Indian Journal of Chemical Technology Vol. 9, July 2002, pp. 316-323

Articles

Morphology of PSIPMMA blends and their solution rheology

S R Vashishtha, Navin Chand & S A R Hashmi*

Regional Research Laboratory (CSIR), Hoshangabad Road, Habibganj Naka, Bhopal 462026, India

Received 24 March 2001; revised received 11 March 2002; accepted 17 April 2002

Polystyrene (PS)lPolymethylmethacrylate (PMMA) blends were prepared by using solution casting method. These polymers show a tendency of mixing each other in compositions where PS is 60 wt. % or above. PS and PMMA mixed partially and formed PS-rich-phase and PMMA-rich-phase in the blend. Morphology of PSIPMMA blend prepared by solution method is discussed with the help of fractured micrographs obtained by Scanning Electron Microscope. Flow curves of solutions of PS and PMMA and their blends in toluene were studied at room temperature. The effects of blend concentration and composition on shear stress and shear viscosity were studied. The viscosity data was obtained for various weight percent of PMMA and PS separately. 15 and 20 wt% solutions of PSIPMMA blends were also studied. Power law model was fitted for shear dependency of viscosity of solutions. Shear stress increases with the addition of PS in PSIPMMA blend solution. Increase in concentration of PMMA decreased the viscosity of the PSIPMMA solution. All these systems behave as pseudoplastic systems.

Development of poly blend by solution technique has an advantage of uniform distribution of each component over the other techniques employed for the preparation of blends. Blends of PS and PMMA have been investigated by several authors for their compatibility l-3, morphology l-3 ,6 and rheology4,5. Both the polymers are incompatible, however it has been reported that in some of the blends, PMMA dissolves in PS phase and some of the PS phase dissolves in PMMA phase. Their morphologies show two distinct phases i.e. PMMA rich phase and PS rich phase l

.

Salami type of morphology has also found in PSIPMMA blend 7. It was also reported8 that compatibility increases more in the region of PMMA rich composition than in the region of PS-rich composition. The morphology of the blend system depends upon the composition, interfacial interactions and processing conditions and therefore, it is interesting to study the rheological properties of the system.

The rheological properties depend upon the structure, which is controlled by the degree of dispersion of the components9

. Mechanical mixing generally leads to the formation of thermodyna­mically non-equilibrium systems however, a uniform distribution of components of the blend can be achieved by using a common solvent lO

• The composition dependence of the rheology of the blends

*For correspondence (E-mail: sarhashmi@rrlbpl.org; Fax: 0755-587042)

is as a rule non-additive, displaying maxima and/or minima on the rheology-composition graphs 11 ,12. In a review, Utracki 12 has classified the polymer blends into three groups: those, which show positive deviations from the additivity, and those, which show negative deviation and the remainder, where both types of deviations ar~ observed. This classification is based on zero shear viscosity-composition dependence. Since these data are not always available, the second best criterion, which has been advocated, is the viscosity at constant shear stress 11 , 12. Shear rate may not be continuous due to formation of discontinuous phases between polymers. Miscible blends and those with strong interdomain interactions have been found to show positive deviations from the additivity rule l 2

. The blends in which the interactions are weak have been found to show negative deviations and the blends in which there is a concentration-dependent transition of structure (phase inversion etc.), positive and negative deviations 12.

In the present study, PS/PMMA blend in toluene solvent has been studied for various rheological properties. The morphology of PS/PMMA blend obtained by solution technique is also studied and reported here.

Experimental Procedure

Materials Commercial grade PS and PMMA were supplied

by Mis Monika Plastics, Indore. Densities of PS and

Vashishtha et at.: Morphology and solution rheology of PSIPMMA blends Articles

PMMA were 1.04 and 1.17 g/cc respectively . The molecular weights of PS and PMMA were 1,33,905 and 1,37,430 respectively. The molecular weights were determined by the viscosity method using Ostwald viscometer and toluene as solvent.

Solution blending Weighed amount of polymers were taken and

dissolved in corresponding volume of toluene. These solutions were kept for 24 h to homogenize the polymers at room temperature. These solutions were kept in airtight flask to avoid the evaporation of solvent. After 24 h this homogenized solution was stirred using glass rod for uniform mIXIng. Compositions of different solutions are given in Table 1.

Solution rheology Solution rheology of PS, PMMA and PS/PMMA

blends were carried out on Bohlin Viscometer (BV-80) at room temperature. For evaluation of above mentioned properties, 17 mL of each solution was taken and transferred to the cylinder having diameter 33 mm. The measuring spindle used in the case had the diameter 30 mm. Rotational speed was varied and corresponding torque was measured. Values of viscosity (T]) shear stress ('r) and shear rate (y) were directly obtained from the instruments.

Scanning Electron Microscopy Solutions of various PS/PMMAlToluene systems

were kept in small glass moulds and toluene was allowed to evaporate slowly at room temperature. After a fortnight, solid PS/PMMA blends were removed for the mould and fractured cryogenically to observe on SEM (JSM-5600 Model) after gold coating.

Results and Discussion

Rheological properties One set of flow curves between log shear rate

(log y) and log shear stress (log 1') is presented in Fig. 1 for different concentration of polystyrene in toluene at room temperature. Shear stress increases with shear rate as well as with PS concentration in toluene. The viscous behaviour of polymer melts and solutions are generally described by the power law. The shear stress relates with the shear rate as per following equation:

l' = ky" .. . (1)

where, k and n are fitted coefficient. Slope of the curve, which is a measure of

pseudoplasticity index (n) of the solution decreases with the increase of PS concentration. The values of n were determined by using Microsoft Excel software in which slope was evaluated from the trend line. The values of the slope of the lines so obtained are given in Table 2. In case of PS, the effect of concentration is significant on n, as compared to PMMA. There is a variation in the second decimal of n values for the PMMA solution, which seems to be an outcome of minor experimental errors such as evaporation of solvent, etc. For 10 wt% PS solution, value of n is

Table I-Compositions of various blends

Sample Code PS (%w/w) PMMA (%w/w)

PS 100 00

SM-I 90 10

SM-2 80 20

SM-3 70 30

SM-4 60 40

SM-5 50 50

SM-6 40 60

SM-7 30 70

SM-8 20 80

SM-9 10 90

PMMA 00 100

Table 2-n value of different compositions

Sample Code 'n' value Sample Code 'n' value

PS (40%) 0.79 PMMA (40%) 0.93

PS (35%) 0.81 PMMA (35%) 0.92

PS (30%) 0.85 PMMA (30%) 0.98

PS (25%) 0.95 PMMA (25%) 0.91

PS (20%) 0.95 PMMA (20%) 0.98

PS (15%) 0.95 PMMA(l5%) 0.97

PS (10%) 0.98 PMMA (10%)

SM-I (15%) 0.94 SM-l (20%) 0.96

SM-2 (15%) 0.94 SM-2 (20%) 0.95

SM-3 (15%) 0.94 SM-3 (20%) 0.92

SMA (15% ) 0.92 SM-4 (20%) 0.93

SM-5 (15%) 0.92 SM-5 (20%) 0.90

SM-6 (15%) 0.93 SM-6 (20%) 0.90

SM-7 (15%) 0.95 SM-7 (20%) 0.91

SM-8 (15%) 0.94 SM-8 (20%) 0.91

SM-9 (15%) 0.94 SM-9 (20%) 0.95

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nearly I . An increase in concentration of PS in toluene changes the behaviour from near Newtonian to non-Newtonian.

Figure 2 represents the graph between the log (shear stress) and log (shear rate) for PMMA solution in toluene at room temperature. Shear stress increases with shear rate. The concentration of PMMA does not show any significant changes in the slope of curves for the studied system.

Variation of viscosity with shear rate of PS solution is shown in Fig. 3(a) at different concentration varying from 10 to 40 wt%. Viscosity decreases slightly with the increased shear rate however, at low concentration (10 and IS wt%) the solution behaves as Newtonian fluid as shown in Fig. 3(a). It is almost a parallel line to x-axis .

Figure 3(b) shows a plot between viscosity and shear rate of PMMA solution at different concentration varying from 15 to 40 wt%. Viscosity is almost unaffected with the increased shear rate. The vi scosity of 10 wt% solution of PMMA could not be measured because of the limitation of the instrument used.

Figure 4 represents the graph between log shear stress and log shear rate for PS/PMMA solution with varying blending ratio. The concentration of the polymers in the solution was fixed at IS wt%. Shear stress increases with shear rate for all the studied blend solutions . Shear stress also increased when concentration of PS increases in PS/PMMA solution. Similar results were observed when the concentration of both the polymers were fixed at 20 wt% as shown in Fig. 5.

The effect of blending ratio on the viscosity at three different shear rates is presented in Fig. 6 for PS/PMMA blend solution when concentration of the polymers in solution was fixed at 20 wt%. Viscosity

(Y\) decreases with increasing concentration of PMMA in the solution. The viscosity shows sudden decrease when 10 wt% PS is replaced by PMMA in the solution . At 20 wt% replacement of PS by PMMA any significant change was not observed however, after every increasing concentration of PMMA in PS , showed a smooth transition till the concentration of PS/PMMA was 20/80. Again, the solution viscosity remained constant up to 10/90 wt% concentration of PS/PMMA solution. Viscosity was again suddenly decreased for PMMA solution.

Figure 6 also compares the experimental values of vi scosity of PS/PMMA blend solution with theoretical values which are based on the rule of the log of

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Indian J. Chern. Techno!. , July 2002

additivity of individual polymers as shown below:

log Y\Blend = log y\ps + log Y\PMMA . .. (2)

where, Y\Blend is the viscosity of the blend system, y\ps is the viscosity of the PS solution and Y\PMMA is the viscosity of the PMMA solution.

The PS/PMMA blend solution belongs to negative deviation blends as can be seen in Fig. 6. This negative deviation can be correlated with viscosity and compatibility of ternary polymer-polymer-solvent systems. The repulsion-interaction may cause shrinkage of random coils of polymer molecules resulting in reduction in viscosity of the blends, when compared with the values calculated from the viscosities of pure components applying the rule of additivity. On the other hand, in the compatible blends, increase in viscosity due to attraction­interaction is expected. It appears that at low fraction of PMMA in PS/PMMA blend, the compatibility is higher as compared to the high fraction of PMMA. The morphology of the blends was studied to confirm the rheological results.

SEM Studies Figure 7(a) shows a cryogenic fractured surface of

PS/PMMA (20/80) blend. Small spherical balls are uniformly distributed in PMMA matrix. Cavities are also observed which may be formed due to the cleavage of PS balls from the PMMA matrix . An addition of PS in the blend i.e. PS/PMMA (30/70) shows the increased number of PS balls in the matrix as shown in Figure 7(b). In addition, the size of the balls also increased with the increased concentration of PS in PMMA. Figure 7(c) shows the dominance of PS phase in the form of balls distributed in the PMMA matrix. In this case, both PS and PMMA are in equal proportion by weight. Size of the balls is significantly higher as compared to 20 and 30 wt% PS in PMMA. The size of balls range from 20 to 40 microns. Figure 7(d) shows a fractured micrograph of PS/PMMA (60/40). This micrograph shows a distinct feature as compared to the micrographs discussed earlier. Inspite of higher percentage of PS in this sample less PS balls are seen in this micrograph. It seems that at this composition PS and PMMA are mixed partially and formed PS rich phase and PMMA rich phase in the blend. The PS rich phase can be still distinguished in the form of balls distributed in PMMA rich phase. This idea is supported by the fact that these blends were prepared by solution method.

Vashishtha et al.: Morphology and solution rheology of PSfPMMA blends Articles

3.5

3

2.5 co a.. p

~ .----~ Ol 2 g I/)

/~ I/) Q) "-

en 1.5 PS "-co

~~ Q) .c CI)

-+-40% -0-35% -6-30% -e-25%

0.5 ---20% -x-15% ~10%

0 1.5 2 2.5 3 3.5

Shear Rate (log y), S·l

Fig. I-Variation of shear stress with shear rate for different PS concentration at room temperature

3.5

3

2.5 co a.. ,.... I-'

Ol 2 .Q -I/) I/) Q) "--1.5 CI)

"- PMMA co Q) .c -0-40% CI)

-0-35% -6-30%

0.5 ~25%

---20% -x-15%

0+-----------.-----------~------------r_----------._----------_4

1.5 2 2.5 3 3.5

Shear Rate (log y), S·l

Fig. 2-Plot between shear rate and shear stress for different PMMA concentration at room temperature

319

Articles Indian J. Chern. Technol., July 2002

2.-------------------------------------------------------------~ PS

--40% -0-35% -tr-30%

0- --25% 0

~ ---20% Ii! -0-15% co

-+-10% ~ A 6 6 6 6 I=" ~ OJ )4 I( II I( .Q 0 ~ . iii

8 :I • • • <IJ • II: 5

-1

-2+--------------------.--------------------.-------------------~ 2 3 4

Shear Rate, log y (S·1)

Fig. 3(a)-Effect of shear rate on the sol ution viscosity of different PS concentration

2 - ._--_ .... .. ~ .. ".-- - --- --_.

PMMA

--<>- 40 wt % -0-35 wt % -tr-30 wt %

0 cr --25wt% - ---<>------<>-II! -0

('II ---20 wt % e:. -o-15wt% ~ Cl ~ 0

~ III 0 A 6 6 6 .6 U )4 M M If III M -*< :>

·1

II • • • • I:

-2 +------2 3 4

Shear rate, log Y(s-1)

Fig. 3(b)-Effect of shear rate on the solution viscosity of different PMMA concentration

320

Vashishtha et at.: Morphology and solution rheology of PS/PMMA blends Articles

(Q a.. -

2 .S~--------------------------------------------------------~ -o-PS (1S%)

-o-SM-1 -.lr-SM-2

2 -tt-SM-3 --SM-4 --SM-S -.-SM-6

~ 1.S .2

-+-SM-8

II) II)

~ en ... C1I Q)

..c:: CI)

--SM-9 -..PMMA (15%)

~SM-7

O.S

O+-----------,-----------~----------_.------------r_--------~ 1 1.5 2 2.5 3

Shear Rate, log y (S·l)

Fig. 4-Plot between shear rate and shear stress for different composition of PS/PMMA ( 15%) blend

2 .5~------------------------------------------------~

2

~ 1.5 .2

-o-PS (20%)

-o-SM-1

iii I/)

~ en ... <0 Q)

..c:: CI)

---A-SM-2 -tt-SM-3

1 --SM-4 --SM-5 -.-SM-6 --SM-7

0.5 -+-SM-8 -+-SM-9 -- PMMA (20%)

O+---------~--------._--------_r--------_r--------~

1.5 2 2.5 3 3.5

Shear Rate , log y (S·l)

Fig. 5-Plot between shear rate and shear stress for different composition of PS/PMMA (20%) blend

3.S

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~ .5~---------------------------------------

-0.6

: ·0.7

e:. ~ -0.8 'iii 8 IJ) :> -0.9

~ :e -1 a (IJ .c f- -1 .1 Shear Rate

-<>-1 .870403905

-1.2 --2.093771781 -e- 2.583538819

Indian J. Chem. Techno!., July 2002

.-------~ -0.5

-0.6

....... IJ)

-0.7 g, ~

-0.8 'co a u IJ)

-0.9 :> S c

-1 ~ .~

a. -1 1 ~

-1 .2

-1 .3 +--+--r----+--I--f---t--+---+-+---+--t---+-+--+-+---+--+---+--+------+ -1 .3 ;g ~ N C') 'f ~ ~ t-- "? m :'j 0 ::E ::E ::E ::E ::E ::E ::E ::E 0 0 !:::!. (/) (/) (/) (/) (/) (/) (/) (/) (/) !:::!. (/)

~ a.. ::E a..

Fig. 6-Comparati ve plots between blend composition with experimental and theoretical viscosity for PS/PMMA blend

322

Vashishtha et al.: Morphology and solution rheology of PSIPMMA blends Articles

Fig. 7-Scanning electron micrographs obtained from cryogenically fractured cross-section surfaces of solution cast samples of PS/PMMA blends: 20/80 (a); 30170 (b); 50/50 (c); 60/40 (d); and 70/30 (e)

PMMA solution has low viscosity as compared to PS solution in toluene. During the process of evaporation of solvent, PS have a tendency to form balls of higher viscosity as compared to PMMA. Therefore, the pOSSibility of the formation of spherical balls, distributed in the matrix shall remain until these ball connected each other to form network due to their increased volume. At that situation PMMA could be seen in the form of balls and on further reduction of PMMA concentration have shown clearly the phase transition which does not occur in the present case as shown in Fig. 7(e). This figure shows a different features as compared to other micrograph in which two phases were always observed distinctly. This micrograph shows the mixed matrix in general with slightly elongated fractured surface. The micrographs

of PS/PMMA (80/20) and (90/10) are not shown here because the two phases are not distinguished. This indicates that PS and PMMA used in this study have a tendency of mixing each other in composition where PS is 60% or above.

Conclusion Power law model was fitted well for shear

dependency of viscosity of solutions of PS/toluene, PMMA/toluene and PS/PMMA/toluene in which weight percent of polymers was varied from 10 to 40. Shear stress increases with the addition of PS in PS/PMMA blend solution. Increase in concentration of PMMA decreased the viscosity of the PS/PMMA solution. PS and PMMA show a tendency of mixing each other in compositions where PS is 60% or above in the blend.

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