Solution Behavior of Metal-Sulfonate Ionomers - ACS Publications

16 Solution Behavior of Metal-Sulfonate Ionomers 1

2

R. D. Lundberg and R. R. Phillips 1

Exxon Chemical Company, Linden, ΝJ 07036 Corporate Research Science Laboratories, Exxon Research and Engineering Company, Annandale, NJ 08801

Downloaded by UNIV OF LIVERPOOL on November 28, 2016 | http://pubs.acs.org Publication Date: March 28, 1986 | doi: 10.1021/bk-1986-0302.ch016

2

Studies on the dilute solution behavior of sulfonated ionomers have shown these polymers to exhibit unusual viscosity behavior in solvents of low polarity. These results have been interpreted as arising from strong ion pair associations in low polarity diluents. Sol vents of higher polarity, such as dimethyl sulfoxide and dimethyl formamide induce classic polyelectrolyte behavior in sulfonate ionomers even at very low sul fonate levels. To a first approximation these two behaviors, ion pair interactions or polyelectrolyte behavior, are a consequence of solvent polarity. Intra molecular association of Lightly Sulfonated Polystyrene (S-PS) results in a reduced viscosity for the ionomer less than that of polystyrene precursor at low polymer levels. Inter-association enhances the reduced vis cosity of the ionomer at higher polymer concentrations. Isolation of the intra- and inter-associated species of S-PS has been attempted (via freeze drying). A comparison of selected properties reveals significant differences for these two conformations.

D u r i n g t h e p a s t 20 y e a r s , t h e r e have been many p u b l i c a t i o n s and p a t e n t s d e s c r i b i n g t h e i n f l u e n c e o f i o n i c groups pendant to a h y d r o c a r b o n polymer c h a i n on polymer p r o p e r t i e s ( 1 - 4 ) . The p r e s e n c e o f as l i t t l e as 1 mol % o f m e t a l c a r b o x y l a t e o r m e t a l s u l f o n a t e groups can have an e s p e c i a l l y p r o f o u n d i n f l u e n c e on t h e melt v i s c o s i t y o f polymers whose backbones a r e c o m p r i s e d l a r g e l y o f e t h y l e n e o r s t y r e n e repeat u n i t s . O t h e r p h y s i c a l p r o p e r t i e s as w e l l as polymer s o l u t i o n b e h a v i o r a r e s i m i l a r l y a l t e r e d ( 5 - 7 ) . The i n f l u ence o f t h e s e i o n i c groups on polymer p r o p e r t i e s i s g e n e r a l l y a t t r i b u t e d to t h e a s s o c i a t i o n o f i o n p a i r s r e s u l t i n g i n an e f f e c t i v e network a t s u f f i c i e n t l y h i g h i o n i c l e v e l s and h i g h polymer concen tration.

0097-6156/ 86/0302-0201 $06.00/ 0 © 1986 American Chemical Society

Eisenberg and Bailey; Coulombic Interactions in Macromolecular Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

COULOMBIC INTERACTIONS IN MACROMOLECULAR SYSTEMS

202

The n a t u r e o f the i o n p a i r a g g r e g a t e s has been examined i n some d e t a i l i n p u b l i c a t i o n s by s e v e r a l a u t h o r s . S p e c i f i c models have been p r o p o s e d to d e s c r i b e the n a t u r e o f the i o n p a i r aggregates (&> .2)> d number o f s t u d i e s have been conducted to e l u c i d a t e the s i z e and morphology o f t h e s e a g g r e g a t e s e x p e r i m e n t a l l y (1, 1 0 ) . W h i l e t h e r e has been some a m b i g u i t y c o n c e r n i n g the n a t u r e o f these i o n - r i c h p h a s e s , t h e r e i s s u b s t a n t i a l e v i d e n c e to suggest t h a t they do e x i s t . T h e i r s i z e and m o l e c u l a r arrangement are s t i l l open to question.


a n

a

Recent s t u d i e s i n o u r l a b o r a t o r i e s have been concerned w i t h the p h y s i c a l p r o p e r t i e s o f s u l f o n a t e d ionomers such as s u l f o n a t e e t h y l e n e / p r o p y l e n e / e t h y l i d e n e norbornene t e r p o l y m e r s ( 4 ) , o r l i g h t l y sulfonated polystyrene (S-PS)(11). These ionomers e x h i b i t pronounced i o n p a i r a s s o c i a t i o n ( a t s u l f o n a t e l e v e l s _> 15 m i l l i e q u i v a l e n t s / 1 0 0 g polymer) to a degree t h a t they appear c r o s s l i n k e d covalently. These i n t e r a c t i o n s can be d i s s i p a t e d by the a d d i t i o n o f a p o l a r a d d i t i v e , t h e r e b y showing t h a t such a s s o c i a t i o n s a r e i n d e e d p h y s i c a l and do not a r i s e due to c o v a l e n t c r o s s l i n k i n g . S e v e r a l s t u d i e s (6, 13) o f the s o l u t i o n b e h a v i o r o f s u l f o n a t e i o n omers have p r o v i d e d a d d i t i o n a l i n s i g h t on the n a t u r e o f the i o n p a i r aggregation. The p o l a r i t y o f the s o l v e n t environment has been shown to have a major i n f l u e n c e on the d i l u t e s o l u t i o n b e h a v i o r o f t h e s e polymers. In the c o u r s e o f these s t u d i e s i t has been o b s e r v e d w i t h s e l e c t e d systems t h a t b o t h m e l t v i s c o s i t y v a l u e s and s o l u t i o n b e h a v i o r can v a r y a c c o r d i n g to the h i s t o r y o f s u l f o n a t e ionomers. T h i s study p r o v i d e s some d a t a and p r o v i d e s one r a t i o n a l e f o r such differences. EXPERIMENTAL The s y n t h e s i s o f l i g h t l y s u l f o n a t e d p o l y s t y r e n e o r S-PS has been d e s c r i b e d i n s e v e r a l p u b l i c a t i o n s and p a t e n t s (5^, j6) . In the c u r r e n t s t u d y o n l y the sodium s a l t o f S-PS has been i n v e s t i g a t e d . The d e t a i l s o f s p e c i f i c i s o l a t i o n p r o c e d u r e s have been d e s c r i b e d i n those r e f e r e n c e s and a r e summarized i n the R e s u l t s s e c t i o n . The m e l t v i s c o s i t y measurements were conducted i n a melt i n d e x rheometer under c o n d i t i o n s w h i c h have been p r e v i o u s l y d e s c r i b e d . The s t a r t i n g p o l y s t y r e n e employed i n t h e s e s t u d i e s was a commercial homopolymer d e s i g n a t e d as S t y r o n 666 marketed by t h e Dow Chemical Company. T h i s polymer had an i n t r i n s i c v i s c o s i t y o f 0.80 i n t o l u e n e a t 25°C, a number a v e r a g e m o l e c u l a r w e i g h t o f 106,000 (as e s t a b l i s h e d by g e l p e r m e a t i o n c h r o m a t o g r a p h y ) . S u l f o n a t i o n / n e u t r a l i z a t i o n r e a c t i o n s were conducted as d e s c r i b e d i n v a r i o u s p u b l i c a t i o n s ( 4 ) . On the b a s i s o f a v a i l a b l e d a t a from GPC and i n t r i n s i c v i s c o s i t y measurements, we c o n c l u d e t h a t the s u l f o n a t i o n / n e u t r a l i z a t i o n p r o c e s s has no s i g n i f i c a n t e f f e c t on the backbone m o l e c u l a r w e i g h t s a t l e a s t f o r p o l y s t y r e n e s h a v i n g s u l f o n a t i o n l e v e l s below about 3 mole p e r c e n t . I n one system a narrow m o l e c u l a r w e i g h t d i s t r i b u t i o n p r o d u c t ( o b t a i n e d from P o l y s c i e n c e s ) was u s e d . T h i s p r o d u c t had M = 220,000 and M /M = 1.06. The r e s u l t s d e s c r i b e d were those o b t a i n e d w i t h commercial p o l y s t y r e n e u n l e s s o t h e r w i s e i n d i c a t e d . n

w

n


16.

LUNDBERG AND PHILLIPS

Solution Behavior of Metal-Sulfonate Ionomers

203

The d i l u t e s o l u t i o n v i s c o s i t y measurements were c o n d u c t e d u s i n g Ubbelohde v i s c o m e t e r s g e n e r a l l y conducted a t 25 + 0.05°C i n a thermostated bath. The reduced v i s c o s i t y o r v i s c o s i t y number [ d e f i n e d as ( η - η ) / n c , where η i s t h e v i s c o s i t y o f t h e polymer s o l u t i o n , 1Ί i s the v i s c o s i t y o f t h e s o l v e n t ( o r mixed s o l v e n t ) , and c i s the concen t r a t i o n o f polymer i n g/100 mL] was c a l c u l a t e d f o r each s o l u t i o n measured. 0

o

0

RESULTS


I.

D i l u t e S o l u t i o n B e h a v i o r o f S-PS

L i g h t l y s u l f o n a t e d p o l y s t y r e n e i s s o l u b l e i n mixed s o l v e n t systems, such as x y l e n e c o n t a i n i n g low l e v e l s o f a l c o h o l s , o r i n m o d e r a t e l y polar solvents. I n low p o l a r i t y s o l v e n t s t h e v i s c o s i t y o f such ionomer s o l u t i o n s can be s u b s t a n t i a l l y h i g h e r than p o l y s t y r e n e o f comparable m o l e c u l a r weight due to i o n p a i r a s s o c i a t i o n at concen t r a t i o n s >1% as shown i n T a b l e I . T a b l e I . V i s c o s i t y ( c e n t i p o i s e ) o f 1.7 M o l % S-PS and P o l y s t y r e n e i n S o l v e n t M i x t u r e ( X y l e n e + 3% Hexanol)

1.7 Mol % SPS Concentration, %

5 3 2 1 0.5 0.25 0.125

(Na s a l t ) V i s c o s i t y , cp

2,066 162 6 1.3 0.87 0.72 0.68

The e f f e c t o f p o l a r a d d i t i v e s i tions. This influence o f polar s p e c i f i c s o l v a t i o n o f the m e t a l and can be viewed as t o a f i r s t (6).

P o l y s t y rene V i s c o s i t y , cp Concentration, % 23 17.5 10 5 3 2 1 0.5 0.25 0.125

275 102 24 6.2 3.3 2.0 1.2 0.9 0.76 0.70

s to moderate the i o n p a i r a s s o c i a c o s o l v e n t s can be d e s c r i b e d as a c a t i o n groups by more p o l a r s p e c i e s a p p r o x i m a t i o n as a s i m p l e e q u i l i b r i u m

ROH + (P - S O ' N a + ^ ^ z z z ^ R O H :

Na^SO" - P)

W h i l e t h i s e q u i l i b r i u m i g n o r e s t h e polymer backbone and t h e m a j o r i t y o f t h e s o l v e n t system, i t does p r e d i c t many c h a r a c t e r i s t i c s o f m e t a l s u l f o n a t e ionomers i n h y d r o c a r b o n s o l u t i o n . M o d e r a t e l y p o l a r s o l v e n t s such as t e t r a h y d r o f u r a n a r e a l s o e f f e c t i v e f o r SPS up t o a s u l f o n a t e l e v e l o f about 5 mole % i n t h e case o f t h e sodium s a l t ( 7 ) . A comparison o f t h e r e d u c e d v i s c o s i t y - c o n c e n t r a t i o n p r o f i l e s o f S-PS a t v a r i o u s s u l f o n a t e l e v e l s i s compared w i t h t h a t o f the PS p r e c u r s o r ( F i g u r e 1 ) . I t i s evident that there i s a regular p r o g r e s s i o n i n Reduced V i s c o s i t y as a f u n c t i o n o f s u l f o n a t e c o n t e n t tfith i n c r e a s i n g v a l u e s a t h i g h e r s u l f o n a t e l e v e l s o b s e r v e d a t h i g h e r



204


concentrations ( ~ 2 % ) . C o n v e r s e l y the h i g h e r s u l f o n a t e d l e v e l s i n SPS l e a d to lower v i s c o s i t y v a l u e s at low polymer l e v e l s . This b e h a v i o r i s r e a d i l y r a t i o n a l i z e d as b e i n g a consequence o f i n t r a a s s o c i a t i o n s at lower polymer l e v e l s and i n t e r - a s s o c i a t i o n s a t the h i g h e r polymer c o n c e n t r a t i o n s . Analogous b e h a v i o r i s o b s e r v e d i n h y d r o c a r b o n s o l v e n t s or those c o n t a i n i n g low l e v e l s o f p o l a r cosolvents. One i m p l i c a t i o n o f F i g u r e 1 r e l a t e s to polymers c o n t a i n i n g h i g h e r l e v e l s of m e t a l s u l f o n a t e g r o u p s . I t might be e x p e c t e d t h a t such polymers might p r o v i d e homogeneous g e l s a t h i g h polymer l e v e l s and y e t d i s p l a y phase s e p a r a t i o n upon d i l u t i o n . Such b e h a v i o r i s o f t e n o b s e r v e d f o r m e t a l s u l f o n a t e ionomers i n h y d r o c a r b o n o r h y d r o c a r b o n a l c o h o l s o l v e n t systems. Few s t u d i e s have been c o n d u c t e d h e r e t o f o r e on s u l f o n a t e d ionomers i n s o l v e n t s which can be c o n s i d e r e d r e l a t i v e l y p o l a r , as d e f i n e d by a high d i e l e c t r i c constant. A r e c e n t s t u d y (13) on a c r y l o n i t r i l e m e t h a l l y l s u l f o n a t e copolymers i n dimethyl-fοrmamide i s a n o t a b l e exception. S-PS i s r e a d i l y s o l u b l e i n a wide v a r i e t y o f s o l v e n t s , some o f them e x h i b i t i n g r a t h e r h i g h v a l u e s o f d i e l e c t r i c c o n s t a n t , such as dimethylformamide (DMF) o r d i m e t h y l s u l f o x i d e (DMSO). The reduced v i s c o s i t y - c o n c e n t r a t i o n b e h a v i o r o f s u l f o n a t e d p o l y s t y r e n e i s markedly d i f f e r e n t i n p o l a r s o l v e n t s from t h a t i n n o n p o l a r s o l v e n t systems. T y p i c a l l y t h e r e i s a marked upsweep i n reduced v i s c o s i t y a t low polymer c o n c e n t r a t i o n s and c l e a r l y a m a n i f e s t a t i o n o f c l a s s i c p o l y e l e c t r o l y t e b e h a v i o r . (7) T y p i c a l p o l y e l e c t r o l y t e b e h a v i o r has been o b s e r v e d i n s e v e r a l s o l v e n t s o f moderate to h i g h p o l a r i t y as d e s c r i b e d i n T a b l e I I . I t i s c l e a r t h a t i n low p o l a r i t y s o l v e n t s i o n - p a i r a s s o c i a t i o n p r e v a i l s , while with high p o l a r i t y solvents p o l y e l e c t r o l y t e behavior i s observed. With s o l v e n t s o f intermediate p o l a r i t y e i t h e r behavior can be observed s u g g e s t i n g s p e c i f i c s o l v a t i o n e f f e c t s . Table

II.

Relation of Dielectric Behavior Solvent

Dimethylsulfoxide Dimethylformamide Cyclohexanone E t h y l e n e g l y c o l monomethyl (2-metho xye thano1) Tetrahydro furan Dioxane

ether

Source: Reproduced w i t h p e r m i s s i o n J . Polym. S c i . , Polym. Phys. Ed.

Constant

and D i l u t e - S o l u t i o n

(2) Dielectric Constant

Polyelectrolyte Behavior?

46.7 36.7 18.2 16.9

Yes Yes No Yes No No

7.6 2.2 from Ref.

7. C o p y r i g h t

1982

W h i l e s o l v e n t s such as THF and Dioxane e x h i b i t i o n p a i r a s s o c i a t i o n w i t h s u l f o n a t e ionomers, the a d d i t i o n o f more p o l a r c o s o l v e n t s can have a marked e f f e c t on the reduced v i s c o s i t y - c o n c e n t r a t i o n p r o files. F o r example,- F i g u r e 2 i l l u s t r a t e s the i n f l u e n c e of v a r y i n g




4r

Solution Behavior of Metal-Sulfonate Ionomers 1

1

1

F i g u r e 1. Reduced v i s c o s i t y - c o n c e n t r a t i o n r e l a t i o n s h i p s of PS and S-PS of d i f f e r e n t sodium s t y r e n e s u l f o n a t e l e v e l s i n tetrahydrofuran. Reproduced w i t h p e r m i s s i o n from R e f . 7. C o p y r i g h t 1982 J . Polym. S c i . , Polym. Phys. E d .

F i g u r e 2. Reduced v i s c o s i t y - c o n c e n t r a t i o n r e l a t i o n s h i p s S-PS i n t e t r a h y d r o f u r a n c o n t a i n i n g v a r i o u s l e v e l s o f methanol. The S-PS c o n t a i n e d 4.2 mole p e r c e n t sodium sulfonate groups.


for

206



amounts of methanol on S-PS of 4.2 mole percent of sodium sulfonate content. At methanol contents of 40 volume percent the i n t r i n s i c v i s c o s i t y of S-PS i s quite s i m i l a r to that of the unfunctionalized polystyrene. To a f i r s t approximation, Figure 2 at varying cosolvent l e v e l s i s s i m i l a r to Figure 1 with varying sulfonate l e v e l s . In effect both curves display the influence of varying degrees of ionic association for ionic groups pendant to a common backbone. The influence of small amounts of water as a cosolvent for THF solutions i s shown i n Figure 3. At water contents of 3% or less the reduced viscosity-concentration p r o f i l e s are s i m i l a r to Figure 2. However, increasing water l e v e l s induces an upsweep i n reduced v i s cosity at low polymer concentrations typical of polyelectrolyte behavior. This behavior has been observed i n other mixed solvent systems and i s c l e a r l y a consequence of s p e c i f i c cation solvation e f f e c t s . Sodium23-NMR studies have shown that water solvates sodium cations at least ten times more strongly at s i m i l a r cosolvent levels and the data i n Figures 2 and 3 are consistent with those findings. II.

Relation of Solution Properties and Bulk Polymer V i s c o s i t y

To a very rough approximation the concentration where reduced v i s cosity-concentration plots of S-PS and PS intersect i n solvents where ion pair associations p r e v a i l can be viewed as that concent r a t i o n where polymer c o i l s overlap, corresponding to C* (Figure 4). In e f f e c t , these studies suggest that S-PS exists i n two different conformations at different polymer concentrations. Furthermore, the very strong associations which can p r e v a i l i n these systems could preclude a rapid interchange between the two species under certain conditions. It can be argued that ±f_ polymer conformation w i l l control polymer properties for ionomers, then a difference i n physical property behavior might be expected for a material isolated at low polymer concentrations as compared to the same material i s o lated from high polymer concentrations ( i . e . , » C*). Typically conventional polymer i s o l a t i o n procedures do not permit product i s o l a t i o n within a s p e c i f i c concentration region; however, under certain conditions the operation of freeze drying ionomer solutions would be expected to o f f e r this option. Thus, freeze drying of S-PS at two d i f f e r e n t concentrations (0.3 and 4.0 weight percent polymer) from appropriate solvents should o f f e r two d i f f e r ent polymer species. This hypothesis, of course, assumes that once solutions of this polymer are frozen, the polymer conformations w i l l be "locked i n " at those concentrations, or the normal changes which might occur under other conditions of polymer i s o l a t i o n w i l l be minimized. A p r a c t i c a l freeze-drying approach with S-PS requires a suitable solvent which freezes at a r e l a t i v e l y high temperature, yet w i l l sublime at a rapid rate. P-Dioxane meets these c r i t e r i a and also o f f e r s the required ion pair association behavior as shown i n Figure 4.




Solution Behavior of M étal-Sulfonate

Ionomers

Polymer Concentration, g/100 ml

F i g u r e 3. Reduced v i s c o s i t y - c o n c e n t r a t i o n r e l a t i o n s h i p s f o r S-PS i n t e t r a h y d r o f u r a n c o n t a i n i n g v a r i o u s l e v e l s o f w a t e r . The S-PS c o n t a i n e d 4.2 mole p e r c e n t sodium s u l f o n a t e groups.

τ

F i g u r e 4. Reduced v i s c o s i t y - c o n c e n t r a t i o n r e l a t i o n s h i p s f o r S-PS (1.7 mole 7 sodium s a l t ) i n D i o x a n e . Reproduced w i t h p e r m i s s i o n from R e f . 13. C o p y r i g h t 1984 J . Polym. S c i . Polym. Lett. Ed. 0


207

C O U L O M B I C I N T E R A C T I O N S IN M A C R O M O L E C U L A R S Y S T E M S

208


In a s e r i e s o f e x p e r i m e n t s , S-PS (1.7 mole % sodium s u l f o n a t e ) was f r e e z e d r i e d a t two polymer c o n c e n t r a t i o n s from d i o x a n e s o l v e n t (0.3% and 4.0%). C h a r a c t e r i z a t i o n was t h e n conducted on the i s o l a t e d p r o d u c t s and on the i n i t i a l S-PS. C h a r a c t e r i z a t i o n o f the samples was attempted by comparing the m e l t v i s c o s i t i e s of the i s o l a t e d p r o d u c t s and by s o l u b i l i t y b e h a v i o r . S i m i l a r l y , PS was f r e e z e - d r i e d under the same c o n d i t i o n s and c h a r a c t e r i z e d i n the same way. The m e l t v i s c o s i t i e s o f the m a t e r i a l s were measured employing a melt rheometer as p r e v i o u s l y d e s c r i b e d (11) where the m e l t v i s c o s i t i e s were o b t a i n e d a t c o n s t a n t s h e a r s t r e s s as a f u n c t i o n o f t i m e . Those r e s u l t s a r e summarized i n T a b l e I I I . The polymer i s o l a t e d by f r e e z e d r y i n g from the more d i l u t e s o l u t i o n e x h i b i t s an i n i t i a l m e l t v i s c o s i t y which i s l e s s than the m a t e r i a l d e r i v e d from the c o n c e n t r a t e d s o l u t i o n ( o r the o r i g i n a l s t o c k S-PS) by a f a c t o r o f 10. Directiona l l y i t appears t h a t the v i s c o s i t y o f the p r o d u c t i s o l a t e d from the d i l u t e s o l u t i o n i n c r e a s e s s i g n i f i c a n t l y w i t h time. The p r o d u c t i s o l a t e d at d i l u t e c o n c e n t r a t i o n e x h i b i t s o t h e r p r o p e r t i e s d i f f e r e n t from t h a t o f the m a t e r i a l i s o l a t e d a t h i g h e r con centration. F o r example, c o n v e n t i o n a l S-PS (1.7%) w i l l form a homogeneous g e l i n x y l e n e a t c o n c e n t r a t i o n >3%, but w i l l phase s e p a r a t e to form a g e l phase i n more d i l u t e s o l u t i o n s , e s p e c i a l l y

Solution Behavior of Metal-Sulfonate Ionomers - ACS Publications

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