Styrene-Polymer Interaction Parameters in High Impact Polystyrene

Bristow and Watson. (6) reported the value 0.43 for PS in toluene, and Boyer and Spencer (7) ... Scott and Magat (9) reported the value 0.30 for PBD w...
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13 Styrene-Polymer Interaction Parameters in High Impact Polystyrene R.

L.

KRUSE

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Monsanto C o . , Polymers and Petrochemicals D i v . , Indian O r c h a r d , Mass. 01051

The separation of polystyrene and polybutadiene into two phases, both containing styrene, was used to measure polymer­ -solvent interaction parameters. By this technique, the mean interaction parameter, X , for polystyrene in styrene is 0.49 and that for polybutadiene in styrene, X , is 0.29. Phase behavior at higher concentrations was calculated from data obtained at concentrations of less than 35%. As a result, phase behavior during polymerization of high impact polystyrene was interpreted. 12

13

T

he polystyrene/styrene/polybutadiene ( P S / S / P B D ) system occurs i n the production of high impact polystyrene. T h e process for making toughened polystyrene as described b y M o u l a u and Keskkula ( 1 ) starts w i t h a rubber i n styrene solution. A s S is polymerized to P S , phase separation results i n imme­ diate formation of droplets of a P S phase. W i t h further polymerization, the P S phase increases i n volume until phase volumes are equal. A t this point, phase inversion occurs—the dispersed P S phase becomes the continuous phase a n d the P B D phase becomes the disperse droplets. Complete conversion of S to PS yields the commercially important high impact polystyrene. One method of quantifying phase behavior is to mix two polymers i n a common solvent and observe the two l i q u i d phase volumes (2, 3). T h e theo­ retical basis for the incompatibility of polymer solutions was discussed b y Scott (4); however, complete phase relationships are rarely measured. T h e poly(methyl methacrylate) /benzene/rubber system was described b y Bristow ( 5 ) , but even he d i d not calculate solubility parameters from the data. Thus, mea­ surement and data interpretation techniques need to be defined. Experimental Samples of linear P B D (M = 149,000; M /M = 1.46) and P S (M„ = 290,000; M /M = 3.0) were dissolved at six levels of total solids i n S i n jars on a roll m i l l . Styrene polymerization was prevented b y adding 0 . 1 % benzoquinone. T h e two-phase mixtures were separated b y centrifugation 15 m i n at 20,000 r p m i n a Beckman model 21 ultracentrifuge, and the solutions were then frozen i n the metal centrifuge tubes. F r o z e n plugs were removed as needed b y warming the outside of the tube. T h e y were then sectioned at the interface, warmed to 25 °C, and the solids level i n each phase was measured b y methanol w

w

w

n

n

141

In Copolymers, Polyblends, and Composites; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

142

COPOLYMERS,

Table I.

A N D COMPOSITES

Phase D a t a for the Styrene/Polystyrene/Polybutadiene System

Polystyrene in Its Phase, wt %

Polybutadiene in Its Phase, wt %

9 15 19 26 29 35

4 8 11 17 20 26

precipitation. I R measurements on the solids from the separated phases i n d i ­ cated an essentially complete separation of the two polymeric components. T h e data are tabulated i n Table I. Duplicate samples agreed within 0 . 5 % . Monodisperse P S (M — 160,000; M /M = 1 . 0 6 ) and a narrow distribu­ tion P B D (M = 78,500; M /M = 1.23) were compared with their broad distribution equivalents at 1 0 % P S and 6 % P B D i n S. Phase volumes were the same, which indicates that polymer molecular weight distribution is not a significant variable. Solutions of P B D i n S and PS i n S, mixed and allowed to separate b y gravity, gave the same results. Measurements at 0 ° - 6 0 ° C also d i d not alter the phase relationships at concentrations over 4 % . Phase separation of a 50/50 mixture of the two polymers at 25 °C occurred at a solid concentration of 2 - 3 % , but the solutions d i d not separate into equal volumes. Dilute solutions separate into equal volume phases if the P S concentration is approximately five times that of the P B D at total solids levels of 3 - 4 % . w

w

w

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POLYBLENDS,

w

n

n

Theoretical T h e change i n chemical potential ( 4 ) , Αμ of S i n the S / P S / P B D system where the components are subscripted 1, 2, and 3, respectively, is: ΐ7

^

= loge (1 - V2 ~ V ) + ( l - ^

V + (l -

Z

V + (%12 t>2 + Xl3 V ) (v + V )

2

Z

t

2

Z

(1)

+ ψν ν 2

ζ

w here v is the fraction of component i, x . is its degree of polymerization, and χ is its interaction parameter w i t h a second component. T h e two h i g h molecu­ lar weight polymers i n S are segregated into separate phases—PS i n S and P B D i n S. T h e chemical potential, Δ/^/ΚΓ for S w i t h dissolved P S is: T

i

{

ΐ ;

A/Lti = loge (1 - v ) + ( l - 1) RT 2

where v is the volume fraction P S and χ The relation for S w i t h dissolved P B D is:

1 2

2

v* + χ v (vz « 0) 1 2

(2)

2

2

is the S - P S interaction parameter.

Αμ. τ = log (1 - v ) + ( l - ^ ) Vz + X13 vz (v « 0) RT e

2

t

(3)

2

where v is the fraction P B D a n d χ is the S - P B D interaction parameter. A t equilibrium, the chemical potentials of S i n the two phases are equal; therefore: 1 3

3

loge 7^

^4 = v - vz + χΐ2 v 2

2

2

— 7.13 v (x2 and xz large)

(4)

2

z

Since v vs. v are measured, the two unknown constants, χ a n d χ can be evaluated b y plots of [v — v + l o g (1 — u ) — l o g (1 — v )]/v vs. (v /vs) Slope, χ ; intercept, - χ . 2

1 2

s

3

2

2

1 2

e

2

3

e

1 3

2

1 3

In Copolymers, Polyblends, and Composites; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

2

3

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13.

KRUSE

143

High Impact Polystyrene

Polystyrene

Polybutadiene

Figure 1.

Triangular phase diagram for the system styrene I polystyrene I polybutadiene , measured tie line;

, calculated tie line

The ratio (1 — υ ) / ( 1 — v ) is a partition coefficient that measures dis­ tribution of the solvent between the polymer phases. T h e partition coefficient is the fraction of S i n the P B D phase divided b y the fraction of S i n the P S phase. F o r concentrated solutions, v and v approach one, and this ratio is a constant given b y 3

2

2

1

1

~



V z

3

e

xi2



xi3

(5)

— V

2

The ratio is the distribution of residual styrene between the two phases. Results D a t a for the P S / S / P B D system i n Figure 1 are replotted i n Figure 2 according to E q u a t i o n 4. T h e 0.49 slope b y least squares is the interaction parameter of P S w i t h S, χ . T h e negative 0.29 intercept b y least squares is the interaction parameter for P B D w i t h S, χ . T h e value 0.29 indicates that S is a better solvent for P B D than for P S . Least squares analysis indicated that the values of χ are precise to ± 0 . 0 1 unit (2 σ ) . Use of volume fractions instead of weight fractions does not change χ , but χ increases from 0.29 to 0.35. Published values for the interaction parameters vary. Bristow and Watson (6) reported the value 0.43 for P S i n toluene, and Boyer a n d Spencer (7) gave the value 0.424 for P S i n S. H i l d et al (8) obtained the value 0.45 + 0.9 v for model crosslinked P S networks i n benzene. O u r mean value for P S i n S, 0.49, is i n this range; our method, however, does not require an estimate of crosslink density. Scott a n d Magat (9) reported the value 0.30 for P B D w i t h toluene, w h i c h is close to our mean value for P B D w i t h S (0.29). 12

1 3

1 2

1 3

2

In Copolymers, Polyblends, and Composites; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

144

COPOLYMERS,

POLYBLENDS,

A N D COMPOSITES

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2.5 r

Figure 2. Calculation of solubility parameters from phase equilibnum data for the system styrene(l)Ipolystyrene(2)/polybutadiene(3) x

u

0.49, xis =

=

0.29; polymer concentrations in their respective phases are reported as (\'i,vs)

Phase relationships at polymer concentrations above 4 0 % are difficult to measure. However, the concentrations of the phases can be calculated numeri­ cally using E q u a t i o n 4, and they are used to position the additional tie lines (dashed) i n Figure 1. Values of the interaction parameters can be used to calculate the partition coefficient of residual styrene i n the final polymer blend. T h e partition coeffi­ cient approaches 1.22 at complete conversion (from Equation 5 ) . The approach to the limiting value w i t h polymer concentration is illustrated i n Figure 3. Some assumptions i n the theoretical development w i l l now be examined. Ignoring the contribution of the P S - P B D interaction parameter (X23A2 X32A3) t ° numerical values of the ordinate i n Figure 2 introduces an error of less than 1 % . T h e magnitude of the interaction parameter, 0.02, was esti­ mated from the concentration of polymer (v + v ) at the point of initial phase separation using the procedure outlined b y Scott ( 4 ) , that is o

t

n

r

e

2

X2

2

(v

2

3

c

+ v) t

e

w

A similar value is calculated b y using the solubility parameters of P S (δο = 8.75) and P B D ( 8 = 8.4) and the relation 3

In Copolymers, Polyblends, and Composites; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

13.

KRUSE

145

High Impact Polystyrene

_ (§2 ~ S ) V x RT ' where V is the molar volume of the repeat unit. A l l e n et al. (10) obtained the value 0.01 b y measuring the phase compositions of high molecular polymers i n carbon tetrachloride. Paxton's data (11) analyzed b y Scott's method give the value 0.1-0.2 for the same two polymers i n toluene, benzene, a n d carbon tetrachloride; however, his P B D s h a d a degree of polymerization of only 20. W i t h the value 0.02, correction terms for the ordinate values are less than 1 % at the concentrations measured. W e assumed complete separation of the two polymers into their separate phases. Scott's dilution approximation for estimat­ ing the P S i n the P B D phase v is X23

2

3

m

K

2

2

|,I 2

=

t>

2

e" 23 2 X

iV

V

(8)

F o r our experiments, v + t; > 0.13 and χ is about 60. Therefore, the calcu­ lated fraction of P S i n the P B D phase, v '/v , is less than 1 % of the total polymer i n that phase. O u r measurements could not detect this small amount. 3

2

2 3

2

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+

H

Discussion Calculation of polymer concentrations at critical points i n the process of polymerizing styrene i n the presence of dissolved P B D is possible with use of V - FRACTION POLYBUTADIENE IN ITS 3

Γ

1.25 h

.2 1

r

1.0 .6 1—ι—ι—ι—ι—Γ—

1.20 h

y

i-v ϊ

7

3

'

1.15 h

^

PHASE

y

y

y '

\

EXTRAPOLATION X12 = 0 . 4 9

1.10

X

' 1.05

1.00 ί 2

Figure 3.

=0.29

-

"/

/

V -

| 3

J

/

.

J .2

ι

ι1

1

1

1—1

.6

FRACTION P O L Y S T Y R E N E IN ITS

1

1.0

PHASE

Partition coefficients for styrene(l) between the polysty rene(2) and the polybutadiene(3) phases

In Copolymers, Polyblends, and Composites; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

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146

COPOLYMERS,

Polystyrene

Figure 4.

POLYBLENDS,

A N D COMPOSITES

Polybutadiene

Phase concentrations during polymerization of styrene with 10% polybutadiene

The concentrations of styrene-π, poly sty rene-xt, and polybutadiene-vs are represented by (xi,Xi,xs)

the phase diagram. F o r example, the path of polymerization of a 1 0 % P B D solution i n styrene is illustrated i n Figure 4. T h e separate P S phase is formed immediately, a n d concentration of P S i n the initial droplets is 1 9 % . T h e volume of dispersed P S phases increases w i t h conversion of S to P S until phase volumes are equal at 1 3 % P S . A t this point, a tie line is bisected a n d the concentrations of P B D and P S i n their respective phases are 1 9 % a n d 2 8 % . A t higher conversions, the P B D phase is disperse. Although partitioning of the monomers at the higher conversions is difficult to measure experimentally, it can be calculated. F o r example, the concentrations of P B D and P S i n their respective phases are 6 1 % and 6 6 % at 5 5 % P S i n the mixture and 8 3 % a n d 8 6 % at 7 5 % P S . Thus, the polymer concentrations i n their phases equalize at higher conversions. T h e partitioning of S, however, increases w i t h conver­ sion; near complete conversion, 1 2 % residual S remains w i t h the 1 0 % P B D . A l l the above calculations have assumed χ values independent of concen­ tration, grafting, and crosslinking. I n high impact polystyrene, a l l three factors can be important. Acknowledgment The interest and encouragement of Q . A . Trementozzi during this research is sincerely appreciated. Literature Cited 1. Moulau, G . E., Keskkula, H., J. Polym. Sci. Part A-1 (1966) 1595. 2. Dobry, Α., Boyer-Kawenoki, F., J. Polym. Sci. (1947) 2, 90.

In Copolymers, Polyblends, and Composites; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

13.

KRUSE

High Impact Polystyrene

3. 4. 5. 6. 7. 8.

147

Kern, R. J., Slocombe, R. J., J. Polym. Sci. (1955) 15, 183. Scott, R. L . , J. Chem. Phys. (1949) 17, 279. Bristow, G . M., J. Appl. Polym. Sci. (1959) 2 (4), 120. Bristow, G . M., Watson, W . F., Trans. Faraday Soc. (1958) 54, 1742. Boyer, R. F., Spencer, R. S., J. Polym. Sci. (1948) 3, 97. Hild, G . , Haeringer, Α., Rempp, P., Benoit, H . , Amer. Chem. Soc., Div. Polym. Chem., Prepr. 14 (1), 352 (Detroit, May, 1973). 9. Scott, R. L., Magat, M . , J. Polym. Sci. (1949) 4, 555. 10. Allen, G . , Gee, G . , Nicholson, J. P., Polymer (1960) 1, 56. 11. Paxton, T. R., J. Appl. Polym. Sci. (1963) 7, 1499.

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R E C E I V E D February 20, 1974.

American Chemical Society Library 1155 16* St. N. W. In Copolymers, Polyblends, and Platzer, N.; WeMaetoii, 0.Composites; £ 20036

Advances in Chemistry; American Chemical Society: Washington, DC, 1975.