixing.

In melt mixing, no solvent is used; therefore solvent removal or a

recycling infrasttucture is not required. Also, on account of recent

3

ecological issues and restrictions, the use of organic solvents is rather

undesirable. Their substitution by solvent-free processing strategies has

thus become increasingly irnportant. Thus considering the high cost of

solvents, poor yield of the final product, carcinogenic and allied pollution

problems with solvents and so on, melt mixing is highly appreciated by

the industry. But, the disadvantages of melt-mixingorc; i) because of large

size of polymer molecule, they don't easily mix and ii) there is chance of

degradation and phase separation at higher temperatures.

1.2.1 Thermoplastic Elastomers (TPEs)

Rubber-plastic blends are commercialised as rubber-toughened plastics

or as thermoplastic elastomers (TPEs). They bridge the gap between

thermoplastics and ela~toniers~~. These are usually phase-separated

systems in which one phase is hard while the other phase is rubbery at

room temperature. Generally, if a relatively large portion of the hard

plastic is used, the corrlpc~sition can be used as an impact resistant

plastic; whereas if a relatively large amount of mbbery phase is used.

blend will be soft and will have at least some of the properties of

elastomers. The improved properties are due to matched surface

energies of plastics and elastomer components, low molecular weight of

elastomer components and sufficient crystallinity for plastic components.

TPEs have a definite advantage over conventional rubbers, as they do

not need curing or cross-l~nking. They can be processed using lower

mold-temperatures and faster molding cycles than conventional rubbers.

Thus by careful selectior~ of component polymerj, their blend ratios and

processing conditions, one tan attain a wide range of desirable properties

of toughened elastomers to high impact thermoplastics. However, most of

the TPEs from rubberlplastil: blends are immiscible and incompatible.

In TPEs wastes and rejects can be reprocessed without change

in properties whereas thermoset rubber scraps are unsuitable for reuse.

Their shorter cycle times and easy fabrication reduces the product- costs.

They also require fewer and simple equipment. These blends do not

require any compounding steps, which are necessaly for thermoset

rubbers. The lower density of TPEs compared to thermoset rubbers

leads to the production of a large number of articles from a given weight.

They are flexible and break at large elongation; they have good vibration

damping characteristi; they can be reinforced w lh fillers as carbon

black and silica. They have good resistance to impact, compressive and

flexural loads and have high fatigue to failure resistance. Reduced

permanent set, improved ultimate mechanical properties, greater fluid

resistance, improved high temperature utility, greater stabilisation of

phase morphology, greater melt strength and more reliable thermoplastic

fabricability7 are the added advantages.

1.2.2 Classification of Thennoplastic Elastomers

On generic classification, thermoplastic elastomers are divided intos (1)

block copolymers, (styrenedienes (styrenics), polyurethanes,

copolyesters and polyamides are block copolymers) (2) olefinic blends,

(EPDM rubber1 polyolefin, nitrile rubber/ polyvinyl chloride blends are

examples of this type) (3) elastomeric alloys with cross-linked rubber

phases, (EPDM rubber1 polypropylene, nitrile rubber1 polypropylene and

natural rubber1 polypropylene, butyl rubber1 polypropylene blends).

Block copolymers consist of alternating soft and hard monomel~

blocks along the polymer chain. Thus, styrenediene TPEs have the

structure S-D-S, where S is a chain of 50 to 80 styrene units and D, a chain

of 20 to 100 diene units. The co-polyester TPEs have ester linkages

between the hard and soft blocks, whereas the polyurethane and

polyamide TPEs have urethane and amide linkages respectively. Olefinic

blends are physical mixtures of thermoplastics such as polypropylene with

a compatible elastomer such as natural rubber. Elastomeric alloys have

highly cross-linked elastomeric phase resulting in a finely divided

dispersion of rubber particles in a continuous thermoplastic matrix.

Because of the synergistic interactions between the thermoplastic and

elastomeric components, these alloys are more rubber-like and are of

higher performance than the blends. Thermoplastic elastomer artides

compete well with themloset rubber items in all non-tyre applications7.

Hence a comparison of these materials is interesting. Unlike thermoset

rubbers. TPEs can be compounded and ready for fabrication. Their

processing is much simpler than thermoset rubber items. So TPE

processing is efficient, speedy and economic. The scrap in TPE

processing is recycled, while that in thermoset rubber is not. Scheme 1.1

depicts the processing steps7 for thermoset rubber and thermoplastic

elastomer articles.

Thermoset rubber article

Gum rubber Filler Mixlng --+ Shaping -+ Vulcanise + Finished Extenders I 1 Rubber

f ~hemicalsJ Scrap Scrap f f Scrap

Thermoplastic elastomer articles

TPE pellets S h a p i n n Finished TPE article

L Recyc le1

article

Scheme 1.1. Processing steps for thermoset ~bbr3-s and thermoplastc elastomers, [Advanced Elastomer Systems, L.P.]

In spite of the advantages cited above, they have the following

disadvantages2. Most commercial TPEs are harder than the hardest of

rubbers, very few of them are only available at lower hardness. At

temperatures above melting point, TPEs lose their rubber like

properties. Even a brief exposure to high temperature can make them

unserviceable due to irreversible deformation. TPEs may require

drying before processing which is not necessary for thermoset

rubbers. In addition, for a rubber product manufacturer it would be too

costly to shift over to the TPE technology.

I. 3 Thermodynamics of Miscibility

Majority of polymers are immiscible at molecular level as given by the

laws of thermodynamics. Given enough time, the internal disorder of

the polymer system will eventually result in phase separation on a

macroscopic scale. The relative miscibility of polymers controls their

phase behaviour, which is of crucial importance for final properties.

The rules governing miscible behaviour of polymer blends are

understood in a thermodynamic sense through the Gibbs free energy

of mixing, AGm. The free energy of mixing can be described in terms

of enthalpic and entropic contributions as

AGm = AH,-TAS, (1.1;)

where, AG, is the free energy of mixing per unit volume and 4 1 ~ is the

volume fraction of component 2, AH, and AS, are enthalpy and

entropy of mixing respectively. AH, is independent of molecular

weight and is a measure of energy change associated witn

intermolecular interactions. As seen in Figure 1.1, AGm for a binary

mixture can vary with composition.

Figure 1.1. Gibbs free energy of mixing for binary mixtures

For a binary blend to be miscible, the following conditions should be

satisfied: (i) The free energy of mixing should be negative or zero and

(ii) the second der~vat~ve of free energy function with respect to

volume fraction of major component should be positive.

AGrn < O (1.2)

These criteria are met by curve B for all compositions. Blends

described by curve A violate equation 1.2 and are completely

immiscible. A system described by curve C is partially miscible, in

which a single amorphous phase can be formed at compositions to

the left and right of the minima of curve C. In miscible polymer blends,

molecular level mixing of the components is obtained and is

characterised by a single-phase morphology. Immiscible blends do not

satisfy the conditions proposed in equations (1 .2 ) and (1.3), and show

a two-phase morphology in the case of partially miscible blends, the

second criterion is not satisfied and will show either two phase or

single-phase morphology. However, the manifestation of superior

properties depends on the miscibility behaviour of homopolymers.

The term immiscible means that the Gibb's free energy of mixing.

AG, is positive whereas "incompatible" is defined with respect to properties

and means that the properties of the blend are inferior to those of pure

polymers. But, most pairs of high molecular weight polymers are

immiscible or incompatible. Polymer-polymer miscibility depends on a

variety of independent variables, viz., composition, molecular weight,

temperature, pressure etc. Another generic term found often in blend

literature is compatibility. Components, which resist gross phase

segregation and show desirable blend properties, are considered to

have a good degree of compatibility, even though they are immiscible

in a thermodynamic sense.

1.3.1 One Phase Polymer Blends

Compatibility is the fundamental property deciding the practical utility

of a blend. In polymer blends, the property P depends on average

properties of the constituents and can be stated as,

where, P is the property of the blend, P1 and P2 the properties of the

isolated components and Cl and C, the representative concentrations

of the constituents, 'I' is the interaction parameter which can be

positive, zero or negative as shown in the Figure 1.2. When 'I' is

positive, the property is synergistic, for zero values, the property is

additive (one phase blend) and when 'I' is negative the property is non

synergistic (two phase blend).

Synergistic

\\

I _ 1 pt C O M P O S I T I O N pt

.i X - L u P.

f &

Figure 1.2. Variation of property with composition for a binary polymer blend [D.R Paul. Polymer blends. D.R. Paul and S. Newman, Academic Press. New Yolk. Vol. l.,Ch.l. (1978)l

\

... 'x.,

'.~ .-.. \, \

x, -.. .> ,,' Nonsynergistic ~

Both miscible and immiscible polymer blends are of concern to

industry When the end property lies between the properties of the

individual polymer components, miscible blends are preferred''. An

example for such a system is poly (2,6-dimethyl-I. 4-phenylene oxide)

(PPO), which is an amorphous polymer, that softens at very high

temperatures and is very difficult to process, but it is miscible with

polystyrene (PS). The addition of PS to PPO leads to a drastic

reduction in melt viscosity, which makes the compound easy to

process; on the other hand. PPO is an effective modifier of PS,

leading to an increased heat distortion temperature. In the case of

immiscible polymer blend systems, synergistic behaviour between the

polymers making up the different phases leads to properties that are

superior to t h o ~ k t h e components. An important example to this class is , .

nrbberm,&ifi~..'thermoplastics. A large number of multiphase polymer

blends have already been developed. These include polyethylene!

polypropylene(PE1PP)". polymethyl methacrylate /chlorinated polyethylene

(PMMA~CPE)'~ and isotactic polypropylene/polybutadiene (~PPIPB)'~.

The blends may be miscible (domain size of the order ot

400 nm) or immiscible (domain size of the order of -100 nm)

depending on thermodynamic requirements. Miscible blends are

thermodynamically stable, molecular level mixtures. Immiscible

blends are separated into macroscopic phases with very minimum

interfacial adhesion and unstable phase morphology. Compatibilised

blends are also macrophase separated (domain size in between that

of miscible and immiscible systems) but the presence of interfacial

agents or chemical bonds stabilise the morphology and increases

interfacial adhesion.

The miscibility behaviour of the blends can be determined by

various techniques such as heat of mixing, glass transition temperature,

dynamic mechanical response, scattering techniques, fluorescence

spectroscopy, other spectroscopic techniques and morphology

determination by transmission electron microscopy, scanning electron

microscopy etc. The interface between immiscible polymers in polymer

blends can be schematically represented in Figure 1.3. Generally, an

interface is considered as a region having a finite distance neighbouring

the dispersed phase. The properties of the interfacial region can differ from

those of pure components. Lack of strong interface between polymer pairs

limits the stress transfer across the phase boundaries. It is clear from this

figure that the interaction between polymers A and B are very weak

resulting in a very thin interfaceq4.

( b l F- -4 THICKNESS OK INTERFACE

Figure 1.3. (a) I n t e h bebeen immiscible polymers and (b) interfxial density profile between immiscible polymers. [J. Noolandi, Polym. Eng. Sci.24. 70 (1984)l

1.4 Compatibility Predictions

1.4.1 Solubility Parameter Approach

According to the solubility parameter approach at predicting

compatibility, two polymers mix well if the difference in the pure

component solubility parameter is small, typically 1.7-~.0'~. For

polymer molecules, the solubility parameter ( 6 ) is best calculated using

the table of molar attraction coefficients. E, as,

where, E is summed over the structural units of the polymer. M the

'mer' molecular weight and 'e' is the densityq6. The solubility parameter

approach is not rigorous, but it allows a useful first approximation to

polymer solubility. The different temperature coefficients for the pure

component solubility parameter has been suggested as the reason for high

temperature phase separation" ~anchez" compared the predictions of

polymer-polymer compatibility from the equation of state theories and the

solubility parameter method. The molecular basis for miscibility is mainly

due to n-rr complex formation, x-rc complex formation, charge transfer

hydrogen bond formation and copolymer effect.

The miscibility of a polymer blend system can be predicted by

calculating the interaction parameter and critical interaction

parameter1*. The interaction parameter us, can be written in terms of

solubility parameter as

where, V, is the reference volume, R the gas constant, and d~ are

the solubility parameters of components A and B, and T the

temperature in the absolute scale. Miscibility can occurs over the

entire composition range only if the critical interaction parameter is

greater than interaction paramete?'.

An illustration of the role of polymer-polymer interaction energy,

B, in the context of Flory Huggin's theo~y~'-~' on blend structure and

hence their properties are given in Figure 1.4. The interaction energy is

defined as described in the equation in box at the top of the figure.

The Gibb's free energy for mixing a unit volume of monodisperse polymer,

A and B are given by the classical result. In this, B is the interaction energy

density, R is gas constant, T, the absolute temperature, PA,, q5,, and MA

represent density, volume fraction and molecular weight of the

component, A. respectively. This scale determines, to a large extent,

the structure and properties of the blend at the end of formulation

process.

j +-*terfadal Thickness dhwion

Interfscial Tuuion Puticlc Size +

~ i s c i b l e e - j ~ ~ - + h m i w i b l e W Incompatible ! i

PF'O/F'S i W A B S i NylodAES or PP I - * 0 Boir -; / ( M A . M B ) 8 = ?

Figure 1.4. Illustration of the role of polymer-polymer interaction energy on blends and hence their properties [D.R. Paul, in Polymer Blends. ed. DR. Paul and C.B. Bucknall. Academic Press. New York. Vol. 2, (2W)I

When the polymer-polymer interaction energy is less than a critical

value. B,, d~ctated by the molecular weight of the components, the

blend will be thermodynamically miscible. For certain reactive systems,

the phase diagram can change such that the mixture goes from

homogenous to phase separated as B,, changes from phase

separated to homogenotls at a fixed temperature. When the interaction

energy exceeds a small, but positive, B,,, a two-phase mixture is

formed. if the value of B is not much greater than B,.,, the interfacial

tens~on 1s small and it I:; possible to achieve a fine dispersion. Further

more, the ~nterfac~al th~ckness will be large and the interface will be

strong. Generally such blends have good properties and can be said

to be compatible,(polycarbonate (PC)/ acrylonitrile butadiene rubber

(ABS) blend. As the interaction energy grows much larger, interfacial

14

tension increases and the sizes of the domains generally get larger;

thus interfacial thickness decreases and the interface becomes

weaker. At some point, the blends must be regarded as incompatible.

Beyond this point, some form of compatibilisation must be utilised to

obtain a finer and more stable morphology and a stronger interface.

1.5 Compatibilisation Strategy

Compatibilisation is very useful for improving the dispersity in polymer

blends. It reduces interfacial tension, facilitate dispersion, stabilise the

morphology against abusive stresses and strains (arising out of

processing), enhance adhesion between phases and improve the

overall mechanical properties25 of the products. The driving forces for

the phase segregation of blend components are gravity and interfacial

tension. The rate of demixing depends on interfacial tension, viscosity

and density Compatibilised blends are not necessarily

miscible blends, but blends that satisfy certain industrial criteria for

usefulness, such as satisfactory mechanical properties.

The key to solve problems of coarse morphology is to reduce

interfacial tension in the melt and to enhance adhesion between the

immiscible phases in the solid state. One solution is to select the most

suitable blending technique so that a co-continuous or interpenetrating

phase morphology can be obtained, which results in direct load

sharing. The second solution is the addition of a third homopolymer or

block or graft copolymer or low molecular reactive compounds, which

is miscible with either of the two phases. This can be considered as

non-reactive compatibilisation. The third way is to blend suitably

functiona~ised~~ polymers, which are capable for specific interactions

or chemical reactions (reactive compatibilisation).

Block or graft copolymers, which act as compatibilisers are of

two types. (i) reactive and (ii) non-reactive. Non-reactive

compatibilisers have segments capable of specific interaction with each of

Table 1.1. Cornpatlbility through non-reactive copolymers

r--Gr 7YF7 Compatibiliser component component

1 PE or PS S-B. S-EP, S-I-S, S-I-HBD. S-EB-S. PS-PE- grafl copolymers I

I

/ S-E-B-S

PMMA EPDM-g-MMA

PSlPA-6 block copolymers or PA-6 or EPDM S-EB-S or PPE I PF PF-g-MMA or PF-g-S

--

PVDF PSlPMMA block copolymer

PVC - --

SAN BWPMMA block copolymer

the blend components. In reactive copolymers, segments are capable

of forming strong covalent or ionic bonds with the blend components.

Copolymers of both A-I3 type and A-C type can act as efficient

compatibilisers in AB system provided one of the components is

miscible with either A or €3. Table 1 . I and 1.2 give a few examples of

polymer systems that are compatibilised through non-reactive and

reactive copolymers. respectively

Table 1.2. Compatibility through reactive copolymers

Major component Minor component I Cornpatibiker I

PP or PS6 PA4 or PP EPMIMA copolymer

PA6 or PA6.6 lonomen. cahnyl functional PE's

PP or PE PPg-A,& carboxyl functional PE

Table 1.3 lists low molecular weight reactive compounds,

which can act as compatibilisers or coupling agents in many polymer

blends. Co-crosslinking, crosslinking, grafting reactions and chain

extension may take place in such systems involving low molecular

weight compounds as compatibilisers.

Table 1.3. Compatibility through low molecular weight reactive compounds

I Major I Minor component component Compatibiliser I

Fluoro rubber, FPM

PVC or LDPE

NBR

NR

PBT

[M. Xanthos, Polym. Eng. Sci., 28, 1392(1988)]

NBR

LDPE or PVC

PP

PA4 or polyolefins

EPDMg-MA or MBS or NBR

Triazine dithiol complex

Polyfunctional monomers plus peroxide

NBR curative and interchain copolymer

Peroxide and or polyfunctional monomers

Oligomers or epoxy silanes or polyfunctional epoxies

Block copolymers do both, emulsify the disperse phase to give

smaller part~cles as well as increase the interfacial adhesion between

the phases. The principle role of block copolymer in controlling

morphology appears to be in preventing coalescence. Preventing

dynamic coalescence leads to size reduction, while preventing static

coalescence results in stability of morphology. Reinforcement of the

interface is primarily accomplished by the copolymer crossing the

interface and entangling with both homopolymers forming "stitchesvzg.

The result is the coupling of the two phases over which stress can be

transferred.

Another strategy of compatibilisation is the generation of

co-continuous morpho~ogy~~. By adjusting viscosity and composition.

co-continuous morphology can be generated so as to have maximum

interfacial contact area In a co-continuous morphology, each of the

blend components takes its part directly in the load sharing process

without transferring stress across the interface. However, this

technique may not be feasible in blends having wide difference in

homopolymer viscositie:;.

1.5.1 Physical Compatibilisation

In physical blending, the compatibilising agent is chemically

synthesised prior to the blending operation, and subsequently added

to the blend components as a non-reactive component. Owing to its

chemical and molecular characteristics, the added agent is able to

locate at the interface, reducing the interfacial tension between the

blend components (emul:;ification effect) and promoting adhesion

between the phases3',32

The copolymers at-e supposed to compatibilise the blends

according to schematic mechanism shown in Figure 1.5. According to

18

this scheme3=, for a blend of two homopolymers. P' and P" and a

symmetric diblock copolymer B"-B', if the block of B' mixes with only

with P' and the blocks of P" only with B", then the copolymer can cover

the interface in a monolayer, with the blocks. B' and B" reaching out into

opposite phases. In an ideally wmpatibilised system, the interfaces are

covered by a continuous copolymer monolayer and there are no

copolymer chains outside the interfaces. Besides promoting adhesion.

the wmpatibiliser acts as a surfactant to stabilise the microstructure of

the blend. This reduces interfacial tension remarkably and a stable

morphology is deve~oped"~~. Compatibilisation in blends can result in

maximisation of phase dispersion and tensile elongation. However,

achievement of the finest phase dispersion may not always result in the

highest value of ultimate elongation confirming the complex nature of

wmpatibilising effect36.

Interface

Figure 1.5. Compatibiliser effect of a monolayer of a block copolymer B'-B" in the interface between the phases of two homopolymers P' and P" [D. Braun. M. Fischer, G.P. Hellrnann. Polymer. 37, 3871(1996])

1.5.1(a) Compatibilisation by Block Copolymers

The screening of the patent literature reveals many citations in the area cf

physically compatibilised blends. Several experimental investigations also

have been reported on the compatibilising action of added block and graft

copolymers in heterogeneous polymer blends. The pioneering studies in

th~s area have been reported by Paul and Barlod7. Teyssie and

co-workerss observed a significant reduction in the dispersed phase size

and an increase in inte~facial adhesion and morphological stability as a

resun of men blending PE and PS with as little as 2 weight% of poly

(butadiene-bstyrene). The copolymer addition also stabilised the system

against coalescence. According to them the copolymer is uniformly

absohd at the interface between the two polymers.

Heikens and coworkers3' have studied the emulsifying action

of such copolymers at the interface. Block copolymer composed of an

incompatible homopolymer pair usually exhibits microphase

separation with various morphological features4'. The morphology of

such a heterophase structure is determined primarily by molecular

characteristics such as molecular weight, composition and chain

architecture of the species involved. The morphological 0bse~ation of

Molau4' clearly demonstrate the ability of block copolymers to emulsify

polymer dispersions irt concentrated solutions and thus to inhibit

phase separation of imrniscible polymers.

Elemans et a ~ . ~ ' found a significant decrease of interfacial

tension between polystyrene and polyethylene by adding a

polystyrene-polyethyler~e diblock copolymer as compatibiliser.

Sundararaj and ~ a c o s k o ~ ~ studied the effect of compatibilising

PS666DlEP blends uslng a commercial diblock P(S-EP) copolymer

and a commercial triblock P(S-EP-S) copolymer. They found the

limiting particles are as the same as that of uncompatibilised blends

and even an increase in particle size with concentration. It was

reported that the diblock copolymer suppresses coalescence at higher

concentration but does not affect the particle size distribution greatly.

Willis et a~.".~* reported a sharp decrease in minor phase size

with the addition of very small amounts of the compatibiliser when an

emulsification agent is used for compatibilising a polymer blend. A leveling

off at higher concentration followed this decrease. The concentration at

which equilibrium is reached is called critical micelle concentration (CMC)

and at this point the interface is saturated with the compatibiliser. This

curve is known as the emulsification curve. Some important contributions

in this fiekl include the works of Favis and coworkefl, Chattopadhyay and

Swaram," Brown et al.*and Tan et a149.

Auschra and Martinso reported improvement in impact toughness

by incorporating poly (s tyrene-b-ethy le~nebrnethy l methacrylate)

triblock copolymer in PS/ PMMA Mends. Zhao and ~uang" studied

compatibilisation of PB and PMMA with (polybutadiene-block-methacrylate).

They found that at higher temperature PB particles in blends tends to

agglomerate into bigger particles. When the molecular weight of PMMA is

close to that of the corresponding block of the copolymer, the best

compatibilisation resuits were achieved. A few studies were carried out to

investigate the effectiveness of the poly(styrene-b-butadiene-b-styrene)

(SBS) triblock copolymer as a compatibiliser for PPIPS blends. The effects

of adding the SBS triblock copolymer to PPIPS blends when PS was a

matrix phase were studied by Santana and ~ulle?'. They reported that the

size of the dispersed PP particles did not change when 2 weight% of SBS

was added. This resulted in a lack of improvement in the tensile and

impact properties of such blends.

Compatibilisation of high-impact polystyrene (PS-HI) and PP

blends was studied by Horak et a ~ . ~ ~ . The effect of di-, tri-, and

21

penta-block types of styrenelbutadiene block copolymers on the

morphology and mechanical properties were studied. These authors

showed obvious differelices in appearance of the PS-HIIPP interfacial

layer in the blends compatibilised with the diblock and those

containing tri-. or penta-block copolymers. Multiblock copolymers

showed higher improvement in impact strength and elongation at

break in comparison with the diblock copolymer. Fortenly and

~ i c h a l k o v a ~ ~ studied the effect of the SBS compatibiliser as well as

the rate and time of m~xir~g on the morphology of PPlPS blends with

the weight ratio 75/25. They found that a mixture of the SBS

copolymer led to the decrease in the average size of the dispersed PS

particles, but, interestingly, it did not lead to an increase in the phase

structure uniformity. Neither the jncreased rate of mixing nor the

increased time of mixirig resulted in a higher uniformity of the phase

structure of the studied PPlPS blends.

Several reports are available on the physical wmpatibilisation of

various polymer Mend system^^'. Hlavata and ~ o r a k ~ investigated the

changes in uystallinlty of PP in Mends with HlPS wmpatibilised with di and

triblock types of styrenelbutadiene block copolymers. They concluded that

the degree of crystallinity of PP in such Mends did not change with the HlPS

wntent and slightly decreased when a styrenic block wpolyrner was added.

Obieglo and ~ o m e r ~ ' showed how the addition of few percent

of SBS could improve the notched impact strength of recycled PPlPS

blends. A recent review of Datta and ~ o h s e ~ ' confirmed that the

compatibilisation of PPlPS blends with SBS influenced the mechanical

properties of polymer blends. Ghaffar et al." showed that SBS could

be a very effective modifier for blends of low-density polyethylene

(LDPE) and PS. All measured mechanical properties (impact strength,

tensile strength, and elongation at break) were considerably improved.

On the other hand, SBS was not so effective when other thermoplastics

like PP or poly (vinyl chloride) was used instead of PS. Similar, but not so

evident improvements in the mechanical properties were found for ternary

PE-LDIPPIPS blends modified with SBS'~. Role of SBS as compatibiliser is

reported in blends of PP and highdensity polyethylene(^^^^)*^^'. The

authors proposed different morphological models for such ternary blends

depending on the processing conditions. They reported that with properly

chosen processing conditions PPIPE-HDI SBS, ternary blends with high

impact resistance can be obtained. Macaubas and Demarquettea studied

morphologies and interfacial tensions of immiscible PPIPS blends modified

with triblodc copolymers, styrene-butadiene-styrene (SBS) or styrene

ethylene/butylene-styrene (SEBS). The addition of compatibilisers to the PS

phase resulted in a reduction of interfacial tension following an emulsion

curve.

Braun et a1.63 reported that block-graft copolymers are good

compatibilisers in polymer blends, if tailored carefully. They have

investigated the efficiency of two block-graft copolymers, SbBgCHMA

and SbBgCMA (CHMA, cyclohexyl methacrylate; MMA, methyl

methacrylate) as compatibilisers in PSIPVC blends. Block-graft

copolymers were found to be excellent compatibilisers in the study, but

they could not decide whether it was superior to the diblock architecture.

Figure 1.6 gives the model of a two -stage compatibiliser effect

by a block- graft copolymer. They reported that naked interfaces in the

uncompatibilised blends are traversed by only a few chains and are

therefore weakly entangled. In contrast, the copolymer-covered

blends are traversed by very many chains. When stress is exerted

during mixing, by the matrix on the compatibilised phase domain

(Figure 1.6a), it is transmitted efficiently through entanglements

between the copolymer chains in the interface. The interface and

therefore the domain can be torn apart by drag forces from the matrix,

which are not totally covered by the copolymer chains (Figure 1.6b).

This leads to produce smaller domains (Figure 1.6~).

Figure 1.6. Model of a two stage compatibiliser effect (a;A phase domain which is totally covered by copolymer chains (1 stage) is (b) torn apart by drag forces from the matrix which are transmitted by an entanglement. This produces (c) smaller domains, which are not totally covered by copolymer chains, as well as dispersed rnicelles of the copolymer ID. Braun. M. Fischer, G.P. Hellmann. Polymer. 37. 3871(1996)].

1.5.l(b) Compatibilisation by Graft Copolymer

Methods of synthesising polymers containing reasonably long

sequences of monomer units are of great interest since they have the

intrinsic potential to exhibit properties much different from that of the

related homopolymers, the corresponding copolymers or even their

physical mixtures of blends. Block or graft copolymers provide bright

examples of having long sequences of constituent monomeric units

linked to each other in the macromolecular structure in linear (axial)

and lateral directions respectively. Graft copolymerisation is still being

vigorously pursued in view of the fact that the outcome of studies in

the area is yet to reach its full potential.

Locke and PaulM studied the enhancement of very poor

mechanical properties of PEIPS blends by the addition of graft

copolymer. In other studies, the addition of graft copolymer was

reported as a means of improving the properties of PPIPS'~-

HDpE/iPprn, PEIPs'~, PSIPMMA" blends. SabbaghS9 recently

reported that the addiiion of a small percentage of EPDM-g-MAH as

compatibiliser in natural rubber (NR)IEPDM blends resulted in

decrease of domain size of the dispersed phase and a consequent

enhancement in properties of the blends. There are several studies in

literature in which the addition of graft copolymer increases the

technological compatibility of immiscible polymer

Contributions to physical compatibilisation by graft copolymers

from our laboratory include the compatibilising action of in PSINR8:'

poly(methy1 methacrylate (PMMA)INR)~ systems. Li et aI.'= studied

the effect of graft copolymer (PP-g-PAC) on polypropylene (PP) and

polyamide6(PA6) blends. They reported that the morphology of

compatibilised blends is developed much faster than that of their

uncompatibilised counterparts. Pa et a\.* studied the preparation,

characterisation and properties of polycarbonatel-g-polystyrene and

also its effects on polycarbonatelpolystyrene blends. Many citations

are available on recent developments in which the addition of graft

copolymer increases the technological compatibility of immiscible

polymer pairs

Paul and coworkersg' reported that conformational issues of

the graft copolymer affect compatibilisation. Figure 1.7 schematically

illustrates the idealised structure of the domains in a nylon 6 matrix or

a styrene acrylonitrile (SAN) matrix after reactive compatibilisation. It

shows the graft copolymer comprised of long nylon 6 side chains

attached with a number of points along the irnidised acrylic polymer

(IA) or styrene acrylonitrile maleic anhydride terpolymer (SANMA)

backbone, located at the interface.

Nylon 6 mabix SbN matrix

(a) SU1 p&lst in a nwn 6 m.bix (b) nylon 6 partirks in a S I N nabi

Figure 1.7. Schematic representation of the conformation of the graft polymers at the interface beween nylon 6 and SAN phase (a) when nylon 6 forms a matrix, (b ) when SAN forms the matrix [N Kitayama. ti. Keskkula, D R . Paul, Polymer, 41, 8058 (2000)l.

For blends with a nylon 6 matrix, the side chains of the graft

copolymer face toward and expand into the nylon 6 matrix. In these

circumstances, there is no crowding problem at the interface. On the

other hand, for blend formed with a SAN matrix, the side chains of the

graft copolymer are more crowded as they try to extend into the nylon

6-domain phase. From the comparison of these two cases it is clear

that the formation of small SAN domains in a nylon 6 matrix is more

favourable in terms of these conformational issues than the reverse

case.

Riess and co-workers reported that block copolymers are more

effective than graft copolymers based on compatibility studies on

PSIPMMA and polystyrenelpolyisoprene blend s y s t e r n ~ ~ ~ . ~ ~ . They

further concluded that solubilisation occurs only when the molecular

weight of the homopolymers are less than or comparable to the

molecular weight of the segment in the block copolymer. Bar and

~homann '~ did characterisation of the morphologies and

nanostructures of blends of poly(styrene) block-poly(ethene-co-

butylene) block-poly(styrene) with isotactic and atactic polypropylene

by tapping-mode atomic force microscopy. They observed

macrophase separation in iPPlSEBs blends caused by incompatibility

of two polymer components. The microphase separation of the SEBs

component depends on the thermal treatment.

Fayt and ~eyss ie '~ have studied the emulsification of LDPE

and commercial HlPS blends by a hydrogenated tapered block

copolymer of butadiene and styrene. Figure 1.8 illustrates the state of

dispersion of HlPS and its internal structure in LDPE upon the addition

of the hydrogenated poly(butadiene-b-styrene)(HPB-b-PS). LDPEl

HlPS blends exhibit coarse phase morphology (Figure 1.8 a). Upon

the addition of the block copolymer (5%) there is a marked decrease

in the particle size and a significant enhancement of the interfac~al

adhesion. PS becomes finely dispersed within LDPE whereas the PB

particles still coated by an adherent layer of PS becomes anchored

within the LDPE matrix by the diblock copolymer (Figure 1.8b). The

blends exhibit stability of the phase morphology and particularly the

stability of the rubber particles against coalescence when the LDPE

content is decreased and improvement of the ultimate mechanical

properties.

(a) 1 LDPE 1 0)

Figure 1.8. (a) Unrnod~fied LDPElHlPS blends showing coarse phase morphology (b) effect of the HPB-b-PS diblock copolymer on the d~spersion of HIPS in a PE matrix [C. Koning et al., Prog. Polym S C I , 23, 707 (1 998)]

1.5.l(c)Compatibilisation by Random Copolymers

Random copolymers have now received substantial attention as possible

interfacial agents. Theoretical work by Balazs and c o - w o r k e r ~ ~ ~ ~ ~ ~

reported the behaviour of a single random copolymer chain at a liquid-

liquid interface. From numerical simulations, they determined that the

random copolymer forms large loops as it weaves back and forth

across the interface, and the maximum interfacial strength should be

obtained for a random icopolymer containing an equal number of A

and B units. Gersappe and Balazsg9 then examined a finite volume

fraction of copolymer at the homopolymer-homopolymer interfaces, as

opposed to a single chain. Using a mean field model, they determined

that the random copolymer was localised at the interface and reduced

interfacial tension. However, the block copolymer was most efficient in

reducing the interfacial tension, with 50150 random and alternating

copolymers showing nearly identical results.

When a third polymer reduces the net interfacial energy, it will

encapsulate the minor phase. This phenomenon has been well explained

using a spreading coefficient, as proposed by Hobbes et al.lW for ternary

polymer blends. Random copolymers having the same monomet-ic

units as the blend components, i.e. AIBIC-ran-B blends, also have a

strong tendency to encapsulate the minor phase101,'02. This is

possible because the interfacial tension between each homopolymer

and the random copolymers is lower than that between the two

homopolymers. Although a random copolymer-encapsulating layer is

effective in reducing the average sizes of the dispersed phase during

melt mixing, the encapsulating layer does not provide stability against

static coales~ence'~~.

Polymer blend systems that show encapsulated structures by a

random copolymer is as follows. The polymers do not have the same

monomeric units as the random copolymer, but rather are miscible, i.e.

C/B/A-ran-B or CIDIA-ran-B blends. Since polycarbonate (PC) is partially

miscible with PMMA'" and poly(pheny1ene oxide) (PPO) is completely

miscible with P S ' ~ , PSIPCISMMA, PPOIPMW SMMA, and

PPOIPCISMMA blends are candidates for encapsulation. This approach

may prove useful for cases where no parent A-B block, graft, or random

copolymer is synthetilly accessible1". Pospiech et al.'" studied the

emulsifying action of block copolymers (BCP) having segments identical to

that of homopolymers by variation of flexibility of blocks. They showed

that stiffness of the second segment of polysulfone (PSU) block copolymer

as well as the PSU segment molecular weight determine the intermixing

between the BCP and the PSU matrix.

1.5.2 Reactive Compatibilisation

Although numerous chemical reactions are encountered in reactive

processing, it is possible to distinguish major classes such as bulk

polymerisation, reactive compatibilisation, controlled degradation,

coupling, grafting and functionalisation. All these types of reactions

can be classified under reactive processing. In reactive compatibilisation,

copolymers can be formed in-situ through covalent or ionic bonding during

melt blending. In this kind of reactive compatibilisation, generally one

phase contains reactive groups inherent in the polymer, while the

other has no inherent functionality. Reactive groups can be

incorporated into the second phase by adding to it a functionalised

polymer, which is miscible. In some cases, both polymers have to be

functional~sed. The in.situ formed copolymer compatibiliser are

preferentially located at the interface where they are most needed,

reducing the size of the dispersed phase, improving the interfacial

adhesion between the phases and physical properties of the blends.

This suppresses the coalescence and reduces the interfacial tension.

As a result of this, a stable and a fine morphology is attained with

enhanced interfacial adhesion between the phases.

In reactive compatibilisation, micelle formation in one or the

other phase is expected to be minimum compared to pre-made block

or graft copolymers which can easily self-organise in the phase where

they are the most energetically stable. This situation is expected to be

more probable in extrusion processes where the residence time is

short so that the mixing equilibrium is often not completely attained.

Since most polymer blends do not have the appropriate functional

groups, a functionalisation of the blend components and reaction with

the other, can be used for the formation of a compatibilising copolymer

at the i n t e r f a ~ e ' ~ ~ . ' ~ ~ . Reactive compatibilisation can also be made

possible by addition of a reactive third polymer of appropriate

functionality acting as a cornpatibiliser.

Since most of these reactions are very fast, they can be

performed within the time scale of an industrial extruder, which makes

reactive blendinhi! industries' option. Reactive compatibilisation, . , ,. ye'sults in "th;.ck interface that shows formidable stability under high

stress and strain. In-situ compatibilisation between two polymers has

gained significant interest during the last few years, the main reason

being the economic advantages it has over compatibilisation methods

using a preformed compatibiliser. The main research efforts have

focused on finding the appropriate functionalised copolymers which

will react during the mixing period.

The basic requirements for the effectiveness of a reactive compatibilising method are:

a high degree of dispersion of one polymer in another,

8 the presence of complimentary groups to form a chemical bond and

strong reactivities of the reactive groups to perform the interact~on

through the polymer melt, the chemical bond formed during the

blending has to be stable to the subsequent processing conditions,

small reaction times, i.e. smaller than the residence time in the

extruder or the mixer.

Grindstaff et al.l1° have reported on the compatibilisation of

poly(dimethylsiloxane)lpoly(oxyethylene-b-oxypropylene) blends by

the addition of poly(dimethy1 siloxane-b-oxyethylene). The basic

principle involved in reactive compatibilisation includes the use of

functionalities present in one or more polymers to form graft or block

co-polymers in-situ during melt processing. This method has been

applied to a number of blends"'-"3. Park et a1.'14 evaluated the

properties of polystyrenelnylon system using maleic anhydride grafted

polystyrene as the compatibiliser. Reactive compatibilisation has

drawn much more importance in recent years and several reports are

,,,. I ' ,. ,;$;i

Y b O f , 31 .. > . I ~.

! 4

' I available on the reactive compatibilisation of y/lop polymer blend A : .:,

:. * . systems"' ' : j4

,,. ,, i />>"

,*..-:~,/ . - 1.6 Mechanism for phase morphology refinement

The phase morphology evolution in physical compatibilisation is a

dynamic process that depends on a very broad spectrum of input

parameters. The interplaying factors, such as the molecular weight of

the compatibiliser, the concentration of the comaptibilisers, the

viscosity of the blend phases and the miscibility of the compatibiliser

with the blend cornponenta are reported to affect the final morphology.

Early researchers attributed the decreases in droplet size to

the block copolymer's ability to lower interfacial tension. However, the

reduction in interfacial tension is not sufficient to explain the size

decrease. The refining and stabilising effect of a compatibiliser on the

blend phase morphology can be explained by the two phenomena of

lowering of interfacial tension upon compatibilisation and the

decreased possibility for coalescence of droplets. Present studies 135,136

however, suggest that the interface becomes too crowded with copolymer

before significant reduction in interfacial tension owxrrs; this suggestion

implies that the dominant mechanism of reducing partide size is the

suwession of dropletdroplet coalescence through steric hindrance by the

copolymer at the interface. This is in agreement with the 0bSe~ati0n of

Sundararaj and ~acosko". They also predicted that the suppression of

coalescence is more efficient with hgh molecular weight Mock copolymers.

M~lner and XI'^' proposed that suppression of coalescence IS

due to a force arlslng from surface tenslon grad~ent (referred as

Marangon1 force) ~nduced by a grad~ent of block copolymer

concentrat~on at interfaces The grad~ent was caused by flow between

Figurel.9. Two mechanisms proposed for blcck copolymer suppression of coalescence (a) sulfate tension gradient (Marangoni force) and (b) steric repulsion. [S. Lyu, T. D. Jones. F.S. Bates, C.W. Macosko, Macromolecules. 35,7845 (2W2)I

the approaching droplets'38. They predicted that the block copolymer

concentration necessary for preventing coalescence was higher at

higher shear rate. An overview of the two mechanisms of coalescence

suppression is illustrated in Figure 1.9.a. When two droplets approach

each other, the matrix between them is squeezed out (film drainage). This

squeezing flow carries the block copolymers at the interface out from the

particle gap forming a concentration gradient. The resistance to

coalescence arises from the steric repulsive force due to compression

of block copolymer layers attached to the surface of two approaching

droplets (Figure 1.9.b).

1.7 Effect of Compatibilisation on Interfacial Thickness

Interfacial thickness13' of some of the polymer systems are given in

Table 1.4 The interfacial thickness of the physically compatibilised

blend is almost tw~ce that of the non-reactive blend, while in reactively

compatibilised blend it is still much higher. The equilibrium interphase

thickness of compatibilised blends ranged from 10 to 50nm depending on

the nature of interaction. This is larger than the radius of gyration of

component polymers'39. When two polymer films are brought together and

heated above their T,s, a broad interface will be developed with time for

miscible polymer pairs. C)n the other hand immiscible systems give thin

and narrow interfacesq4' If two polymers are strongly immiscible their

chains repel each other and the penetration of each chain ends from either

phase into the other across the interface is highly unfavourable. In such a

case, the interfacial tension will be hgh and interfacial thickness small. For

immiscible systems, both mean field and lattice theories predict that some

inter diision of polymer segments occurs at the interface to minimise the

interfaaal energy. It is postulated that the thickness of the interface is

proportional to while the interfacial tension behaviour of both polymers

is proportional to X 0 5 where, x denotes the Flory-Huggin's segment

interaction parameter. Immiscible polymers do have large and posnie X ,

as a result the interface thickness is very thin and the interfacial tension is

very high.

Yukioka and no we':^ have performed time resolved ellipsometric

investigations to analyse the interfacial thickness between amorphous

nylon and styren~c polymers during annealing at high temperature

(170-210°C). According to them, interface established at later stage was

tremendously thick ranging from 10 to 5Onm. The thickest interface was

several times the coil size of the component polymer due to the drawing

of the whole graft copolymer chain into the interface to establish a

smooth concentration gradient at the interface.

Table: 1.4. lnterphase thickness

[S. Yukioka, T. Inoue. Polymer. 35, 1182 (1994)l

Type of blend

Immiscible

Block copolymer

Polymer1 copolymer

Reactive compatibilisation

Radius of gyration

It is interesting to look into the phase behaviour of a block

copolymer. Theoretical analysis indicates that three parameters in

particular determine the phase behaviour of a block copolymer; N, its

overall degree of polymerisation (DP); XAB, the Flory Huggin's

interaction parameter, which characterises the repulsive interactions

between the two blocks A and B; and f ~ , the volume fraction of A,

which in the case of equal effective monomer volume, i. e. VA= VBq4' is

NA/N, where NA is the degree of polymerisation of block A. As for

immiscible blends of poly(A) and poly(B), the driving force for phase

demixing is the repulsion between dissimilar monomers, which increases

as bs increases, opposed by the decrease in configurational entropy

of mixing, which increases as N decreases. The net effect can be

expressed in terms of the product N;@.,, ~f ND~, is too small, then

poly(A-b- B) will tend to form a uniform mixture.

Thickness (nm)

2

4-6

30

30-60

5-35

A schematic overview of the interface between immiscible

homopolymers containing a diblock copolymer is given in Figure 1.10.

In Figure I.lO(a), some of the copolymers settle at the interface while

some are randomly distributed among the homopolymer phases. In

addition to the first one. Figure l.lO(b) shows the micelle formation

due to copolymers.

:igure 1.10. Schematic representation of the blend interface between an immiscible blend in presence of a compatibiliser (diblock copolymer): (a) diblock copolymer present at the interface as well as in the bulk and (b) micelle formation in the bulk. [J. Noolandi and K.M. Hong. Macromolecules. 15, 482 (1982)l.

1.8 Interfacial Stability of the Copolymer

Since polymer blends are very often subjected to different processing

operations for the fabrication of end-use products, the interfacial

stability of the copolymer is of high concern. The interfacial stability of

the copolymer means whether or not the copolymer stays at the

interface as a function of time under quiescent or dynamic conditions.

One of the important advantages of blending is the stabilisation of the

blend phase morphology in the melt. The generated morphology

should be preserved against coalescence during melt-processing, on

account of the fact that very often polymer blends are annealed during

fabrication and processing operations which can lead to coalescence.

The phenomena of phase instability and coalescence are very

important in blends having a very high interfacial tension, especially in

low shear rate regions with long residence times such as in moulding

operations with thick sections where coalescence has time to occur

prior to solidification. The phase instability was shown to be dramatic

for blends of PPlnylon 6. PElPC and PSInylon, as reported by White

and co- worker^'^"'". These studies were carried out in the barrel of a

capillary rheometer where annealing was conducted. The influence of

reactive compatibilisation on phase stability of blends in the melt can

be understood from Table 1.5.

Table 1.5. Average dimensions of phases in annealed polymer melt blends

Dedecker and Groeninckx14' compared the morphological

Blend system

PSINylon6 HDPEINylon6 HDPElNylonll PSI Nylon6lSAN PSI Nylon61SMA HDPEI Nylon6 LDPEY Nylon6 HDPU Nylon6tMA-g-PP

stability of reactively compatibilised PAG/(PMMA/SMA) blends with

uncompatibilised PA6IPMMA blends. The morphology of the

SAN = styrene acrylonitrile copolymer, SMA = styrene-methacrylic acid copolymer, HOPE = high density polyethylene, MA-g-PP = maleic anhydride grafted polypropylene. [G. Groeninckx, C. Harrats, S. Thomas. Reactive Polymer Blending, eds. W.E. Baker, C.E. Scott, G.H. Hu, Hanser, Munich (2001)l

ratio

60140 50150 50150

5713815 5713815 50150 50150

47.5147.515

Mean phase dimensions (pm)

30min

90.2 163 50.9 131

4.02 163 4.53 8.81

60 min

140 248 20.2 104

4.65 248 6.90 11.9

-

90 min

310 2 78 229 117 4.58 278 13.0 10.9

compatibilised blends was unchanged after thermal annealing at a

temperature of 260°C for 15 minutes; however, the phase morphology

of the uncompatibilised blends was highly unstable. The effect of the

extrusion time on the phase morphology of the reactively

cornpatib~llsed PAG/(PPO/SMA) 75/(20/5) blend has been reported by

Dedecker and ~ roen inckx '~~ . At very long extrusion times

(30 minutes) a b~modal particle size distribution was found for the

blend system with SMA.8 as reactive compatibiliser. This has been

accounted for by the fa'd that the graft copolymer already formed at

the interface has left the interfacial region by the applied shear forces

upon prolonged mixing. The shear forces must have overcome the

thermodynamic compatibility between the SMA-g-PA6 copolymer and

the respective homopolymer phases. The separated graft copolymer

gave rise to the formation of micelles, which were detected by SEM as

very small particles. As a result of this, the size of the dispersed phase

increased drarnatlcally Interestingly the graft copolymer left the

interface only in the case of blends containing SMA8, while the blends

with SMA2 as reactive c~mpatibiliser did not show such instability; this

is associated with the limited miscibility of SMA8 with PPO. Dedecker

and ~roeninckx'" had reported on morphology coarsening in

reactively compatibilised polyethylene terephthalatelelastomer blends.

1.9 Factors Affecting the Efficiency of Compatibiliser

1.9.1 Chemical Nature of the Compatibiliser

The compatlbiliser should carry segments identical to the respective

homopolymer phases in the blend or they have segments miscible

with one of the phases. The copolymer itself should be immiscible in

the blend. In immiscible polymer blends, the compatibiliser functions

just like surfactants or detergents do in oil-water emulsions. In an

ideally compatibilised system, the interfaces are totally covered by a

continuous copolymer monolayer and there are no copolymer chains

outside the interface. In such conditions, interfacial tension is lowered

and morphology gets stabilised.

Two strategies of physical compatibilisation have so far been

developed. (i) addition of small quantity of a third component which is

either miscible in both the phases or is a copolymer whose one

segment is miscible with one of the phases while the other is miscible in

the remaining phase. The recommended amount of copolymer is only

0.5-2 weight percent with respect to minor phases and preferentially

block copolymers, rather than grafted ones (ii) addition of a large

quantity of a core-shell copolymer, which serves as compatibiliser as

well as impact modifier and approximately less than or equal to 35

weight percent of a core-shell copolymer is the required amount.

1.9.2 Copolymer Chain Microstructure

For a block copolymer, the compatibilising action depends upon the

number of blocks (di block, tri -block etc. and the nature of the block

(normal or tapered). It has been shown that tapered block copolymers

are superior to normal block copolymers. For a graft co-polymer to be

effective, the number of segments should be minimum.

1.9.3 Molecular Mass and Composition of the Copolymer

The copolymer should have high molecular mass compared to

homopolymers for obtaining better compatibilisation. Best cornpatibllising

action would be obtained when the composition is 50150 as the copolymer

needs to be located at the interface. Reiss and Jolivet14' studied the effect

of molecular weight on solubilisation. When the homopolymer molecular

weight is larger than the corresponding block copolymers, the long

segments are able to anchor the immiscible phases firmly. Besides 311

these, there should be an optimum molecular weight when the cost benefit

of the resultant product is concerned. According to Macosko et the

optimal molecular chain of blodc copolymer may be short enough to diffuse

quickly to the interface but also have to be long enough to entangle

sufficiently with homopolymers to prevent coalescence during blending.

Chattopadhyay and ~ivaram" studied the effect of addition of

block copolymer on the domain size of the dispersed phase as a function

of molecular weight. They reported an increase in block copolymer

molecular weight of poly(styrene)-block-poly(isoprene)(PS-b-) in

(composition 50150 P:S/NR) resulted in progressive particle size

reduction as seen in Figure 1.1 1

Figurel.11. Influence of molecular weight of pdy(styrene)kkck- pdy(iirene)(PS+PI) on the morphology of 50150 weigh% PSlNR blend.[S. Chattopadhyay and S. Sivaram. Polym. International, 50, 67 (2001))

The desired amount of block copolymer required to saturate unit

volume of the interface: was also reported to decrease with the

decrease of molecular weight.

1.9.4 Concentration of the Copolymer

There is an optimum concentration of the copolymer needed for

saturating the interface and attaining maximum efficiency (CMC).

Above this concentration, the copolymer will form micelles in the

homopolymer phases. Figure 1.12 illustrates the number average

domain diameter of the dispersed nylon phase as a function of

compatibiliser, EPM-g- MA in EPMlnylon6'" blends. These graphs are

generally called emulsification curves. At the equilibrium concentration

of the compatibiliser (about 1.5 weight %) leveling off occurred.

Weight percent of EPM-g-MA

Figure 1.12. Emulsification curves for EPMlnylon blends where EPM forms the dispersed phase. IS. Thomas, G. Groeninckx. Polymer, 40. 5799(1999)]

Generally CMC is estimated from the plot of interfacial tension

versus copolymer concentration. As the interfacial tension is directly

proportional to domain size, CMC can be estimated from the plot of

domain size versus copolymer concentrati~n'~'. This type of analysis

has been extensively reported in l i te ra t~ re '~~ . It is interesting to note

that in most of the cases a 5wt. % of the graft copolymer is sufficient

for interfacial saturation.

1.9.5 Mode of Incorporation of the Cornpatibiliser

It is found that the blend properties are highly sensitive to the

sequence of m~xing. The mode of incorporation of the wmpatibiliser in the

blend has a strong effect 011 the phase morphology development. Studies

in this direction clearly indicated that the finest morphology is obtained by

preblending the compatibiliser in the minor phase and then mixing with the

major continuous phasea3. Willis et aLu, Asaletha et aka3, Thomas and

Groen inck~ '~~ and Chattopadhyay and sivaram4' have reported that

the two-step mtxing is more effective in reducing the dispersed phase

size than the one step mixing during compatibilisation. A speculative

model illustrat~ng the compatibilisation efficiency under different mode

of addition of the copolymer is given in Figure 1.13. By pre-blending

the modifier with the dispersed phase, it was possible to localise the

co-polymer at the interface and increase the interaction between the

copolymer and the dispersed phased5.

Cimm~no et a1.lS3 studied the effect of mixing procedure on PA-6-

EPM (ethylene-propylene-random copolymer) blends stabil~sed with an

EPM copolymer functionalised with succinic anhydride (EPM-g-SA). They

considered (a) one step mixing, in which all the components were added

into the mixer directly, and i:b) two step mixing, in which the EPM and

EPM-g-SA were premixed prior to mixing with PA-5. They reported that

the blends w~th one- step mixing showed a very coarse morphology and

poor impact strength while those prepared by t w ~ step mixing had a very

fine morphology and excellent impact strength.

TWO STEP WOSTW PROCESS

Figure 1.13. Speculative model illustrating the compatibilisation efficiency under different mode of addition of the copolymer. [R. Asaletha, M.G. Kumaran and S. Thomas. Rubber Chem. Technol., 68. 679 (1995)]

1.10 Organisation of the Copolymer at the Blend Interface

An immiscible thermoplastic blend AIB can be compatibilised by

adding a diblock copolymer, poly(A-b-B) whose segments are

chemically identical to the dissimilar homopolymers, or poly(X-b-Y) in

which each block is chemically different but thermodynamically

miscible with one of the blend components. Experimental evidences

suggest that the addition of pre-made block copolymer leads to a

considerable reduction of interfacial tension. By properly choosing the

type and molecular weight of block copolymers, so as to lower the

interfacial tension in highly incompatible homopolymers, one can achieve

outstanding mechanical properties. Kawai and coworker^'^^'^^ studied

the compatibilising action of associated block copolymer on ternary

blends of polystyrene-polyisoprene. The efficiency of a copolymer, either

block or graft, acting as the compatibiliser depends on its structure. One

of the primary requirements to get maximum efficiency is that the

copolymer should locate preferentially at the blend interface. Figure 1. 14

shows the organisation of the graft, diblock and triblock copolymers at the

blend interface respectwely.

It is found that conformational restraints are important and on this

basis a block copolymer tan be expected to be superior to graft

copolymer. In the case of graft copolymer, multiple branches should be

avoided. Otherwise it would restrict the penetration of the backbone into

the homopolymer phases. Arnong block copolymers, a diblock copolymer

will be more effective than a triblock copolymer. Teyssie and co-workers

have studied the compatibilisation of PSIPE system with tapered and

pure block copolymer^'^^^'^' They found that the copolymer is uniformly

adsorbed at the interface between the two homopolymers. They have

also shown that a tapered diblock is more efficient than a pure diblock

with the same composition arid molecular weight.

Figure 1.14. Physlcal models supposedly representing the conformation of copolymer at the interface of a heterogeneous polymer blend (a) graft (b) diblock and (c) triblock copolymers.

Pure diblock copolymers contains highly incompatible sequences.

These sequences segregate into domains and less mixing occurs. But

tapered block copolymers do not form domains of their own and

therefore provide strong adhesion. Compared to diblock, the tapered

block copolymers can be easily dispersed due to their low viscosity.

Chemical identity of the copolymer segment with the homopolymer

phase is important. Symmetric diblock copolymer was found to be

more efficient than the asymmetric diblock copolymer of the same

molecular because this copolymer is more or less located at

the interface.

Architecture and sequence distribution of the copolymer are

important parameters that affect the solubility of a copolymer and the

ability of a copolymer to compatibilise a polymer blend. The sequence

distribution of a linear copolymer can vary continuously from

alternating to random to blocky and plays important role in the ability

of the copolymer to modify the biphasic interface. Both theoretical and

experimental s t ~ d i e s ' ~ ~ . ' ~ ~ have shown that the diblock copolymer

arranges itself across an interface.

The concept of probable alignment of the block copolymer at the

interface is given in Figure 1.15 .If the interface is sharp, diblock

copolymer chain will cross the interface only once. However, as the

number of blocks increases, the number of interface crossings also

increases. For example, a triblock will cross the interface twice, a

pentablock has four interfacial crossings, and a heptablock stitches

the interface six times. The random copolymer is also believed to

cross the interface multiple times although the exact number is not

known. Eastwood and ~ a d m u n ' ~ ~ have reported the role of multiblock

copolymer architecture in compatibilisation of PSIPMMA blends.

Optimum strengthening was observed with maximum number of

blocks that are longer than a critical block length.

Figure 1.15. lllustrat~on of the probable alignment of block copolymers at an ~nterface [ E.A Eastwood and M.D. Dadmun, Macromolecules,

1.11 Physical Versus Reactive Blending

Reactive blending versus physical blending with respect to

compatibilisation has both very many similarities and differences in

common which are summed up as follows. In physical blending the

compatibilising agent is chemically synthesised prior to the blending

operation, and subsequently added to the blend components as a

non-reactive component I,I reactive blending, the cornpatibilisation of

immiscible polymers is ascertained by a chemical reaction initiated

during the process of meit mixing. Both methods reduce the

interfacial tension between the immiscible blend components also

enhances the adhesion between the phases, imparts to the blend

acceptable mechanical properties. In both cases, compatibilising

agents are expected to be located at the interface between the

phases. For industrial purposes melt-extrusion is used as the main

compounding operation for both types of blending.

The products formed in reactive blending are difficult to be

identified, separated and characterised. In reactive blending, one has

to employ NMR and FTlR spectroscopy investigations for the

characterisation on selectively extracted polymers. Reactive blending

is met with limitations resulting mainly from the solvent extraction

efficiency. There are problem of swelling and the partial solubility of

the blend components. But micelle formation in one or the other

phases is expected to be minimum. It is a very cost-effective process

and allows the formulation of new multiphase polymeric materials.

There is only very little reaction and 4% reactive component is only

needed.

1.12 Theories o f Cornpatibilisation

Theoretical mode~s'~'~'" have been developed to describe the

molecular mechanisms of emulsification and compatibilisation by

block copolymers. The generated loss of conformational entropy is

compensated by the gain in enthalpy due to the segregation of the

block in the corresponding compatible homopolymer phases so that

the overall interfacial free energy is lowered. The interfacial tension

and, hence, the phase- separated domain sizes decrease. In any case

the improvement of the mechanical phase adhesion requires a good

thermodynamic andlor physical interpenetration of the block

copolymer segments with respective homopolymers. To act as an

efficient mechanical reinforcement of a blend NB, a block copolymers

poly(A-b-B) should then have a molar mass, high enough to form

entanglements with the two immiscible polymer^'^^^'^^. The tendency

towards micelle formation of diblocks, in one phase is concurrently

favoured so that as compromise between molar mass and

concentration has to be reached. As neat materials, most block

copolymers phase separate into ordered nanoscopic domains due to

the immiscibility of the component blocks and the covalent linkage

between segmentslS5.

Noolandi et al. developed thermodynamic theories concerning

the emulsification of copolymers (A-b-B) in immiscible polymer blends (AB)

and LeiMerlM for semi-compatible blends. Noolandi and Hong developed a

general theory to evaluate the free energy function for the immiscible

systems. This is then modified to get the meanfield equations for a system

of two immiscible homopolymers diluted with a solvent in the presence of a

diblock copolymer. They studied reduction in interfacial tension by

interaction energy of block copolymer at interface, taking into account the

associated entropy loss of polymer chains. According to them localisation

of some block copolymer at the interface results in a lowering of the

interaction energy between two immiscible homopolymerr, broadening of

the interphase between homopolymers, a decrease in free energy,

and a small decrease in entropy that ultimately limits the amount of

copolymer present at the interface.

The difference between the total free energy and that of the

bulk polymers was used to evaluate the interfacial tension reduction.

where, d = width at half height of the copolymer profile reduced by

Kuhn statistical segment length. +c = bulk copolymer volume fraction,

4, = bulk volume fraction of polymer A or B, x is the Flory Huggin's

interaction parameter and Z,= degree of polymerisation of copolymer.

Noolandi and ~ o n g ' ~ ' also pointed out that both copolymer

concentration and molecular weight are equally important in reducing

the interfacial tension. However, it is noted that the theoretical

treatment is applicable only for concentrations well below the critical

micelle concentration. L e i b ~ e r ' ~ ~ developed a mean field formalism to

study the interfacial properties of nearly compatible systems. The free

energy was expressed in terms of monomer concentration correlation

functions that were calculated in a self-consistent way within the

random phase approximation. According to the first mechanism it is

the surface activity of the block copolymer chains that causes the

interfacial tension reduction. This is applicable to completely

incompatible systems having concentration less than the CMC. The

second mechanism applicable to nearly compatible systems suggests

that presence of copolymer molecules dissolved in the bulk

homopolymer phases causes the compatibility behaviour. Noolandi16'

further modified the expression for interfacial tension reduction. Ay, for

a system without any solvent. The modified equation says,

A? =~(I,[(~&+I/z, - I/& exp Z, ~121 (1.8)

Since interfacial tension reduction is directly proportional to the

particle size reductionB8, it can be argued that,

where, AD is the particle size reduction or increment upon the addition

of compatibiliser and K is the proportionality constant. Accord~ng to

this theory, the interfacial tension reduction should decrease linearly

with copolymer content at low concentration followed by a leveling off

at higher concentration.

Koberstkn and u>workers3 and Thomas and co-workers 83.84. 169.169

have testified the theory of Noolandi and Hong. Koberstien and co-

workers3 have reported the compatibilising effect of block copolymers

added to the polymer polymer interface. The compatibiliser used in

PSlPB blends was poly(styrene -block-1,2-butadiene).They observed

a decrease in interfacial tension followed by a leveling off at higher

concentration of the copolymer(above CMC) as seen in Figure 1.16.

Figure 1.16. Interfacial tension reduction versus copolymer volume fraction for the system PS/PB/P(S-bB) at 145'~. [S.H. Anastasiadis. I. Gancarz. J.T. Koberstein, Macromolecules. 22, 1449 (1989))

Thomas and prud'homme6' investigated quantitatively the

effect of molecular weight, composition and concentration of diblock

copolymer of PS and PMMA on the morphology of PSIPMMA binary

blends by optical and electron microscopes. A sharp decrease in

dispersed phase dimension was observed with the addition of a few

percent of block copolymer having equal segment mass (50150

PSIPMMA), followed by a leveling off as the copolymer content was

increased above the critical micelle concentration. For concentrations

below the crit~cal value, the particle size reduction was linear with

copolymer volume fraction. The experimental results were in

agreement with the predictions of Noolandi and ~ o n g ' ~ ~ . The

conformation of the copolymer at the interface was discussed with the

help of models.

It is possible to obtain low interfacial tension by proper

selection of copolymers. The interfacial tension, 7 is the difference of

two contributions and can be represented as follows,

where, yo represents the interfacial energy due to non homogeneity of

the overall monomer content. The second term r , is the decrease of

interfacial tension due to the effect of preferential location of the

copolymer at the blend interface.

For a flat interface with the surface area 'a', the interfacial thickness

'D' and interfacial tension r are given by

where. x is the interaction parameter and a is the monomer length. Leiber

and Noolandi discussed the formation of thermodynamically stable droplet

phase, in which one of the homopolymer is solubilised and protected

by an interfacial film. In order to get such a system, the copolymer

should be highly symmetric. The condition for a symmetric copolymer is

v3a - v,' 2 2 R <;A R,,

VA and VB are the molar volumes and RGA and RGB are the radius of

gyrations of A and B respectively.

The large copolymers are found to be more effective than

shorter ones for the same surface area per chain. At equilibrium, the

number of adsorbed copolymers is determined by a competition

between enthalpic effects, which tend to reduce the overall number of

A-H contacts in the systttm and the loss of entropy associated with an

acc:umulation of chains at the interface.

1.13 Other Routes of Compatibilisation

1.13.1 Solution Casting Method

Two incompatible polyrners are dissolved in a common so~vent"~,

keeping the concentration of the solution as approximately 5%, either

under ambient or elevated pressures and temperatures. After the

complete dissolution, the solvent is removed, usually by freeze-drying

or sublimation. This technique creates a very large interfacial area,

which allows even very weak polymer1 polymer interactions to stabilise

to a pseudo-homogenous system. In this way much finer dispersions

can be obtained in comparison with the more conventional melt

blending techniques.

1.13.2 Dynamic Crosslinking

Dynamic crosslinking is another method of reactive compatibilisation.

In this process, the elastomer phase is preferentially crosslinked using

selective crosslinking agents. The crosslinking agents are low

molecular weight chemicals. Depending on the crosslinking agents,

they acquire distinct characteristics. Peroxides, bifunctional chemicals

that form block copolymer or a mixture of peroxides and bifunctional

chemicals can be added as low molecular chemicals. The high degree

of selective crosslinking prevents the dispersed phase from

undergoing coalescence, which affects the kinetics of blend

morphology developnlent. In this method, compatibilisation is

achieved not by reducing the interfacial tension, but by freezing in

non-equilibrium morphology. Upon dynamic crosslinking the viscosity

increases and this leads to an increase in shear stress and at a

sufficient degree of curing, phase inversion occ~rs '~ ' . This is shown

schematically in Figure 1.17 shown below.

thermoplastic 1 elastomer

dynamic vulcaniratiun

phase inversion

Figure 1.17. Morphology of a TPE blend before and after dynamic vulcanisation showing phase inversion. [C. Koning, M. Van Duin, C. Pagnoulle, R. Jerome, Prog: Polym. Sci., 23. 707(1998)]

It is assumed that with increasing crosslinking of the molecules, the

high viscous rubber phases can no longer be deformed by the local

shear stress and then finally breaks up into small particles. The thin

thermoplastic layers between the crosslinked rubber particles are

somehow elastic and act as a kind of glue between the crosslinked

rubber domain^"^. These materials have the ability to be melt-

processed as crosslinked thermoplastics and have outstanding

elastic recovery after much deformation. Dynamic vulcanisation leads

to reduced permanent set, improved ultimate mechanical properties,

greater fluid resistance, improved high temperature utility, greater

stabilisation of phase morphology, greater melt strength, more reliable

thermoplastic fabricability and improved fatigue res is tan~e"~~"~. This

technique promotes dispersive mixing which leads to finer particle

size. The schematic mechanism accompanying dynamic vulcanisation is

illustrated in Figure 1 .I 8.

Plasbc matrrx phase 1 Plastic mabix phase 7 I

Dispersed rubber parlicles t

Cross-linked rubber parlicles

Figure 1.18. Morphology of a TPE blend before and after dynamic vulcanisation. [S Thomas, A. George.. Eur. Polym. J., 28, 1451(1992)]

Thermoplastic/rubber dynamic vulcanisates combine

thermoplast~c and elastic properties as a result of the final

morphology, and are superior to thermoset rubbers obtained by static

vulcanisation, with respect to processability and recycl~ng. These

blends are called thermoplastic vulcanisates (TPVs) and belong to the

same class of thermoplastrc elastomers (TPEs) as, for instance,

hardtsoft segmented block copolymers and elastomeric ionomers.

Dynarn~c vulcanisation has been applied to several TPEs with

important contributions from Coran et a1.'75. ~resge'" and Siriwardena et

a1.'77 It is to be remembel-ed that static vulcanisation .e. normal

vulcanisation after the stock has been blended does not show superior

morphology and property characteristics unlike in dynamic vulcanisation.

Setua et a ~ . ' ~ ' studied the physicomechanical and thermal properties of

different grades of TPEs ot nitrile rubber (NBR) and high density

polyethylene (HDPE) by chemical treatment of HDPE and dynamic

vulcanisation. There are many recent reports in l i t e ra t~ re "~~ '~~ on

compatibilisat~on by dynamic: vulcanisation of blends.

Groeninckx and coworkers 189-191 developed a novel method to

prepare TPV based on nylon 61 EPDM blends with a high amount of

rubber using both reactive compatibilisation and dynamic

vulcanisation in an extruder by melt mixing. Peroxide was used as

crosslinking agent, which was first, dissolved in oil to make a fine

dispersion. They got very fine dispersions of rubber with good strain

recovery and mechanical properties.

1.14 Phase Morphology Development in Physical Blending

Morphology development is the evolution of the blend morphology

from pellet-sized or powder-sized particles to the sub micrometer

droplets, which exists in the final blend. Final morphology has a

controlling influence on the properties and end use of the blend.

Knowledge of mechanism is also useful for design of intensive mixers

with better dispersive mixing capabilities for reactive blending and also

to understand the kinetics of creation of interfacial area during

blending.

The mechanisms governing morphology development are;

fluid drops stretching into threads.

break-up of the threads into smaller droplets1vz and

coalescence of droplets into larger ones

Balance of these competing processes determines the final particle

size, which is formed upon solidification of the blend. While drop

break-up is not dependent on the content of the dispersed phaselg3.

coalescence is strongly influenced by the blend cornposition'94~'95.

Particle size reduction is the first step in morphology development for

a polyb~end"~.

Scott and ~ a c o s k o ' ~ ~ ~ ' ~ ~ have performed model experiments in

order to investigate the initial stage of mixing at very short times.

Schematic representation of morphology development in compatibilised

and uncompatibilised blends is shown in Figure 1.19. At first, pellets or

powder of the minor phase soften and then stretch out in the flow.

These sheets then thin into approximately lpm and then breakup by

tearing or developing holes. The regions between the holes then

stretch out into fibres, which eventually break down into submicron

droplets. Without compatibiliser in the interface (upper right of Figure)

coalescence is rapid and final size is determined by equilibrium

between breakupTQ9 and coalescenceTgz. In fact the primary mode of

morphology developmer~t is at short mixing times. A phase inversion

mechanism has been shown in literature when the minor component

melted or softened at a lower temperature than the major component*.

Figurel.19. Schemat~c representation of morphology development in compatibilised and uncompatibilised blends. [C.E. Scott and C. W. Macosko, Polymer, 35. 5422 (1994)l

When two polymers arfl melt- blended in an extruder, the polymer with

the lowest melting or flow temperature tends to form, in first instance,

the matrix. After the second polymer has melted, phase inversion may

occur depending on the viscosity ratio and blend composition. The lower

right figure illustrates the role of compatibiliser. If enough copolymer can

diffuse into the new interface, it will reduce interfacial tension and

prevents drop coalescence. Shih200 reported a sequence of four

morphological stages during phase inversion:

Solid major component pellets are suspended in a molten minor- component matrix.

The surfaces of the pellets heat up and are sheared off the

solid core as sheets.

The major component continues to be dispersed in the

minor component. This decreases the minor component

concentration in the melt until it reaches the percolation

threshold, the minimum concentration at which a

component forms a continuous path across the sample.

The blend phase inverts

Figure 1.20 illustrates melt mixing of PSlLLDPE for different

mixing times. Lazo and ~ c o t t ~ ~ ' have studied phase inversion under

steady shear conditions. They have applied shear stress to an initial

morphology of solid pellets of major phase in a matrix of minor phase,

prepared by compression moulding of thin slides. During shearing,

the heated major pellets are sheared off the solid core as sheets.

When the minor component film thickness is thin enough, the

sheets and ribbons of the major phase coalesce and phase inversion

occurs. Shearing during the phase inversion process increased the

thinning speed of the elongated minor component and therefore

affected the phase inversion process.

Figure 1.20. Schematic illustration of morphology development in PSILLDPE blends. There is a change of scab between (c) and (d). The short bar beside the 3-rnrn scale bar in (c) is the 100- pm scale bar. [N.B.D. Lazo, C. E. Scott, Polymer,42,4219(2001)]

1.15 Effect of Physical Blending on Phase Morphology

The dispersity in melt mixed immiscible blends is influenced by

material parameters like viscosity and polanty ratios, blend composition

and processing cor~ditions~"~~. The mechanisms governing morphology

development are drop breakup and coa~escence'~. While drop breakup is

not dependent on the content of the dispersed pha~e''~, is

strongly influenced by the blend

Tay~or"~ and Willis et a1.1g5 made pioneering works on

theoretical investigation of deformation and its eventual breakup of

Newtonian droplets in Newtonian media. According to Taylor, the

droplet size of the emuls~on depended on the interfacial tension and

viscosity ratio between the dispersed phase and continuous phase.

He thus modeled the Newtonian drop size using two dimensionless

parameters, the viscosity ratio (qr = qdq,,,) and critical Weber number,

(We) as

where, y is the shear rate, q, is the matrix phase viscosity. D,, is the

dispersed phase size, and T is the interfacial tension. After critical

particle size, no further reduction in particle size occurs. On additior,

of compatibiliser. We decreases and a reduction in dispersed particle

size occurs.

Thus assuming Newtonian material behavior and simple

shear- flow, drop breakup occurs when the shear forces deforming the

droplet are higher than the interfacial forces. From this balance,

Taylorisg obtained a relation for the drop to be at the maximum stable

state. As coalescence is not included in this relation, the Taylor

diameter (DT) can be considered as the value for the lower limit of

particle size (Taylor-limit). According to them,

where, D,. = drop diameter according to Taylor. F interfacial

tension, .I = viscosity ratio = qd /TI,,, , qd = viscosity of dispersed

phase, q, = matrix phase viscosity and f ' = shear rate. Thus it IS

concluded that drop- breakup cannot occur when viscosity ratio is

greater than 4 or less than 0.003. But for viscoelastic systems, drop-

break up occurs even when the viscosity ratio is greater than 4. The

theories of Taylor have been verified by several authors and later

extended to polymer blends by WU"'. WU obsewed in

polyamidelrubber blends that the smallest particles are generated

when the viscosity of the c:omponents are similar (A =I) . The viscosity

and interfacial tensions of the blend components, which were varied

over a wide range, were correlated with the final particle diameter

(Dw,, Wu- diameter) as

with exponent = +0.84 for A >I and -0.84 for A <l.Here y is the

shear rate adopted in preparing the blend, and q is the melt viscosity

of the matrix phase. This expression was derived on the basis of

balance between the shearing force, which tends to deform the minor

phase into droplets, and the interfacial tension, which tends to restore

the spherical shape of the minor phase into droplets.

1 . 6 Blends based on PSlPB

High-impact polystyrene is one of the most important graft copolymers

formed from PS. Various elastomers can be added to PS to improve

its impact toughness. The two most common rubbers for this purpose

are butadiene rubber and styrene butadiene rubber. The method of

making high-impact polystyrene (HIPS) is to dissolve the rubber in

styrene monomer and then to polymerise the styrene by conventional

means. The resulting product contains not only the rubber and PS but

also a copolymer in which short PS side chains are grafted onto the

polymer. These graft copolymers improve the compatibility of the PS

and rubber and the entire mixture becomes an alloy. The impact

toughness of HIPS made by this copolymer method can be as much

as seven times greater than unmodified PS, but tensile strength.

hardness and softening temperature are all lower.

In patent literature, many kinds of polymeric blends based on PS

and thermoplastic or elastomeric polymers have been r e p ~ r t e d ~ . ~ ~ ' : ! .

This include the works of Anastasiadis et a ~ . ~ in which there is sharp

decrease in interfacial tension by the incorporation of few weight % of

PS-b-PB in PS/PB blends. The interfacial tension leveled off at higher

copolymer concentration. The CMC was evaluated from the plot of interfacial

tension versus block copolymer concentration. Gaillard et aI.'O5 have made

reports on the reduction of interfacial tension of PS/PB/styrene ternary

blends by the addiion of poly(styrenebbutadiene) copolymers. Chu et aL2"

correlated viscosity, morphology and compatibility of solution cast

PSlPB blends. The effect of styrene butadiene styrene triblock copolymer

in PS/PB blend was studied and found that the domain size decreased

with increase of wmpatibiliser. They also reported that blending methods

influenced the morphology due to the difference in the extent of mixing.

Further addition of the copolymer did not modify the interface any more.

Yang et a~.~'' studied the miscibility and mechanical properties

of sulphonated polystyrene (SPS)IPB blends. They found that as the

SPS concentration increases, a significant improvement of miscibility was

observed. At a lower SPS concentration, the blend behaved like a rubber,

while higher SPS concentration in the blend resulted in a briile failure

before yield. An increase in the sulphonation level of SPS in the blends

gave improved miscibility. A significant enhancement of tensile strength

was observed with increase in sulphonation level. Horak2" et al. studied

the effect of selected structural parameters of styrene-butadiene block

copolymers on their tnmpatibilisation efficiency in PSIPB blends by

electron microscopy and small angle X-ray scattering.

1.17 Scope and Objective of the Work

Styrene polymers have [unique properties of their own, which make

them useful in a wide range of products. PS is an amorphous glass

like solid approximately below 1 0 0 ~ ~ . ~ b o v e this temperature, the

polymer chain has rotational freedom, which imparts segmental

mobility, which makes processing easy. Below T,, it possesses

considerable mechanical strength, good dielectric properties and

superior processing characteristics. Albeit these desirable properties,

disadvantage5of PS mi ts inherent brittleness, low impact strength.

poor chemical resistarlce and poor environmental stress resistance.

PB is characterised b y good resilience, resistance to abrasion, low

heat build up, but poor chemical resistance and processability. TPEs

from PS and PB comb~ne the superior processability characteristics of

PS and elastic properties of PB. In spite of the positive aspects

described above for PS and PB, the performance of PSlPB blend is

not up to the expectations. The unmodified blends are immiscible and

incompatible. The blending of the PS and PB phase will result in a

high impact resistant plastic or toughened rubber depending on the

composition of the blend.

From the foregoing discussions on the experimental and

theoretical aspects of compatibilisation of various polymer blends, it is dear

that some studies have already been done in the compatibilisation of

PSIPB system. But no systematic study has been carried out to evaluate

the effects and efficiency of compatibilisers by physical compatibilisation

routes on these blends. The compatibilisers used are a block copolymer

poly(styrene-bbutadcewbstyrene) (SBS) and a random copolymer

styrene butadiene rubber(SBR). Both the copolymers are non-

reactive. It has segments identical with each of the blend components

The most important challenging part in this research problem is to

analyse the effect of these two compatibilisers SBS and SBR on the

morphology and properties of PSIPB blends. Major objectives of the

present work are:

1. To prepare new high performance thermoplastic elastomer from

PS and PB by melt mixing.

2. To prepare compatibilised blends using triblock copolymer.

SBS and random copolymer, SBR.

3. To evaluate the effects and efficiency of compatibilising action on the

morphology and properties of the blends. The properties of the blends

have been evaluated by scanning electron microscopy, static

mechanical studies, rheological measurements, thermogravimetric

analysis, differential scanning calorimetly, dynamic mechanical

analysis and solid state nuclear magnetic resonance spectroscopy.

1.18 References

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