Morphology, morphology development and mechanical properties of polystyrene/polybutadiene blends

Morphology, morphology development andmechanical properties of polystyrene/polybutadiene blends

Susan Joseph a, Sabu Thomas b,*

a St. Stephen’s College, Pathanapuram 689695, Kerala, Indiab School of Chemical Sciences, Priyadarshini Hills P.O., Mahatma Gandhi University, Kottayam 686560, Kerala, India

Received 23 January 2002; received in revised form 14 May 2002; accepted 11 July 2002

Abstract

Polystyrene/polybutadiene (PS/PB) blends with different plastic/rubber ratios were prepared by melt mixing. A

detailed investigation on phase morphology development of 30/70 wt.% PS/PB blends as a function of processing

conditions was quantitatively analyzed. Morphology is developed at the initial stages of mixing. Suitable blending

conditions resulting in optimum phase morphology were obtained at 180 �C, 60 rpm and at 8 min mixing time. Phase

morphologies of the blends were also studied as a function of composition. Mechanical properties of the blends were

measured. Attempts were made to correlate the morphologies with the properties. Parallel–Voids model has been

applied to characterize phase morphology of these blends.

� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Blends; Morphology development; Morphology; Mechanical properties

1. Introduction

Polymer blending is a means of improving deficient

properties of some traditional polymers. The attainment

of desirable properties depends mainly on the extent of

molecular interactions between the blend components.

Thermoplastic elastomers (TPEs), for example, from

rubber–plastic blends are such materials that combine

the excellent processing characteristics of plastics at high

temperature and the wide range of physical properties of

elastomers at service temperature [1,2]. By careful se-

lection of component polymers, their blend ratios and

processing conditions, one can attain wide range of de-

sirable properties in TPEs.

Blends of polystyrene (PS) and polybutadiene (PB)

combine the superior processability characteristics of PS

and elastic properties of PB. PB is characterized by good

elasticity, excellent resistance to abrasion, low heat build

up, but poor chemical resistance and processability. PS

exhibits high modulus, good dielectric properties and

superior processing characteristics, but it is extremely

brittle. The temperature range of PB applications can be

increased through blending with PS.

For blends, the morphology depends on composition,

rheological and physical characteristics of the compo-

nents, relative compatibility, and the nature and intensity

of mixing. Detailed investigations on phase morphology

development, phase co-continuity and phase stability of

polymer blends [3,4] have been undertaken. The corre-

lations between morphology and mechanical properties

have been established [5,6]. In fact, it is well known that

by using different mixers, and/or by varying the mixing

parameters, it becomes possible to control phase mor-

phology [7–9]. 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 con-

trolling influence on the properties and end use of the

blend. Knowledge of mechanism is also useful for design

European Polymer Journal 39 (2003) 115–125

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* Corresponding author. Tel.: +91-481-730003/730015/

598303; fax: +91-481-561190.

E-mail address: [email protected] (S. Thomas).

0014-3057/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0014-3057 (02 )00180-5

mail to: [email protected]

of intensive mixers with better dispersive mixing capa-

bilities for reactive blending and also to understand the

kinetics of creation of interfacial area during blending.

The mechanism governing morphology developments

are; fluid drops stretching into threads, break-up of the

threads into smaller droplets [9] 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

phase [10], coalescence [9] is strongly influenced by the

blend composition [8,11,12]. There are several good re-

ports depicting the relationship between morphologies

and these factors [13–18].

The influence of time of mixing in a Haake Rheocord

on the size of dispersed phase has been studied by a

number of workers. Particle size reduction is the first

step in morphology development for a polyblend. Favis

[19] observed that for polypropylene/polycarbonate (PP/

PC) blends, the most significant particle size deforma-

tion and disintegration process took place within the

first 2 min of mixing. After 2 min very little reduction in

the size of the dispersed phase was observed upto 20 min

mixing time. Based on these results it was suggested that

melting process might be important in generating the

final morphology of an immiscible blend. Plochocki

et al. [20] studied the processing of PS/LDPE blends in

several industrial mixers and observed that the initial

dispersion mechanism might be due to abrasion of solid

or partially softened pellets on the wall of the processing

equipment. Sundararaj et al. [21] studied the processing

of both polyamide/PS and polypropylene/PS blend sys-

tems in a twin screw extruder and showed that major

reduction in domain size occurred during the melt-

softening step. Continued mixing reduces the size of

large particles and rounds off the particles. Morphology

of the dispersed phase is rapidly modified under dynamic

conditions in the melt.

Thomas et al. [22] studied the effect of processing

conditions on the morphology development of nylon 6/

EPM blends. The dispersed phase morphology as a

function of mixing time was investigated for aromatic

amorphous nylon/Zytel (Zytel 330)/EPM and PS/EPM

by Scott and Macosko [23]. In this study most of the

reduction in the dispersed phase size was observed at

shorter mixing times in conjunction with the melting and

softening process. At intermediate mixing time, the

morphology consisted of a large number of small par-

ticles along with a small number of very large particles in

the size distribution. The effect of subsequent mixing

reduced the size of the largest particles in the size dis-

tribution. Other reports in the literature regarding the

influence of mixing time on the dispersed phase size in-

clude the studies of Karam and Bellinger [24], Karger-

Kocsis et al. [25], Schreiber et al. [26] and Laokijcharoen

et al. [27].

The main objective of this paper is to critically con-

sider the rate of morphology development of PS/PB

blends under dynamic conditions in a Haake mixer. The

effect of blend ratio on the mechanical and morphological

properties is studied in detail. The results of tensile ex-

periments at room temperature are interpreted with re-

gard to blend morphologies, as determined by scanning

electron microscopy (SEM). The effects of mixing time,

mixing temperature and rotor speed on the phase mor-

phology development of PS/ PB (30/70 wt.%) blends were

also investigated. Dobkowski model was applied to char-

acterize phase morphology of these blends.

2. Experimental

2.1. Materials used

PS (POLYSTRON 678 SF-1, Crystal grade) supplied

by Polychem Limited, India, and cis-1,4, PB supplied by

IPCL (Indian Petrochemical Company Ltd.) Vadodara

under the trade name Cisamer G.P. were used in this

work. Characteristics of the materials used are given in

Table 1.

2.2. Blend preparation

The polymer blending was performed in a Haake

Rheocord 90 mixer. PS was melted for 2 min to turn it

into a melt and then PB was introduced. The mixing

time was recorded from the moment that PB has been

introduced. However, in order to study the effect of

mixing time on phase morphology, experiments were

performed at constant rotor speed of 60 rpm at 180 �C.

The mixing time was varied from 30 s, 1, 2, 4, 8, 15 and

25 min. The longest mixing time was confined to 25 min

to avoid any possible thermal decomposition. To study

the influence of rotor speed on phase morphology, the

rotor speed was varied from 20, 40 and 60 rpm. Samples

were also prepared by varying the temperature of mixing

i.e.; 140, 180 and 200 �C.

2.3. Physical testing of the samples

After mixing, the molten materials were sheeted out

through a lab mill at nip setting of 2.5 mm. The sheets

were removed and pressed in a mold in the hot condition

itself. It was then compression molded in an electrically

heated hydraulic press at 180 �C for four minutes in

Table 1

Characteristics of the materials

Material Density (g cm�3) Molecular weight (Mw)

PS (atactic) 1.04 3.51� 105

PB 0.94 5.3� 105

116 S. Joseph, S. Thomas / European Polymer Journal 39 (2003) 115–125

specially designed mold, keeping the specimens still

under compression. The molded samples were then cut

into sheets of 15 cm � 15 cm � 0:2 cm. Samples for

tensile testing were punched out from the molded sheets.

After optimizing the processing conditions, blends for

mechanical testing was prepared at the optimized condi-

tions. The melt mixed samples are denoted as S00, S20,

S30, S40, S50, S60, S70, S80 and S100, where ‘S’ stands for PS

and the subscripts indicate content of PS in the blend.

Tensile testing of the samples was performed at 25 � 2

�C according to ASTM D412-80 test method using

dumb-bell shaped test specimens at cross-head speed of

50 mm/min using a Zwick Universal testing machine

(Model 1445). The tear strength was determined as

per ASTM D624-81. The instrument and experimental

conditions were the same as in the case of tensile testing.

Hardness was measured according to ASTM D 2240-81

test method, using a Shore D durometer for PS rich

blends and Shore A, using Zwick model 3114 instrument

for PB. The testing were conducted at room temperature

(25 � 2 �C). Hardness values quoted were an average of

eight readings taken at random over the entire specimen

surface.

2.4. Phase-morphology analysis

The cryogenically fractured samples were examined

by a scanning electron microscope, Philips XL20 model,

operating at 20 kV. To facilitate identification of phases

and to enhance morphological features, the cryogeni-

cally fractured samples were etched with n-heptane and

butan-2-one as etchants for dissolving the minor PB and

dispersed PS phases respectively. The samples were then

dried and coated with gold and examined under SEM.

The diameters of dispersed phase were measured from

several micrographs at random. The photographs were

quantitatively analysed by counting the size of dispersed

domains from different fields of view of the specimen

based on different domain diameters [28].

The number-average domain diameter,

Dn ¼P

NiDiPNi

ð1Þ

The weight-average domain diameter,

Dw ¼P

NiD2iP

NiDið2Þ

and volume average diameter,

Dv ¼P

NiD4iP

NiD3i

ð3Þ

In these equations, Ni is the number of domains having

diameter Di. The poly dispersity index, which is a mea-

sure of domain size distribution, was calculated as:

PDI ¼ Dw

Dnð4Þ

3. Results and discussion

3.1. Influence of mixing time

Fig. 1(a–f) shows the SEM photographs of the blend

morphologies of PS/PB (30/70) blend after various mix-

ing times of 30 s, 1, 2, 4, 8 and 15 min respectively. The

influence of dependence of particle size on various do-

main diameters on the time of mixing is shown in Fig. 2.

It is seen that during blending, solid pellets with dough

consistency of approximately 200 lm are broken down

into finer dispersion of �2 lm in short mixing times.

After 30 s of mixing, dispersed PS domains are seen in

the continuous PB matrix. After 2–4 min of mixing, all

the particles are of nearly of uniform size. The results

indicate that the most significant domain break-up

occurs within the first 1–2 min of mixing time where,

melting and softening occurs. At intermediate mixing

time of 4 min, the morphology consisted of a large

number of small particles along with a small number of

very large particles. Further increase in mixing time does

not have any major influence on particle size and the

smallest particle size was obtained at 8 min of mixing

time. The invariant morphology is due to the rapid es-

tablishment of equilibrium between domain breakup

and coalescence. The effect of subsequent mixing re-

duced the size of the largest particles in the size distri-

bution. But at longer mixing times of 15 min, phase

dimension increases. This is due to shear induced phase

coarsening. It is established that contact time needed

for drop coalescence increases when drop diameter in-

creases, matrix viscosity increases and density difference

between the drop and matrix phase increases. Avgero-

poulos et al. [29] have made similar observations. Ac-

cording to them, in melt mixed elastomer blends, early in

the mixing process, sheets or ribbon-like structures of

the softened dispersed phase are formed. The dispersed

phase appears as elongated structures. Then it becomes

drawn into fibrous domains. Upon further mixing, the

elongated droplets are broken into smaller particles or

droplets. Due to interfacial tension and flow character-

istics, the sheets formed become unstable and holes are

formed, which coalesce rapidly.

3.2. Influence of mixing temperature

Reports regarding influence of temperature on parti-

cle size are contradictory. Tokita [30] observed a de-

crease in particle size with increase of temperature in

ethylene propylene diene copolymers (EPDM) and nat-

ural rubber (NR) (EPDM/NR) blends, which is ex-

plained based on the reduction in interfacial tension with

S. Joseph, S. Thomas / European Polymer Journal 39 (2003) 115–125 117

increase in temperature. The SEM micrographs of S30

blends prepared by varying the mixing temperature (140,

180 and 200 �C), keeping the rotation of rotor and

mixing time constant are seen in Fig. 3(a–c). The various

domain diameters and polydispersity values calculated

according to Eqs. (1)–(4) from micrographs of these

samples are given in Table 2. The number average do-

main diameter at 60 rpm and 8 min mixing time de-

creases from 4.27 lm at 140 �C to 2.40 lm at 180 �C.

This may be associated with the reduction in interfacial

tension on account of increase of temperature. Further

increase in temperature to 200 �C does not have much

influence on reduction in particle size. Instead, the do-

main diameter observed is of size 2.60 lm. The different

domain diameters, Dw Dv, and polydispersity index were

also minimum at the optimized conditions. Thus the

temperature was optimised at 180 �C further for all the

mixes, keeping other conditions the same. The marginal

increase in domain diameter with increase in tempera-

ture is due to shear induced phase coarsening.

3.3. Influence of rotor speed

Keeping temperature and time of mixing constant,

rotor speed was varied to 20, 60, and 80 rpm respec-

tively. Effect of rotor speed on the phase morphology

was quantitatively assessed by morphological observa-

tions. SEM of the mixes at 20 and 80 rpm are seen in

Fig. 4(a) and (b) respectively. These are compared with

the micrograph of the mix at rotor speed 60 rpm seen in

Fig. 3(b). The various domain diameters quantified from

micrographs of these samples are given in Table 2. The

dispersed phase size of 2.95 lm at 20 rpm got reduced to

2.25 lm at 60 rpm. Further increase in rotor speed to 80

rpm could not further reduce the particle size; instead

there is an increase in dispersed domain diameter to 2.54

lm. The different domain diameters, Dn Dw and Dv and

polydispersity index were also minimum at 60 rpm

mixing conditions. Since at 60 rpm, minimum in particle

domain diameter was obtained, further for all the mixes

rotation of rotor was optimized for 60 rpm. According

Fig. 1. SEM showing blend morphologies after various mixing times for (30/70) PS/PB blends (a) 30 s, (b) 1 min, (c) 2 min, (d) 4 min,

(e) 8 min and (f) 15 min.


to Min [31] the changes in shear stress has considerable

influence in particle size. It appears as a droplet struc-

ture rather than as elongated structures; which can be

explained based on sufficient stress for disrupting the

dispersed PS phase by high viscous PB matrix. At a

rotor speed as fast as 60 rpm, the PS phase appears to be

completely dispersed. Taylor model of droplet break-up

offers a remarkably accurate and simple picture of the

shear response in polymer blends. Above a critical shear

rate, drop breakup is not possible. As shear rate is in-

creased, viscosity decreases, but drop elasticity increases

as reported by Sundararaj et al. [21].

3.4. Sheath/core structure

Several authors have reported about phase segrega-

tion (division into skin and core) during extrusion of

polymer blends. For observing the stratification phe-

nomenon if any, the skin of the extrudate was analyzed

by SEM. Stratification phenomenon in S30 blend system

is shown in Fig. 5. The low viscosity PS phase is seen to

migrate towards high viscosity PB phase and encapsu-

late it by giving it the lowest rate of viscous dissipation.

Similar reports were made by Danesi and Porter [32] for

EPM/PP blend, where, low melt viscosity PP was seen to

encapsulate the high melt viscosity EPM phase.

3.5. Effect of blend ratio

SEM of extracted S40, S50, and S70 blends shown in

Fig. 6(a–c) and of S30 in Fig. 3(b) demonstrate two-

phase morphology. The SEM photographs show that

upto 30% PS concentration, PS is dispersed as domains

in the continuous PB matrix. When the proportion of

PS is 40%, there is onset of co-continuity. An inter-

penetrating co-continuous morphology is obtained for

50/50, PS/PB system. At this particular composition,

both immiscible phases are completely continuous.

Similar behavior of two-phase morphology has been

reported in NR/epoxidised NR-25 blends by Thomas

et al. [41]. This is followed by phase inversion. In S70

the minor phase of PB is dispersed as droplets in the

continuous PS matrix as seen in Fig. 6(c). When PS is

the dispersed phase, rate of coalescence is lower due to

high viscosity of PB phase. Coran and Patel [33–36]

published a series of articles on rubber–thermoplastic

Fig. 2. Influence of dependence of the particle size on the time of mixing for (30/70) (PS/PB) blends.


blends and attempted to correlate their physical

properties with the fundamental characteristics of the

elastomer and thermoplastic components. In our labo-

ratory Thomas and co-workers [37–40] reported mor-

phology and mechanical properties of various TPEs.

The major factors determining the blend morphology

are the blend ratio, the shear viscosity of the compo-

nents and shear rate during mixing.

Fig. 3. SEM showing the effect of mixing temperature on

morphology development studies of (30/70) (PS/PB) blends at

(a) 140 �C, (b) 180 �C and (c) 200 �C.

Table 2

Influence of mixing parameters on different domain sizes of PS/

PB (30/70) blends

Mixing conditions Dn (lm) Dw (lm) Dv (lm) PDI

140 �C, 60 rpm, 800 4.27 7.02 6.55 1.68

180 �C, 60 rpm, 800 2.4 2.81 3.58 1.17

200 �C, 60 rpm, 800 2.60 3.22 6.45 1.23

180 �C, 20 rpm, 800 2.95 4.71 4.01 1.60

180 �C, 80 rpm, 800 2.64 5.55 6.55 2.10

Fig. 4. SEM of (30/70) PS/PB blends at different rotor speeds

(a) 20 rpm and (b) 80 rpm.

Fig. 5. SEM showing the phenomenon of stratification in

(30/70) (PS/PB) blend.


As the PS content in the blend increases, domain size

also increases from 2.40 lm in S30 to 2.93 lm in S40. This

can be attributed to the coalescence phenomenon oc-

curring at higher concentration of one of the phases as

reported by Heikens [42] and Dao [43]. In S70, the par-

ticle size still increases to 4.52 lm. In PS rich blends, PB

is dispersed as big domains in the continuous PS matrix

(droplet-matrix structure). The bigger domain size is

attributed to reagglomeration or coalescence of the

dispersed rubber particles. The high diffusional mobility

of PB domains in the low viscosity PS matrix is re-

sponsible for the high extent of coalescence phenomenon

of rubber domains. The various domain diameters and

PDI values calculated according to Eqs. (1)–(4) from

micrographs of these samples given in Table 3 also

supports the above observations. Blends of PB dispersed

in PS, (S70) showed much bigger particle size with

composition than the blend of PS dispersed in PB, i.e.

(S30). This asymmetric behavior can be explained by the

lower melt viscosity of the PS phase as compared to PB

matrix phase. As a result, the equilibrium between do-

main breakup and coalescence is shifted more in the

direction of coalescence in blends where PB is dispersed

in PS. Thus deformation of the two colliding drops is

lower at higher dispersed phase viscosities. Schematic

representation of the drop break-up in high and low

viscosity matrices are shown in Fig. 7. When the dis-

persed phase is highly viscous than the matrix phase

(gd > gm), it is poorly dispersed. On the other hand,

when the dispersed phase has lower viscosity than the

matrix, it is finely dispersed. It is observed that viscosity

ratio plays a crucial role in the droplet break-up process;

higher viscosity ratios hamper the droplet breakup.

From rheological studies already made for this system,

the viscosity of the PS and PB phases are 756 Pa s is 3528

Pa s respectively at 180 �C and 100 s�1 [44]. The flow

field of low viscous PS is not able to sufficiently transfer

the applied stress to the highly viscous dispersed phase.

In the present study, at high viscosity ratio of >4 (i.e.

viscosity of the dispersed PB phase/viscosity of PS

phase), the minor PB phase is coarsely dispersed. On the

other hand when PB forms the matrix phase and PS is

the dispersed phase, the viscosity ratio becomes <0.5

and we obtain fine dispersion of PS domains in the high

viscosity PB matrix.

Fig. 6. SEM of the blends; (a) (40/60), (b) (50/50) and (c) (70/

30) after extraction of minor phase.

Table 3

Various domain diameters calculated according to Eqs. (1)–(3)

for PS/PB blends

Blends Dn (lm) Dw (lm) Dv (lm) PDI

S30 2.40 2.81 3.58 1.17

S40 2.93 3.69 5.02 1.26

S70 4.52 4.86 10.02 1.02

Fig. 7. Schematic representations of the drop break-up in high

and low viscosity matrices.


3.6. Mechanical properties

A plot of stress strain behaviour is shown in Fig. 8.

The PB and high PB blends (S20 to S40) show typical

behavior of uncrosslinked elastomers, while PS behaves

similar to that of a brittle plastic material. Addition of

PS to PB changes the nature of the curves considerably.

The S50 blend shows intermediate behavior. It is ob-

served that up on the addition of PS, the strain de-

creases, and the stress increases. The extremely low

values of strain at break have been noted for composi-

tions where PS forms the continuous phase (S70 and S80).

A plot of variation of tensile strength with weight

percentage of PB is seen in Fig. 9. Tensile strength in-

creases with incorporation of PS into PB and sharp

increase in tensile strength is observed when the PS

content exceeds 40 wt.%. This sharp increase in tensile

strength is associated with the predominance of PS

phase as the continuous phase. The negative deviation

observed in the curve is due to the poor interfacial ad-

hesion between PS and PB phases, which causes

poor stress-transfer between the matrix and the dis-

persed phase. Similar studies of mechanical properties

like tensile, impact and hardness of melt blended nylon

6(PA6)/acrylonitrile butadiene–styrene copolymer made

by Jang et al [45] reported incompatibility between the

phases.

Fig. 10 shows the variation of elongation at break and

Young’s modulus as a function of blend ratio. PB and

PB rich blends, exhibit very low Young’s modulus val-

ues. Modulus gradually increases with PS incorporation.

Beyond 50% of plastic content, there is sharp increase in

modulus. This is due to the reinforcement of rubber

phase by addition of plastic phase. The modulus is a

measure of the strength of the material at low strains.

Fig. 8. Stress–strain curves of PS/PB blends.

Fig. 9. Variation of tensile strength with weight percentage of

PB.

Fig. 10. Variation of elongation at break and Young’s modulus

with weight percentage of PB.


Therefore, it can be inferred that PS rich blends exhibit

good mechanical properties. Young’s modulus values

followed a trend similar to the stress at break, increasing

with increasing PS content. Reverse trend for elongation

at break is observed for the blends; lower values for PS

rich blends. Danesi and Porter [32] have reported similar

observations. The properties show a negative deviation

indicating incompatibility of the system. Schurmann

et al. [46] studied mechanical properties and morphology

of polyethylene (HDPE)/polypropylene (iPP) blends.

Fig. 11 is a plot of tear strength and hardness values in

Shore D units with weight percentage of PB. The tear

strength increases upon the incorporation of PS into

rubber. Tear strength values also show a negative devi-

ation, which again is due to high interfacial tension and

poor interfacial adhesion between the phases. It is found

from the figure, that blends with larger PS content

(above 60 wt.%) show considerable increase in tear

strength due to the continuous nature of the PS phase. A

wide range of hardness could be attained for the blends;

viz. Shore D values ranging from 2 Shore D for S20 to 83

Shore D for S100. Shore D values exhibited a sigmoidal

curve. A slightly higher than expected increase in Shore

D hardness occurred in S50 to S60, indicating a change in

the continuous phase possibly. A significant increase in

hardness values was observed for S70 and S80 blends and

hardness is directly related to strength [47]. A hardness

value of 29 in Shore A units is obtained for PB. In this

case the curve shows a positive deviation. Since hardness

is inherently a surface property, positive deviation can-

not be always interpreted as a sign of good interfacial

adhesion and miscibility between the phases. Since use-

ful working range [48] of the Shore hardness measure-

ments is between 10 and 90 for Shore A and between 30

and 90 for Shore D, reliable results for Shore D values

were obtained for compositions from S50 and upto S80.

3.7. Application of model

Xie et al. [49] applied various theoretical models to

correlate morphology and mechanical properties of PS/

polypropylene blends, in terms of component strength

and composition. The Parallel–Voids model proposed

by Dobkowski [50] when applied to modulus-composi-

tion data is a more helpful tool to understand the

microstructure of the blends. According to this, the

fraction of reinforcing material ðnÞ is given by [51],

n ¼ ð1=rÞ þ ð1=rÞðe=V Þ ð5Þ

where, r is the ratio of the tensile moduli of the pure

polymers, V is the volume fraction of the reinforcing

material and e is the reinforcing factor,

e ¼ ðE=EmÞ � 1 ð6Þ

where, E is the modulus of the blends and Em that of the

polymer matrix. If n ¼ 1, voids are not present and there

is perfect adhesion between matrix and filler. If n < 1,

there are voids in the filler space and the filler does not

adhere to the surface of the matrix. On the other hand, if

n > 1, the reinforcing material occupies more space than

its original volume.

The values of reinforcing factor obtained for different

blends are given in Table 4. For all the blend systems, nvalues are found to be less than one, indicating varying

amount of voids. PS-rich compositions, S60, S70 and S80

and PB-rich composition, S20 have considerable amount

of voids as seen in the table. This in turn can adversely

affect blend properties. The reinforcing factor for S40 is

moderately high, indicating onset of co-continuity as

discussed earlier. The maximum value for the 50/50

blend confirms the co-continuous nature. Earlier reports

[52] in PS/NBR blends from our laboratory have given

similar data for the reinforcing factor <1 indicating

Fig. 11. Effect of blend ratio on tear strength and hardness

values in Shore D units of PS/PB blends as a function of weight

percentage of PB.

Table 4

The reinforcing factor ðnÞ of PS/PB blends

Samples Reinforcing factor ðnÞS20 0.0089

S30 0.0130

S40 0.0470

S50 0.0520

S60 0.0014

S70 0.0008

S80 0.0012


varying amount of voids and weak interactions between

phases.

4. Conclusions

In the present paper, influence of processing history

upon the morphologies of PS as the minor phase was

investigated in the PS/PB blends. Blending condi-

tions were chosen carefully by variation of rotor speed,

blending temperature and mixing time. The suitable

blending conditions resulting in optimum phase mor-

phology were found to be 60 rpm, 180 �C and 8 min. The

influence of mixing time on phase morphology indicated

that the most significant particle size deformation and

disintegration took place within the very beginning of

the mixing process, indicating the importance of melting

process in final blend morphology generation. Influence

of viscosity ratio on droplet-break-up indicated that

blends with a low viscosity ratio (q < 1) gave stable

phase morphology. Morphology of PS/PB blends indi-

cates a two-phase structure. A co-continuous morphol-

ogy was obtained between 40 and 60-wt.% of PS

content.

Mechanical properties of the blends prepared under

optimised mixing conditions were correlated with the

morphology. Strain at break increased with increase in

PS content. It is observed that changing the structure

from a droplet-matrix to a co-continuous structure at a

given composition can result in a quite significant in-

crease in tensile strength. Mechanical properties such as

tensile strength, tear strength and hardness is higher for

blends containing higher percentage of PS. Parallel–

Voids model have been applied to modulus composition

data.

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Morphology, morphology development and mechanical properties of polystyrene/polybutadiene blends

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