Phase Equilibria and Fluid Properties in the Chemical Industry

6 Polymer Equilibria

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 10, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0060.ch006

A. BONDI Shell Development Co., Houston, Tex. 77001

The purpose of the present paper i s to provide a g e n e r a l i z e d view of those phase e q u i l i b r i u m c h a r a c t e r i s t i c s of polymers o r polymer dominated systems which a r e unique to the "polymer w o r l d " . This uniqueness may flow from the h i g h molecular weight of t e c h n i c a l l y important polymers o r from the a s s o c i a t e d high v i s c o s i t y o r from the g l a s s y c o n d i t i o n s which c h a r a c t e r i z e the u s e f u l s o l i d s t a t e o f many members of that c l a s s . Another purpose o f t h i s paper i s to assess the t e c h n i c a l and the economic s i g n i f i c a n c e of ignorance i n t h i s area o f r e s e a r c h , because the purpose o f t h i s conference i s served more by e x h i b i t i n g what we do not know than by the proud d i s p l a y o f a seemingly impregnable, coherent body o f v a l i d a t e d t h e o r e t i c a l understanding. The scope of t h i s p r e s e n t a t i o n i s a sweeping survey w i t h a few in-depth excursions i n t o v a l l e y s of s i g n i f i c a n t ignorance. General P r i n c i p l e s The dominant problem of phase e q u i l i b r i a i n v o l v i n g a polymer phase i s the measurement problem: When has (or can) e q u i l i b r i u m be c a l l e d " e s t a b l i s h e d " . The usual means f o r r a p i d e q u i l i b r a t i o n , f o r c e d c o n v e c t i o n , i s not only d i f f i c u l t t o implement, i t may even be i m p o s s i b l e to do, when the r e q u i r e d energy i n p u t would a c t u a l l y break chemical bonds along the polymer molecule's backbone chain. Another problem i s that o f data c o r r e l a t i o n , o r o f data genera l i z a t i o n by means of w e l l founded theory. A t the rough approximat i o n l e v e l we a r e (thanks t o the work by F l o r y , Huggins, P r i g o g i n e , P r a u s n i t z , and o t h e r s ) i n good shape, and h e r e t o f o r e that has been q u i t e adequate. But we s h a l l see that recent s t u d i e s o f concent r a t e d systems e x h i b i t v a p o r / l i q u i d e q u i l i b r i a which are not even described q u a l i t a t i v e l y by e x i s t i n g theory, assuming that the measurements are r e l i a b l e . In t h i s d i s c u s s i o n we s h a l l take f o r granted that the reader i s f a m i l i a r w i t h the body of theory, according to which polymers 118

Storvick and Sandler; Phase Equilibria and Fluid Properties in the Chemical Industry ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


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Polymer Equilibria

119

are c h a r a c t e r i z e d by t h e i r molecular weight, t h e i r molecular weight d i s t r i b u t i o n , t h e i r cohesive energy d e n s i t y (or other measure of p a i r p o t e n t i a l between unbonded neighbors) and the f l e x i b i l i t y of c h a i n s , measured e m p i r i c a l l y or c h a r a c t e r i z e d by barr i e r s to i n t e r n a l r o t a t i o n and energies of r o t a t i o n a l i s o m e r i z a t i o n of main c h a i n bonds or bond groupings, the c r o s s l i n k d e n s i t y , i f any, and e l e c t r o s t a t i c charge d e n s i t y , e s p e c i a l l y i n the case of p o l y e l e c t r o l y t e s . The l a t t e r w i l l not be d e a l t w i t h i n t h i s survey. S i n g l e Component, Two Phase Systems S o l i d / S o l i d E q u i l i b r i a i n C r y s t a l l i n e Polymers Polymorphism i s f a r l e s s common among c r y s t a l l i n e polymers than among c r y s t a l s composed of s m a l l molecules. The reason f o r t h i s p a u c i t y i s , of course, the c o n s t r a i n t imposed by t h e i r onedimensional i n f i n i t y upon r o t a t i o n a l d i s o r d e r , the primary cause of polymorphism among organic s o l i d s . The few known cases have been assembled on Table 1. The s i t u a t i o n i s q u i t e d i f f e r e n t , i f we admit a s s o c i a t i o n polymers as l e g i t i m a t e polymers. An outstanding example i s the c l a s s of the a l k a l i and e a r t h a l k a l i metals soaps of long chain f a t t y a c i d s . T h e i r polymorphic behavior i s w e l l e s t a b l i s h e d ( 1 ) , and i s a s c r i b e d to the i n c r e a s i n g m o b i l i t y of the alkane groups w i t h i n c r e a s i n g temperature. T y p i c a l examples are shown on Table 2. Polymeric c r y s t a l l i n e long c h a i n e s t e r s of v i n y l compounds are, of course, the C-C backbone chain e q u i v a l e n t of those association-polymers . S o l i d / L i q u i d E q u i l i b r i a . Few polymer e q u i l i b r i a have been s t u d i e d as thoroughly as those a t the m e l t i n g p o i n t of c r y s t a l l i n e polymers. In l a r g e measure t h i s i n t e r e s t may be caused by the d e s i r e to understand the p e c u l i a r c h a i n f o l d i n g p r o p e n s i t y of many c r y s t a l l i n e polymers. The c u r r e n t l y accepted theory (2) a s s o c i a t e s chain f o l d i n g w i t h an entropy phenomenon, such that chain f o l d i n g should disappear w i t h i n c r e a s i n g temperature; and, indeed, i f one r a i s e s the m e l t i n g temperature high enough by conducting the s o l i d i f i c a t i o n from the melt at high enough h y d r o s t a t i c pressure, chain f o l d i n g can be avoided, and completely s t r a i g h t chain c r y s t a l s are formed without chain f o l d s ( 3 ) . Thus the l i q u i d / s o l i d , pressure/ temperature phase diagram of polymeric c r y s t a l l i n e s o l i d s , i l l u s t r a t e d on F i g u r e 1, i s not q u i t e comparable w i t h those of s i m p l e r compounds, because the two ends of each curve represent m a t e r i a l s i n two q u i t e d i f f e r e n t c r y s t a l morphologies. As t h i s change i s gradual, and r e l a t e d to the molecular weight d i s t r i b u t i o n , the T vs. pressure curves on F i g u r e 1 are without d i s c o n t i n u i t y . A unique aspect of h i g h polymer s o l i d / l i q u i d e q u i l i b r i a i s the o r i e n t a t i o n - i n d u c e d c r y s t a l l i z a t i o n a t temperatures above T (4). The b e s t known i n s t a n c e s of c r y s t a l l i z a t i o n of T > T under m

m

m


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EQUILIBRIA A N D FLUID PROPERTIES IN C H E M I C A L INDUSTRY

TABLE 1 EXAMPLES OF POLYMORPHISM AMONG CRYSTALLINE POLYMERS

Crystal Morphology

poly-l-butene

rho ter ortho

o t , c m 135/6 122/4 106

poly-l-pentene

mono port

130 80

poly-4-methylpentene-l

tet 2 2

235/50 125 75

poly 1,3 butadiene, trans I

phex mono phex hex

100 96 141 148

Polymer Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 10, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0060.ch006

3

II

mono ortho

23 14

tri mono

67/73 62/77

v i n y l c y c l o pentene

tri tet

292 270

poly-p-xylylene, a

mono ortho

375 420

poly a r c y l o n i t r i l e , synd.

hex ortho

317 341

poly v i n y l c h l o r i d e

ortho mono

273 310

poly v i n y l f l u o r i d e

hex ortho

200 230

p o l y 8-amino c a p r y l i c a c i d , a

mono phex

185 220

poly c i s isoprene poly-cyclo-octene,

trans

'From Polymer Handbook, Brandrup, J . and Immergut, E. H., John Wiley, New York, 1975.


6. BONDI

Polymer Equilibria

121

TABLE 2 POLYMORPHISM OF ASSOCIATION POLYMERS EXAMPLE:

THE ALKALI SALTS OF LONG CHAIN FATTY ACIDS*


S o l i d / S o l i d T r a n s i t i o n Temperatures and F i n a l M e l t i n g P o i n t s , °C Carbon Number o f F a t t y A c i d : Cation

C^

C

7

C

8

C

9

c

io

L i m.p.

Na m.p.

K m.p.

Rb m.p.

C

s

m.p.

a)

C

12

C

14

C

16

C

18

237

233

223

226

227

208

215

215

197

191

361

363

360

355

348

329

311

302

283

235

242

243

243

245

246

246

251

255

-

-

-

-

218

218

215

212

208

210

198

189

185

181

179

171

168

165

140

141

138

136

135

113

117

116

>400

>400

>400

>400

>400

395

375

362

348

305

301

291

282

277

273

271

269

267

-

195

170

^400

380

375

358

300

291

284

281

385

370

358

345

295

290

279

273

Demus, D.; and Sackmann, H. From Baum, E.; 37. H a l l e (1970), 19, (5)

Wiss . Z. Univ.


122

P H A S E EQUILIBRIA

A N D FLUID PROPERTIES I N C H E M I C A L INDUSTRY

250

200 -


o o Journal of Macromolecular Science

Figure 1. Phase diagrams of crystalline polymers (6) Figure 1A. Phase diagrams of polyethylene. Melting (solid points) and crystallization (open points) temperatures of: (• Oj fee, (A A) ecc, (| Q) unknown structure

150 -

0 2 4 6 PRESSURE (xl0 kg/cm ) 3

2

(A)

Figure IB. Pressure dependence of the melting temperature, T , of polyethylene-chain crystals ( ) and foldedchain crystals ( ) m



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Polymer Equilibria

400 T

C

(D) Figure ID. Pressure dependence of the melting temperature, T , of Penton for samples (a) crystallized at atmospheric pressure, and (b) crystallized at the measurement pressure m

c o n d i t i o n s of t e n s i l e e l o n g a t i o n are those of n a t u r a l rubber and, o c c a s i o n a l l y , of melt s p i n n i n g . S i m i l a r l y , o r i e n t a t i o n - i n d u c e d c r y s t a l l i z a t i o n has been observed i n melt flow, such as i n i n j e c t i o n molding, a t high shear r a t e s . A very high degree of s t e r e o r e g u l a r i t y or " t a c t i c i t y " i s necessary, so that l o n g , u n i n t e r rupted p a r a l l e l a l i g n e d s t r e t c h e s of molecule chains can form s t a b l e c r y s t a l c r y s t a l n u c l e i that w i l l grow r a p i d l y i n t o macroscopic c r y s t a l s . A t h e o r e t i c a l treatment of s t r a i n induced c r y s t a l l i z a t i o n w i t h q u a n t i t a t i v e p r e d i c t i v e power i s s t i l l i n i t s infancy. The r e l a t i o n of T or b e t t e r of AS and AH (and AV ) to molecular s t r u c t u r e of c r y s t a l l i n e polymers has been thoroughly discussed elsewhere (5) and i t s d i s p l a y would go beyond the scope of t h i s review. S u f f i c e i f to say here, that T a t atmospheric pressure i s measured so e a s i l y that i t s e s t i m a t i o n by p r e d i c t i v e methods seems an uneconomical t h i n g to do. The e s t i m a t i o n of the slope of the T vs. P l i n e , (6) the s o - c a l l e d m e l t i n g curve of c r y s t a l l i n e polymers seems no more d i f f i c u l t than of other substances. However, given the high v i s c o s i t y of the polymer melt, e s p e c i a l l y a t high pressures, n u c l e a t i o n may be so r e t a r d e d , to make experimental m e l t i n g p o i n t determination q u i t e u n c e r t a i n . m

m

m

m

m

m

The Glass/Rubbery State " E q u i l i b r i u m " Glassy polymers are of such t e c h n i c a l (and economic) importance that i t would seem pedantic to ignore the very obvious p h y s i c a l change at the g l a s s t r a n s i t i o n temperature. Morphologically, both phases are l i q u i d l i k e , i . e . h i g h l y disordered on an atomic s c a l e . Furthermore the dramatic change i n v i s c o s i t y at the g l a s s t r a n s i t i o n temperature (Tg) i s not much more gradual than i t i s a t


124

PHASE

EQUILIBRIA

A N D FLUID

PROPERTIES

IN CHEMICAL

INDUSTRY

T . However, i t s strong dependence upon the c o o l i n g r a t e (Figure 2) c l e a r l y d i f f e r e n t i a t e s T from T . The curve of Tg v s . P (Figure 3) has q u a l i t a t i v e l y the same appearance as the m e l t i n g curve, but the slope i s w e l l a p p r o x i mated, but not p r e c i s e l y d e s c r i b e d as that of a second order t r a n s i t i o n depending on the d i f f e r e n c e s i n the slopes of V v s . T and V vs. P between rubbery and g l a s s y s t a t e . Hence F i g u r e 3 does not q u a l i f y as a genuine phase e q u i l i b r i u m , even i f i t looks and a c t s l i k e one. m


g

m

The Rubbery S t a t e / L i q u i d E q u i l i b r i u m There i s now i n c r e a s i n g evidence f o r the r e a l i t y of the b l i p s i n the d i f f e r e n t i a l thermal a n a l y s i s a t T > Tg, a t a temperature i d e n t i f i e d as T ^ i o r l i q u i d / l i q u i d t r a n s i t i o n by Boyer ( 7 ) . The p h y s i c s o f t h i s phenomenon have y e t to be s t u d i e d both phenomonol o g i c a l l y as w e l l as i n terms of molecular motions. According t o some conjectures ( 8 ) T ; Q i s the temperature (range) c h a r a c t e r i z i n g the change from entanglement of neighboring chains to a s t a t e o f f r e e r i n t e r m o l e c u l a r movement. The p a r a l l e l change i n T ^ and i n v i s c o s i t y w i t h moelcular weight (shown on F i g u r e 4 ) i s the source of t h i s c o n j e c t u r e . E x i s t e n c e of a r e l a t i o n between the v i s c o s i t y

V

-10

10

0 T - Tg °

20

°C

Figure 2. Effect of cooling rate f on the observed glass-transition temperature and specific volume according to the theory of Saito et al. The rate 10 C/sec was chosen for the standard for which T = T ° and V = 1.000; dV /dT was assumed independent of cooling rate. (A. Bondi, "Physical Properties of Molecular Crystals, Liquids and Glasses" Wiley, New York, N.Y., 1968) s

g

g

g


g

6.

BONDI

125

Polymer Equilibria

T (°C) g


260r

lOOl

1

"

1

1

1

0

2

4

6

8

10

P (Kb) g

Figure 3. Vitrification Phase diagram of polystyrene. Note that vitrification takes place over a pressure range. 4

I

0

Ht

I

I

40 Nt

I

I

l

80 Ar T

b

i

t

120 Kr

!

I

I

1

160 0 40 Xe H t Nc

,in°K

i — I I

80 Ar

I

1

120 Kr

I

160 Xc

T ,in°K b

Figure 4. Logarithm of the solubility coefficients of rare gases in various homopolymers vs. the boiling point, T , of the gas. [Lundstrom, J. E., Bearman, R. J., J. Polym. Sci., Polym. Phys. (1974) 12, 97] PVA = polyvinyl acetate), SR = silicone rubber, SRDC = silicone rubber, SRGE = 5 % phenyl silicone rubber, TREGEM = poly(tetraethyleneglycol dimethacrylate), PE = polyethylene, NRUA = natural rubber. b


126

P H A S E EQUILIBRIA AND

F L U I D PROPERTIES IN

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INDUSTRY

of polymer melts and c h a i n entanglement i s f a i r l y w i d e l y accepted


Two Component, Two-Phase System Melt/Gas The p r e d i c t a b i l i t y of the s o l u b i l i t y of gases and vapors i n polymer m e l t i n g i s s t i l l j u s t as e l u s i v e as i t i s f o r simple l i q u i d s . Some hoary g e n e r a l i z a t i o n s about the r e l a t i v e e f f e c t s of the m o l e c u l a r f o r c e constants (e and r ) of gases or of t h e i r c r i t i c a l c o n s t a n t s , and the s o l u b i l i t y parameter of the polymer melt can be used f o r i n t e r p o l a t i o n purposes i n a narrow group of systems. But a good theory, or even a r e l i a b l e engineering c o r r e l a t i o n f o r a wide range of such systems have y e t to be developed. The s o l u b i l i t y of polymer melts i n h i g h l y compressed gases (at T > T and P » P ) i s of p o t e n t i a l i n t e r e s t as a f r a c t i o n a t i o n medium f o r polymers by m o l e c u l a r weight, because under some c o n d i t i o n s pressure i s a more convenient v a r i a b l e than s o l v e n t composition, and a l e s s d e s t r u c t i v e v a r i a b l e than temperature. However so f a r only e x p l o r a t o r y experiments have been p u b l i s h e d i n t h i s f i e l d (10). Q

c

c

Polymer Melt/Monomer L i q u i d ( s ) The s o l u b i l i t y of polymer melts i n s i n g l e component and i n b i n a r y monomeric l i q u i d mixtures i s so w i d e l y known and f r e q u e n t l y reviewed both w i t h r e s p e c t to p r a c t i c a l a p p l i c a t i o n s , e s p e c i a l l y f o r polymer f r a c t i o n a t i o n , and w i t h r e s p e c t to theory that l i t t l e need be s a i d here. The p r o p e r t i e s of monomer l i q u i d s and of p o l y mer melt which determine t h e i r mutual m i s c i b i l i t y have been assemb l e d on Table 3 and are seen to be q u i t e s i m i l a r to those which determine the i n t e r a c t i o n between gases and polymer melts on F i g ure 4. Four cases are of s p e c i a l i n t e r e s t , the p r e c i p i t a t i o n of a d i s s o l v e d polymer melt by a d d i t i o n of a non-solvent, the separat i o n of c h e m i c a l l y d i f f e r e n t polymers by mixtures of s e v e r a l s o l vents (below t h e i r CST), (11) the d i s s o l u t i o n of a polymer melt by mixture of two non-solvents, and the s w e l l i n g of a c r o s s l i n k e d polymer melt by monomeric s o l v e n t s . The i n c i d e n c e of the f i r s t three cases i s described i n F i g u r e s 5 and 6, r e s p e c t i v e l y , f o r nons p e c i f i c i n t e r a c t i o n s between polymer and s o l v e n t ( s ) . Specific i n t e r a c t i o n s a r e q u a l i t a t i v e l y j u s t as p r e d i c t a b l e as i n the case of s p e c i f i c i n t e r a c t i o n s between monomeric l i q u i d s , say m o l e c u l a r compound formation or acid/base i n t e r a c t i o n . But q u a n t i t a t i v e pred i c t i o n s are even more d i f f i c u l t here than i n the monomer case. Although polymers are commonly mixtures w i t h a molecular weight d i s t r i b u t i o n of f i n i t e w i d t h we t r e a t them as s i n g l e components, or b e t t e r as q u a s i - s i n g l e components. I f we do so, a l l the consequences of the phase r u l e are found to be v a l i d . Moreover,



± .03

Impossible

Too few d a t a so f a r on g l a s / g l a s s phase b o u n d a r i e s

P r e d i c t i o n from Experimental Data

P r e d i c t i o n from E s t i m a t e d Data

F of F u n c t i o n a l Group C o m b i n a t i o n

2

6^;6

Estimated

Data

N o t h i n g , most prop e r t i e s cannot be measured i n t h e glassy state.

2

A v a i l a b l e from Experiment

x

0

g

2

= f(T)±.2

6

S i n g l e phase properties only very approximately.

Phase B o u n d a r i e s No

2

.03

= f(T)

1' log n

n

g

T (1);T (2)

T ( l ) , T (2),

6 (1),6 ( 2 ) , M 6 ;6 D/A c h a r a c t e r i s t i c s

Data

Required

Time and P r o x i m i t y of Tg

Time and P r o x i m i t y o f Tg

Constraints

Glass/Glass Blends

The p r o b l e m i s too new f o r m e a n i n g f u l f o r m u l a t i o n of i t s specific character.

S i z e of I n d i v i d u a l Blocks; P r o x i m i t y to Block: Tg

Phase Diagrams and P r o p e r t i e s o f M i x t u r e s o f Gas, Glass P l a s t i c i z e r Solvents with Block Systems Copolymers

For d i l u t e s o l u t i o n s o f non-polar p o l y m e r s - f a i r . For h i g h l y p o l a r i n t e r a c t i o n s - i r r e v e r s i b l e . For concentrated s o l u t i o n a b s o r p t i o n i s o f t e n domin a t e d by dynamic phenomena. P r e d i c t i o n s not achievable now.

Polymer C o n c e n t r a t i o n Geometry o f A d s o r p t i o n S u r f a c e ; Time

Adsorption Equilibria

TABLE 3. PREDICTABILITY OF TWO VERSUS MULTICOMPONENT - TWO PHASE SYSTEMS



A N D FLUID PROPERTIES

IN C H E M I C A L INDUSTRY


128


6. BONDI

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the i n c i d e n c e and shape o f m i s c i b i l i t y gaps i s p r e d i c t e d q u a l i t a t i v e l y c o r r e c t l y by molecular theory. But no e x i s t i n g theory can p r e d i c t q u a n t i t a t i v e l y the i n t e r e s t i n g m i s c i b i l i t y gap geometries shown on Figures 6 through 8 (12). The s w e l l i n g o f c r o s s l i n k e d polymer melts i s j u s t a l i m i t e d d i s s o l u t i o n . The only d i f f e r e n c e i s t h a t the e f f e c t i v e molecularweight f o r i n t e r a c t i o n w i t h the s o l v e n t i s (M ) t h a t of the chains between c r o s s l i n k s . Hence m i n i m i z a t i o n of s w e l l i n g i s achieved by the choice of polymer and s o l v e n t chemistry that minimizes mutual compatibility. There i s i n c r e a s i n g evidence that the a c t i v i t y of the s o l v e n t such as p l a s t i c i s e r s i n h i g h l y concentrated systems i s not adequately represented by e x i s t i n g f o r m u l a t i o n s , such as the F l o r y Huggins r e l a t i o n s and t h e i r v a r i o u s higher approximations (13). In view o f the i n c r e a s i n g l y c o n v i n c i n g evidence f o r the aggregation of polymer molecules i n t o "superaggregates" i n concent r a t e d s o l u t i o n s (14,15) these d e v i a t i o n s a r e not s u r p r i s i n g . "Concentrated' means concentrations i n excess of F a r more work needs t o be done on the systematics of the aggregation cons t a n t s as f u n c t i o n of c o n c e n t r a t i o n , solvent-polymers i n t e r a c t i o n , and temperature before t h e o r i e s o f the e q u i l i b r i a from concentrated polymer s o l u t i o n s can even be formulated.


c

1

M e l t / M e l t I n t e r a c t i o n . The s i m p l e s t f o r m u l a t i o n of polymer s o l u b i l i t y i n terms of r e g u l a r s o l u t i o n theory on Table 3 o r F i g ure 4 makes i t very c l e a r that h i g h molecular weight (or volume) of the s o l v e n t must be compensated f o r by s m a l l d i f f e r e n c e s i n t h e i r s o l u b i l i t y parameter, i f mutual c o m p a t i b i l i t y i s to be maintained. When both components of the mixture are h i g h polymers, compatib i l i t y i s commonly achieved only when t h e i r s o l u b i l i t y parameters d i f f e r by l e s s than .05 (cal/cm^)!' 2 i n other words, most polymer melts a r e incompatible w i t h each other. Even d i l u t e s o l u t i o n s of two d i f f e r e n t polymers i n a s i n g l e s o l v e n t w i l l separate i n t o d i l u t e phases. The r a r e exceptions of compatible yet d i f f e r e n t polymers a r e shown on Table 4. Hence most s o - c a l l e d "polyblends" are h i g h l y d i s p e r s e d two phase systems which are prevented from s e p a r a t i n g i n t o l a r g e s c a l e phases by p o t e n t i a l energy b a r r i e r s impeding flow (16) o r by the s k i l l f u l i n t r o d u c t i o n of c r o s s l i n k s (19). #

Glass/Gas (or Vapor). Polymers i n the g l a s s y s t a t e are p l a s t i c i z e d by most monomeric substances w i t h which they a r e compatible. Hence we are concerned here w i t h those s o l u t i o n s o f substances i n the g l a s s f o r which Tg > T ( o b s e r v a t i o n ) . Measurements of the part i a l pressure of the v o l a t i l e s o l u t e over such g l a s s y s o l u t i o n s a r e g e n e r a l l y b e t t e r represented by a combination of a Langmuir i s o therm w i t h Henry's law. The common e x p l a n a t i o n i s that the b u l k o f the v o l a t i l e s o l u t e i s adsorbed on the surfaces of minute voids i n the g l a s s , r a t h e r than being d i s s o l v e d i n the g l a s s (18,19). These


130


A N D FLUID PROPERTIES I N C H E M I C A L INDUSTRY

513 37000

503 O


o

2700000

37000 0.1

0.2 POLYSTYRENE-OCETANE

POLYSTYRENE -CYCL0HEXANE British Polymer Journal

Figure 6. Experimental examples of the various types of miscibility gaps in polymer solutions (12)

British Polymer Journal

Figure 7. Experimental example of the miscibility gap for polyvinyl alcohol-water, a hydrogen bonding system (12)

Figure 8. Lower critical solution temperature observed in glassy state by mechanical relaxation spectroscopy [Akiyama, S., et al, Kob. Roab. (1976) 5, 337.]

0

0.5

WT. FRACTION

PVN


6.

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Polymer Equilibria

131 TABLE 4

COMPATIBLE POLYMER PAIRS*


Polymer 1 Polystyrene

Polymer 2 poly-2-methyIs tyrene

ti

p o l y - m e t h y l v i n y l ether

ii

poly-2,5 d i a l k y l - p h e n y l e n e oxide

it

benzyl-cellulose

poly caprolactam

polyvinyl chloride v a r i o u s poly ethers

II

nitrocellulose

polyvinylidene fluoride II

polymethyl methacrylate^^ "

ethyl

b u t a d i e n e / a c r y l o n i t r i l e copolymer

polyvinyl chloride II

e t h y l e n e / v i n y l a c e t a t e copolymer

Polymethyl methacrylate ( i s o )

same s y n d i o t a c t i c nitrocellulose

From data by S. Davidson and by D. R. P a u l . ^ C o n f i r m e d by B r i l l o u i n - S c a t t e r i n g , P a t t e r s o n , G. D. e t a l . , Macromol. (1976), 9 603. 9



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v o i d s are s a i d to be the r e s u l t of the i n a b i l i t y of the g l a s s to acquire i t s e q u i l i b r i u m d e n s i t y i n the face of v i s c o u s r e s i s t a n c e to b u l k flow. The p r a c t i c a l importance of t h i s o b s e r v a t i o n and of the i n c r e a s i n g evidence f o r the o r i e n t a t i o n dependence of gas or vapor s o l u b i l i t y i n g l a s s y systems d e r i v e s from the i n c r e a s i n g l y widespread use of g l a s s y polymers as gas and vapor b a r r i e r f i l m s . Their p e r m e a b i l i t y f o r the gas i s , of course, the product of gas s o l u b i l i t y and d i f f u s i o n c o e f f i c i e n t . Given t h i s p r a c t i c a l importance, i t i s o b v i o u s l y awkward that the e q u i l i b r i u m constants of the dual mode s o r p t i o n equations cannot be c o r r e l a t e d w i t h the p r o p e r t i e s of the polymer and of the permeant. Such c o r r e l a t i o n s are reasonably s u c c e s s f u l i n the rubbery s t a t e . Owing to .the e f f e c t s of mechanical and thermal h i s t o r y of the g l a s s on the i n c i d e n c e and e f f e c t i v e s u r f a c e area of the v o i d s , a r e l i a b l e g e n e r a l i z e d c o r r e l a t i o n f o r p r i o r e s t i m a t i o n s of the three constants f o r gas s o r p t i o n i n g l a s s y polymers may never be p o s s i b l e . Yet more complicated i s the s o r p t i o n e q u i l i b r i u m of more s o l u b l e substances i n g l a s s y polymers. Since they p l a s t i c i z e the g l a s s f a r more than the l e s s s o l u b l e gases do, s o r p t i o n causes d r a s t i c changes i n the polymer, i n c l u d i n g the b u i l d i n g up of s t r e s s e s along the s o r p t i o n f r o n t . S o r p t i o n e q u i l i b r a t i o n under such circumstances i s w i t h a glass only at very low s o l v e n t a c t i v i t i e s , w h i l e at higher s o l v e n t a c t i v i t i e s i t would be w i t h a rubbery substance. The " v i t r i f i c a t i o n c o n c e n t r a t i o n " of the s o l v e n t which separates these two regimes i s , of course, w e l l known from p l a s t i c i z a t i o n experiments, but has r a r e l y been systematized f o r other s o l v e n t s . C r y s t a l / S o l v e n t . Given the d i f f i c u l t y of f i n d i n g s u i t a b l e s o l v e n t s f o r the t e c h n i c a l l y important high m e l t i n g c r y s t a l l i n e polymers, i t i s perhaps not s u r p r i s i n g that only l i t t l e work has been done on the discovery of e u t e c t i c s between c r y s t a l l i n e p o l y mers and s o l v e n t s of n e a r l y s i m i l a r m e l t i n g p o i n t s . These s o l v e n t s then must be completely m i s c i b l e w i t h the polymer i n t h e i r respect i v e melt s t a t e s , and completely i m m i s c i b l e as s o l i d s . Such systems have acquired p r a c t i c a l s i g n i f i c a n c e because of the unique f i b r i l l a r morphology of c r y s t a l l i n e polymer which p r e c i p i t a t e s at or near the e u t e c t i c p o i n t . In the c u r r e n t context i t i s important to note that the e x p e r i m e n t a l l y observed phase diagram, F i g u r e 9, d i f f e r s s u b s t a n t i a l l y from that p r e d i c t e d by the Flory-Huggins theory. At present i t i s not c l e a r whether that d i f f e r e n c e i s r e a l or whether i t i s due to retarded c r y s t a l l i z a t i o n of the p o l y mer (12,20). G l a s s / P l a s t i c i z e r . B a s i c a l l y t h i s system i s j u s t a v a r i a n t of the l i q u i d / l i q u i d system. However, the e f f e c t of phase separat i o n on the g l a s s t r a n s i t i o n temperature i s not only time dependent, as expected, but can a l s o be q u i t e unique. This i s best


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r

Figure 9. Eutectics in quasi-binary systems: crystalline polymer plus crystalline, high-melting-point solvents British Polymer Journal

1101—•—i—i—i—I—i

i

•

i

0.5

0

i 1.0

Figure 9A. Phase diagram of the system linear polyethylene-l,2,4,5-tetrachlorobenzene. ( ) Flory-Huggins theory calculations, (%) melting temperatures of mixtures quenched at 87 C.(20)

British Polymer Journal

100 '—I—i—J I

SOLVENT! I)

i i

-*

Phase Equilibria and Fluid Properties in the Chemical Industry

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