Styrene-Polymer Interaction Parameters in High Impact Polystyrene

13 Styrene-Polymer Interaction Parameters in High Impact Polystyrene R.

L.

KRUSE

Downloaded by DICLE UNIV on November 8, 2014 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0142.ch013

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


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


2

3


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


144

COPOLYMERS,

POLYBLENDS,

A N D COMPOSITES


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


13.

KRUSE

145


_ (§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


+

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



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.


13.

KRUSE


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.


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.

Styrene-Polymer Interaction Parameters in High Impact Polystyrene

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