Coulombic Interactions in Macromolecular Systems - ACS Publications

15 Modification of Ionic Associations by Crystalline Polar Additives 1

2

3

4

I. Duvdevani , R. D. Lundberg , C. Wood-Cordova , and G. L . Wilkes 1

Corporate Research Science Laboratories, Exxon Research and Engineering Company, Annandale, NJ 08801

2

Exxon Chemical Company, Linden, ΝJ 07036

3Allied Corporation, Hopewell, VA 23860 4

Downloaded by FUDAN UNIV on February 27, 2017 | http://pubs.acs.org Publication Date: March 28, 1986 | doi: 10.1021/bk-1986-0302.ch015

Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

Ion

containing

polymers

such

as

sulfonated

ethylene-

propylene-diene (EPDM) polymers which strongly associate in the bulk, require ionic-plasticization for melt processing. It was found that a crystalline additive such as zinc stearate can strongly affect material properties in addition to being a highly effective ionic-plasticizer. Zinc stearate is compatible with sulfo-EPDM even at high loadings (over 30% by weight) and i t enhances physical associations as reflected in mechanical properties and swelling characteristics. The morphological structure of zinc-stearate/sulfo-EPDM blends was investigated and zinc stearate was found to microphase separate into small crystallites (less than about 5000 angstroms) which act as a reinforcing f i l l e r .

I o n - c o n t a i n i n g polymers c a n develop s t r o n g a t t r a c t i v e networks even at low i o n i c c o n t e n t ( 1 - 4 ) . Polymers which c o n t a i n s u l f o n a t e groups n e u t r a l i z e d by m e t a l c o u n t e r i o n s were shown t o form p a r t i c u l a r l y s t r o n g networks ( 5 ) . C o n s i d e r a b l e a t t e n t i o n was g i v e n t o s u l f o n a t e d Ethylene-Propylene-Diene (Sulfo-EPDM o r S-EPDM) polymers, due t o t h e i r e l a s t o m e r i c n a t u r e ( 6 ) . The i n c o r p o r a t i o n o f s u l f o n a t e groups n e u t r a l i z e d by m e t a l c o u n t e r i o n s i n EPDM c a n r e n d e r m a t e r i a l s which approximate a c r o s s l i n k e d EPDM a t lower temperature and can be melt p r o c e s s a b l e a t h i g h temperature. T h i s approach t o a so c a l l e d t h e r m o p l a s t i c e l a s t o m e r i s d i f f e r e n t from p r e v i o u s approaches u s i n g b l o c k copolymers h a v i n g h a r d and s o f t segment such as i n p o l y u r e t h a n e s ( 7 ) , s t y r e n e - b u t a d i e n e b l o c k copolymers, ( 8 ) o r p o l y e s t e r b l o c k copolymers ( 9 ) . I n o r d e r t o e n a b l e melt p r o c e s s i n g o f i o n c o n t a i n i n g polymers, such as S-EPDM, i t i s n e c e s s a r y t o I n t r o d u c e a mechanism t h a t weakens the i o n i c interactions. This c a n be a c h i e v e d by t h e a d d i t i o n of a p o l a r i n g r e d i e n t t h a t would " p l a s t i c i z e " i o n i c domains a t e l e v a t e d temperatures o n l y . A v a r i e t y o f such i o n i c - p l a s t i c i z e r s were d e s c r i b e d by Makowski and Lundberg (10). A particularly a t t r a c t i v e c o m b i n a t i o n was found t o be z i n c stéarate w i t h a z i n c s a l t o f S-EPDM. I t was shown t h a t f o r such a c o m b i n a t i o n melt 0097-6156/ 86/ 0302-0184506.00/ 0 © 1986 American Chemical Society

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

15.

DUVDEVANI ET AL.

Modification of Ionic Associations

185

r h e o l o g y was i n d e e d s u i t a b l e f o r melt p r o c e s s i n g and i n a d d i t i o n m e c h a n i c a l s t r e n g t h was h i g h l y enhanced (10,12). T h i s a l s o enabled f u r t h e r d i l u t i o n or h i g h l y extended compounds of S-EPDM w i t h o t h e r i n g r e d i e n t s (such as o i l s , f i l l e r s , and o t h e r p o l y m e r s ) to o b t a i n t h e r m o p l a s t i c e l a s t o m e r s of v a r i o u s p r o p e r t i e s . Many such compounds a r e d e s c r i b e d i n the p a t e n t l i t e r a t u r e (11) and an i n v e s t i g a t i o n of S-EPDM/Polypropylene b l e n d s was r e p o r t e d by Duvdevani, Agarwal and Lundberg ( 1 2 ) .


T h i s paper attempts t o f u r t h e r e x p l o r e the m o d i f i c a t i o n of i o n i c a s s o c i a t i o n s by a c r y s t a l l i n e i o n i c p l a s t i c i z e r , such as z i n c stéarate, at the s o l i d s t a t e . M e c h a n i c a l p r o p e r t i e s , s w e l l i n g beh a v i o r , and m o r p h o l o g i c a l a s p e c t s were s t u d i e d i n o r d e r to b e t t e r u n d e r s t a n d the r o l e of such c r y s t a l l i n e p o l a r a d d i t i v e s .

Experimental Materials

and P r e p a r a t i o n

A z i n c s a l t of s u l f o n a t e d EPDM (Zn-S-EPDM) was p r e p a r e d i n our l a b a c c o r d i n g to methods d i s c u s s e d i n e a r l i e r p u b l i c a t i o n s ( 6 , 1 3 ) . The Zn-S-EPDM used i n the p r e s e n t study was d e s i g n a t e d TP-303 and i t had a backbone c o n t a i n i n g about 55 wt% e t h y l e n e , 40 wt% p r o p y l e n e , and 5 wt% diene (ENB) w i t h a weight average m o l e c u l a r weight of about 90,000 and a M /M r a t i o i n the range of 2-2.5. I t was s u l f o n a t e d to a l e v e l of about 30 m i l l i - e q u i v a l e n t s per 100g (about 1 mole %). A g e n e r a l d e s c r i p t i o n of Zn-S-EPDM i s d e p i c t e d i n F i g u r e 1. The z i n c i o n s may be b a l a n c e d by a c o m b i n a t i o n of s u l f o n a t e and a c e t a t e groups s i n c e a 100% molar excess of z i n c a c e t a t e was used f o r n e u t r a l i z a t i o n i n o r d e r t o a s s u r e complete n e u t r a l i z a t i o n . w

n

A t e c h n i c a l grade of z i n c stéarate was used f o r preparing b l e n d s of the Zn-S-EPDM w i t h z i n c stéarate. Such t e c h n i c a l grades c o n t a i n about 90% stéarate. The m i x i n g was done both by i n t e r n a l mixers (Brabender or Banbury) and by m i l l m i x i n g a t about 150-200°C, y i e l d i n g e s s e n t i a l y i d e n t i c a l r e s u l t s as e x p l a i n e d below. The z i n c stéarate l e v e l (based on the t e c h n i c a l m a t e r i a l ) was v a r i e d from 0 to 50g of z i n c stéarate per 100g S-EPDM ( o r 0-33.3 w t % ) . Materials were d e s i g n a t e d 303-0 t o 303-50 i n d i c a t i n g TP-303 (Zn-S-EPDM) and the level of z i n c stéarate i n p a r t s per 100 p a r t s of TP-303. Samples were p r e p a r e d by compression molding a t 150°C. Mixing zinc stéarate w i t h Zn-S-EPDM i s not as c r i t i c a l as b l e n d i n g two h i g h m o l e c u l a r weight i n g r e d i e n t s . T h i s i s due t o the s o l u b i l i t y of m o l t e n z i n c stéarate i n p o l y o l e f i n s such as EPDM. I t s h o u l d be noted t h a t when z i n c stéarate i s melt mixed i n t o EPDM, c o m p a t i b i l i t y i s e v i d e n t d u r i n g the m i x i n g p r o c e s s . However, a f t e r c o o l i n g t o ambient temperatures z i n c stéarate tends t o bloom out of the b l e n d and s e t t l e on the s u r f a c e . On the o t h e r hand, t h e r e was no such blooming out of the Zn-S-EPDM b l e n d s even a t the h i g h e s t l o a d i n g of z i n c stéarate. T h i s a s p e c t was a l s o p a r t of the study presented here.


186

COULOMBIC INTERACTIONS IN MACROMOLECULAR SYSTEMS


Measurements M e c h a n i c a l p r o p e r t i e s ( s t r e s s - s t r a i n ) were measured on an Instron t e s t e r at a c r o s s head speed of 10 mm/min. Dumbbells shaped samples were cut w i t h a d i e and had an 8 mm t e s t l e n g t h and a t h i c k n e s s of no more than 0.5mm. A d d i t i o n a l d e t a i l s may be found i n the t h e s i s work of one of the c o a u t h o r s (C. W. C.) ( 1 4 ) . Swelling measurements were done In h e p t a n e . Disc shaped samples of about 0.5 mm i n t h i c k n e s s which were immersed i n heptane were weighed p e r i o d i c a l l y a f t e r w i p i n g a l l l i q u i d from the sample surface. R e s u l t s were c o n v e r t e d t o volume s w e l l i n p e r c e n t . Scanning-transmission electron microscope measurements were done on microtomed or r a z o r cut samples, w i t h o u t s t a i n i n g . D i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y measurements were done i n a P e r k i n - E l m e r DSC-2 c a l o r i m e t e r a t a s c a n n i n g r a t e of 20 °C per minute. Results

and

Discussion

The i n t r o d u c t i o n to this paper o u t l i n e d the need f o r a polar a d d i t i v e to S-EPDM f o r melt p r o c e s s i n g . I t s h o u l d be s e l f e v i d e n t t h a t i f such an a d d i t i v e p r e v e n t s good network f o r m a t i o n a t the use temperature, the m a t e r i a l p r o p e r t i e s c o u l d d e t e r i o r a t e to those of the backbone a l o n e (with no i o n i c g r o u p s ) . T h i s would be e q u i v a l e n t to a reduction i n sulfonate content as was shown by Agarwal, Makowski and Lundberg ( 1 5 ) . One needs, t h e r e f o r e , t o f i n d a p o l a r a d d i t i v e t h a t c o u l d d e s t r o y or weaken the network a t melt p r o c e s s i n g temperatures but would e n a b l e r e - e s t a b l i s h m e n t of networks at use temperatures. A c o n v e n i e n t way of d o i n g j u s t t h a t i s the use of a p o l a r m a t e r i a l which i s h i g h l y c r y s t a l l i n e as a r e some of the stéarates and p a r t i c u l a r l y z i n c stéarate. Such a p o l a r a d d i t i v e i s i n d e e d a h i g h l y e f f e c t i v e agent f o r melt v i s c o s i t y r e d u c t i o n as shown by the v a r i o u s workers mentioned i n the i n t r o d u c t i o n . On the o t h e r hand, the s t r o n g tendency of a h i g h l y c r y s t a l l i n e m a t e r i a l as zinc stéarate to c r y s t a l l i z e below i t s m e l t i n g point, helps to remove most of the a d d i t i v e from s h i e l d i n g the i o n i c groups which can then r e - e s t a b l i s h the network. The added advantages, as mentioned above, are the non m i g r a t o r y b e h a v i o r of z i n c stéarate i n Zn-S-EPDM and the p r o p e r t y enhancements. T h i s i s what w i l l be d i s c u s s e d here i n view of the r e c e n t measurements. I t was p r e v i o u s l y shown by Wagener and Duvdevani (16) t h a t z i n c stéarate i n c o p o r a t e d i n t o Zn-S-EPDM e x i s t s In the b l e n d i n a h i g h l y c r y s t a l l i z e d form. F i g u r e 2 shows t h a t the wide a n g l e x - r a y p a t t e r n i n a f i b e r which i s melt spun out of a 303-50 b l e n d i s e s s e n t i a l l y t h a t of the z i n c stéarate powder p a t t e r n . F i g u r e 3 shows t h a t the DSC thermogram of 303-50 i s a l s o very c l o s e t o t h a t of z i n c stéarate w h i l e 303-0 shows no endothermic peaks. The x - r a y s t u d i e s as w e l l as DSC studies shown i n F i g u r e 4 i n d i c a t e t h a t the crystalline s t r u c t u r e s a r e p u r e r or l a r g e r as the z i n c stéarate l o a d i n g i n the blend i n c r e a s e s . T h i s can be seen from m e l t i n g p o i n t s h i f t s and peak w i d t h i n the DSC thermograms or the s h a r p n e s s of the x - r a y r i n g patterns. I t was a l s o shown by Wagener and Duvdevani t h a t z i n c stéarate can o r i e n t i n a s t r e c t h e d f i b e r and t h a t the o r i e n t a t i o n r e l a x e s s l o w l y under l o a d ( F i g u r e 5 ) .


DUVDEVANI ET AL.


. . . -^-CH -CH 2

;


CH

3

* Zn

Figure 1· G e n e r a l representation m o l e c u l e (Zn-S-EPDM).

of

a

Zinc-Suifo-EPDM

F i g u r e 2. Wide Angle X-ray (WAXS) p a t t e r n o f 303-50 e x t r u d e d f i b e r ( r i g h t ) and o f z i n c stéarate powder ( l e f t ) (adapted from Ref. 1 6 ) .


COULOMBIC INTERACTIONS IN MACROMOLECULAR SYSTEMS

188

15.H -ι

DSC Scon SCAN RATE.

â Ul

Zn(S02

20.00 d.g/mln

(4.7 mg)

2nd Me 11i ng

303-5^.

(7.3 mg) / (J UJ

_)


U

303-0

\ 318. «

4 m «

35LM

371 • TEMPERATURE

F i g u r e 3. Differential and z i n c stéarate powder

39LM

^

411. •

(9.3 ma)

43L«

s c a n n i n g thermograms o f 303-0, i n a second m e l t i n g pass*

F i g u r e 4· D i f f e r e n t i a l s c a n n i n g thermograms v a r i o u s l o a d i n g s of z i n c stéarate*

303-50

o f Zn-S-EPDM w i t h


15.

DUVDEVANI E T A L .


189


Properties In this i n v e s t i g a t i o n we c a r e f u l l y studied the stress strain c h a r a c t e r i s t i c s as a f u n c t i o n of z i n c stéarate l o a d i n g and aging. F i g u r e 6 compares s t r e s s - s t r a i n b e h a v i o r of 303-0, 303-50 and the v u l c a n i z e d EPDM backbone. I t can be seen t h a t 303-0 i s s t i f f e r and s t r o n g e r than a v u l c a n i z e d backbone due to the f o r m a t i o n of a denser network by i o n i c a s s o c i a t i o n than by s u l f u r c r o s s - l i n k i n g . A l a r g e number of s i t e s on d i f f e r e n t c h a i n s can be a g g r e g a t e d i n an i o n i c " c r o s s - l i n k " w h i l e o n l y two s i t e s are c o n n e c t e d i n s u l f u r v u l c a n i zation. T h i s p o i n t was previously explained by Agarwal and coworkers (15) f o r the case of S-EPDM. F i g u r e 6 a l s o shows a l a r g e increase i n modulus o b t a i n e d by the a d d i t i o n of z i n c stéarate. F i g u r e 7 shows the s t r e s s s t r a i n b e h a v i o r as a f u n c t i o n of z i n c stéarate l e v e l f o r w e l l aged samples w h i l e F i g u r e 8 shows the e f f e c t of sample a g i n g f o r no z i n c stéarate and 50 p a r t s per hundred of z i n c stéarate. From these c u r v e s and from F i g u r e 9 i t i s c l e a r t h a t modulus changes s i g n i f i c a n t l y not o n l y by i n c r e a s i n g z i n c stéarate l e v e l but by a g i n g as w e l l . T h i s i n d i c a t e s t h a t z i n c stéarate a c t s more than j u s t a f i l l e r and t h a t a s t r o n g r e i n f o r c i n g mechanism s e t s up w i t h time. However, s i n c e a g i n g e f f e c t s were more pronounced within the z i n c stéarate c o n t a i n i n g systems, the time dependent b e h a v i o r may a l s o be due to some a d d i t i o n a l c r y s t a l l i z a t i o n of the additive. The r e i n f o r c e m e n t p r o p e r t i e s of z i n c stéarate i n Zn-SEPDM are comparable t o those of the h a r d phase i n segmented p o l y u r e t h a n e s and much h i g h e r than s i m p l e f i l l e r s as shown i n F i g u r e 10. The lower f o u r c u r v e s i n F i g u r e 10, u r e t h a n e and s a l t (17), d i s c s p h e r u l i t e s i n a rubbery m a t r i x (18) and g l a s s beads i n r u b b e r (19) are representing discrete particle fillers dispersed in a matrix. Such b e h a v i o r can be p r e d i c t e d by v a r i o u s models s u g g e s t e d i n the l i t e r a t u r e l i k e those of K e r n e r (20) and of Guth (21) and Smallwood ( 2 2 ) . However, i n the case of w e l l aged samples of z i n c stéarate i n Zn-S-EPDM the b e h a v i o r i s c l o s e r to the segmented p o l y u r e t h a n e case were the h a r d phase i s c h e m i c a l l y l i n k e d to the s o f t portions. I t i s a l s o c l o s e to the r e i n f o r c i n g b e h a v i o r of s m a l l s i z e , s u r f a c e a c t i v e f i l l e r s such as c a r b o n b l a c k s and fumed s i l i c a o r to the upper u r e t h a n e c u r v e i n F i g u r e 10 which i s a b l e n d w i t h 0.1 m i c r o n s i z e f i l l e r , (compared to 15 microns f o r the s a l t i n the c u r v e below i t ) . T h i s b e h a v i o r i s above t h a t p r e d i c t e d by a s e r i e s model or some of the o t h e r models such as mentioned above. It a c t u a l l y comes much c l o s e r t o the b e h a v i o r of a c o n t i n u o u s h a r d phase r a t h e r than a d i s p e r s e d one. I t i s n e c e s s a r y to c l a r i f y t h a t e l a s t i c p r o p e r t i e s beyond the range of low s t r a i n or low s t r e s s a r e somewhat d e t e r i o r a t e d by the a d d i t i o n of z i n c stéarate, as i s the case w i t h o t h e r r e i n f o r c i n g fillers. Since a possibility of " i o n - h o p p i n g " or " i n t e r a c t i o n h o p p i n g " can e x i s t i n a "physical c r o s s - l i n k , " higher stiffness which brings about higher stresses for similar strains can a c c e l e r a t e the "hopping" p r o c e s s . T h i s can be seen i n F i g u r e s 11 and 12 d e s c r i b i n g e l o n g a t i o n s e t and s t r e s s r e l a x a t i o n as a f u n c t i o n of z i n c stéarate l o a d i n g . The amount of e l o n g a t i o n s e t a t a g i v e n s t r a i n i s h i g h e r at h i g h e r l o a d i n g s but so i s the s t r e s s . However, i n s p i t e of a sharp s t r e s s r e l a x a t i o n w i t h h i g h z i n c stéarate l o a d i n g s , the s t r e s s l e v e l which the m a t e r i a l can s u s t a i n at l o n g e r time and a t a g i v e n s t r a i n (100% i n the case shown i n F i g u r e 12) i s s t i l l



190

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

F i g u r e 5. Wide a n g l e x-ray (WAXS) p a t t e r n o f a 303-50 f i b e r a f t e r i t was: ( a ) s t r e t c h e d t o 200% e l o n g a t i o n and k e p t under s t r a i n f o r t h e WAXS experiment, ( b ) A f t e r 14 days a t 200%, ( c ) u n l o a d e d from 200% s t r a i n and a l l o w e d t o r e l a x , ( a f t e r Wagener et a l (16)).

τ

1

1

Γ

500

Strain, % F i g u r e 6. Engineering s t r e s s - s t r a i n a t ambient c o n d i t i o n s o f 303-0, 303-50 and o f the s u l f u r v u l c a n i z e d EPDM backbone. ASTM d i e C samples were s t r e t c h e d a t 20 i n . / m i n .


DUVDEVANI ET AL.


14— .303-M •

12-

303-40 «

ν N

/

N ^^^y/^ s

10—

0*

i ·s —

X*

\—-303-10

s'

^/^X^XC

^

4— / S^* ^


/VO*"**^

0

100

• 303-10

\ — 3 0 3 - 0

0

2 -

>

Ambleiit

200

300

400

Conditio**

500

600

E l * * .

Coulombic Interactions in Macromolecular Systems - ACS Publications

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