Elastomeric Polydiene ABA Triblock Copolymers with Crystalline End

5 Elastomeric Polydiene ABA Triblock Copolymers with Crystalline End Blocks 1

MAURICE MORTON, N.-C. LEE , and E. R.

2

TERRILL

Downloaded by COLUMBIA UNIV on November 1, 2017 | http://pubs.acs.org Publication Date: July 19, 1982 | doi: 10.1021/bk-1982-0193.ch005

The University of Akron, Institute of Polymer Science, Akron, OH 44325

ABA t r i b l o c k copolymers having c r y s t a l l i n e end blocks and elastic center blocks were prepared by anionic polymerization of butadiene and isoprene, followed by hydrogenation. The end blocks con sisted of hydrogenated high-1,4 polybutadiene while the center block was either a high-1,4 polyisoprene (H 2 -BIB) or a hydrogenated 45%-1,2 polybutadiene ( H 2 - B B B ) . The hydrogenation could be carried out to over 99% with minimal chain scission (~1-2% ). At 30% end block content, both types of polymer exhibited thermoplastic elastomer behavior, but higher end -block content led to plastic-type behavior. The hydrogenated polybutadiene end blocks showed some degree of spherulitic c r y s t a l l i z a t i o n , about 50% crystallinity and a T m of 1 0 7 ° C . , close to that of high pressure polyethylene. Tensile strengths of 17 to 32 MPa were obtained at 30% end block content, but this dropped sharply with increasing temperature. None of the polymers were soluble at room tempera ture, showing only mild swelling in benzene. ABA triblock copolymers of the styrene-diene type are well known, and owe their unique properties to their heterophase morphology. This arises from the incompatibility between the polystyrene A blocks and the polydiene Β blocks, leading to the formation of a dispersion of very small polystyrene domains within the polydiene matrix. This type of e l a s t i c network, held together by the polystyrene "junctions", results i n thermoplastic elastomer properties. There has been considerable interest recently i n an alterna tive type of ABA t r i b l o c k structure, where the end blocks could form c r y s t a l l i n e domains, by c r y s t a l l i z a t i o n , rather than amorphous domains by phase separation. It was felt that, since such a c r y s t a l l i z a t i o n process need not depend on the incompati bility between the blocks, it should be possible to have a homo geneous melt, which should exhibit a much lower v i s c o s i t y , and hence much easier processing, than the heterogeneous media of the conventional t r i b l o c k copolymers. Furthermore, thermoplastic Current addresses: 1 Universal Energy System, Dayton, OH 45432. Ε. I. du Pont de Nemours Co., Inc., Wilmington, DE 19898.

2

0097-6156/82/0193-0101$06.00/0 © 1982 American Chemical Society

Mark and Lal; Elastomers and Rubber Elasticity ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


102

ELASTOMERS A N D R U B B E R ELASTICITY

elastomers based on c r y s t a l l i n e domains should a l s o e x h i b i t an advantageous r e s i s t a n c e t o s o l v e n t s . Since the a n i o n i c t r i b l o c k copolymers a r e based on monomers s u s c e p t i b l e t o t h i s mechanism, one r e c e n t approach to t h i s synthes i s has been to prepare butadiene-isoprene-butadiene t r i b l o c k copolymers, which are then hydrogenated so t h a t the high-1,4 polybutadiene end b l o c k s become c r y s t a l l i z a b l e , s i m i l a r t o h i g h pressure p o l y e t h y l e n e (1.-5.)· Recent work i n t h i s l a b o r a t o r y has been concerned w i t h the p r e p a r a t i o n and study o f two d i f f e r e n t v a r i e t i e s o f t h i s type of t r i b l o c k copolymer. Both o f these t r i b l o c k s had high-1,4 p o l y butadiene end b l o c k s , which were then hydrogenated to a "pseudo p o l y e t h y l e n e " s t r u c t u r e . However, the e l a s t o m e r i c center b l o c k s differed, i n t h a t one c o n s i s t e d o f a high-1,4 p o l y i s o p r e n e w h i l e the other was comprised o f a 45%-l,2 polybutadiene. The p o l y i s o p r e n e - c o n t a i n i n g t r i b l o c k was hydrogenated s e l e c t i v e l y , l e a v i n g the p o l y i s o p r e n e untouched, w h i l e the o t h e r polymer was hydrogenated t o t a l l y , so t h a t the 45%-l,2 polybutadiene was transformed i n t o an ethylene-butene-1 copolymer. These two t r i b l o c k copolymers were designated ( a f t e r hydrogénation) as H -BIB and H -BBB. The molecular " a r c h i t e c t u r e " o f these two types o f t r i b l o c k s i s l i s t e d i n Table I . 2

2

Table I M o l e c u l a r A r c h i t e c t u r e o f T r i b l o c k Copolymers Type

End Block

46-190-46 48-140-40 62-120-62

34 41 53

H BIB-34 H BIB-41 H BIB-53

19- 25-19

60

H BBB-60-19

18- 85-18 54- 72-54 55-257-55

30 60 30

H BBB-30-l8 H BBB-60-54 H BBB-30-55

3

H 1,4B-I-H 1,4B 2

2

ff ff

Η 1,4Β-Η (45*1,2)ΒH 1,4B 2

2

Designation

Mol. wt. (xlCT )

%

2

2

2

2

2

ff ff ff

2

2

2

I t can be seen from Table I t h a t these polymers v a r i e d both i n t h e i r end b l o c k content and molecular weight. I n the case o f the BIB polymers, a l l the end b l o c k s were o f "high" molecular weight (^50,000), w h i l e the BBB polymers contained both h i g h and low (^20,000) molecular weight end-blocks. The v a r i a t i o n o f these two parameters i s important, as w i l l be seen l a t e r i n connection w i t h c r y s t a l l i z a t i o n and mechanical behavior o f these materials.


5.

MORTON

ET AL.

Triblock Copolymers with Crystalline End Blocks

103


Experimental The base polymers were prepared by a n i o n i c p o l y m e r i z a t i o n using high-vacuum techniques. sec-Butyl l i t h i u m was used as i n i t i a t o r . The BIB t r i b l o c k s were synthesized by s e q u e n t i a l a d d i t i o n o f the monomers i n cyclohexane as s o l v e n t , while the BBB t r i b l o c k s were synthesized by f i r s t preparing the f i r s t p o l y b u t a diene end b l o c k , then adding 5% d i e t h y l ether before adding the second charge o f butadiene, f o l l o w e d by coupling of the AB d i b l o c k s w i t h the c o r r e c t s t o i c h i o m e t r i c amount o f d i m e t h y l d i c h l o r o s i l a n e . Both polymers were near-monodisperse i n molecular weight, as i n d i c a t e d by the GPC curves i n F i g u r e s 1 and 2 (%/M ^1.05 ), the BBB type showing a small r e s i d u a l peak o f up to 5% u n l i n k e d d i b l o c k . Hydrogénation was c a r r i e d out w i t h the a s s i s t a n c e o f an η-butyl l i t h i u m / c o b a l t octoate c a t a l y s t ( 6 ) . I t was necessary t o determine the proper c o n d i t i o n s f o r e f f i c i e n t hydrogénation w i t h minimal degradation ( 7 ) . For the BIB polymer the L i / C o r a t i o used was 5/1 to o b t a i n s e l e c t i v e hydrogénation of the polybutadiene, w h i l e f o r the t o t a l hydrogénation o f the BBB polymer, a r a t i o o f 2.2/1 was s a t i s f a c t o r y . NMR a n a l y s i s showed b e t t e r than 99% hydrogénation. Since the hydrogenated polymers were i n s o l u b l e a t room temperature, i t was not found p o s s i b l e t o determine the extent of chain degradation caused by hydrogénation, since t h i s would i n v o l v e molecular weight measurements. Instead, p o l y i s o p r e n e and a 45% 1,2-polybutadiene were used as c o n t r o l s f o r t h i s purpose, s i n c e they represented the center b l o c k s o f t h e r e s p e c t i v e polymers, and a l s o r e t a i n e d t h e i r s o l u b i l i t y a f t e r the hydrogénation treatment ( t h e polyisoprene was not expected to, and d i d n o t , become hydrogenated). Osmometric molecular weight measurements showed that minimal chain s c i s s i o n had occurred i n the case o f b o t h polymers during the hydrogénation. Thus Table I I shows t h a t the hydrogénation method used l e d to about 1% degradation of 1,4-polyisoprene. S i m i l a r s t u d i e s on the 45% 1,2-polybutadiene showed t h a t l e s s than 5% of t h e chains were cleaved. This was considered acceptable, s i n c e t h a t amount of f r e e d i b l o c k s would not be expected t o a f f e c t the mechanical p r o p e r t i e s t o any extent. Samples o f ,the polymers f o r p h y s i c a l e v a l u a t i o n were prepared by f i l m c a s t i n g from toluene s o l u t i o n a t 90°C. and a l l o w i n g the c r y s t a l l i z a t i o n to occur by c o o l i n g the melt. I t was observed that phase s e p a r a t i o n occurred i n the melt i n the case o f the H -BIB but not f o r the H -BBB. These m a t e r i a l s could a l s o be compression molded a t 140°C, b u t optimum r e s u l t s appeared to be obtained w i t h the f i l m - c a s t samples. n

2

2


104



Figure 2.

Gel permeation chromatogram of BBB.


5.

MORTON ET A L .


105

Table I I Degradation o f P o l y i s o p r e n e During S e l e c t i v e Hydrogénation o f Polybutadiene ( O r i g i n a l Mol. Wt. = 209,000) Hydrogénation Temperature


(°)

69 62 52

Reaction Time (nr.) 10 10 4.5

M ( Osmometry ) n

189,000 197,000 207,000

% Chain Scission*

11 6.1 1.0

* % o f chains cleaved R e s u l t s and D i s c u s s i o n Morphology. Observations w i t h the l i g h t microscope, under p o l a r i z e d l i g h t , showed t h a t the end b l o c k s i n the case o f both types o f polymers c r y s t a l l i z e d i n the form o f the u s u a l spherul i t e s , but not as w e l l as t h e analogous homopolymer, H - l , 4 polybutadiene. The f o r m a t i o n o f the s p h e r u l i t e s was improved w i t h i n c r e a s i n g end-block content and/or h i g h e r molecular weight o f t h e end b l o c k s . The morphology, as r e v e a l e d by l i g h t microscopy, i s shown i n F i g u r e s 3 t o 7. Thus F i g u r e s 3 and 4 show photomicrographs o f the hydrogenated 1,4-polybutadiene and a commercial l o w - d e n s i t y p o l y e t h y l e n e (Dow 991), r e s p e c t i v e l y . The s i m i l a r i t y between the two i s obvious. F i g u r e s 5 and 6 show the e f f e c t o f t h e endb l o c k content on the c r y s t a l l i z a t i o n o f two H2-BBB polymers, both having end-blocks o f h i g h molecular weight (^50,000-60,000). The more d i s t i n c t s p h e r u l i t e f o r m a t i o n i n F i g u r e 5 i s c l e a r l y seen. S i m i l a r l y , t h e e f f e c t o f end-block molecular weight on c r y s t a l l i z a t i o n i s demonstrated i n F i g u r e 7 f o r an Hf^BBB polymer having end b l o c k s o f o n l y 19,000 molecular weight, where s p h e r u l i t e formation i s q u i t e poor even a t h i g h end-block content. As s t a t e d p r e v i o u s l y , phase s e p a r a t i o n occurred i n the melt i n the case o f t h e H -BIB polymers, and t h i s i s shown i n F i g u r e 8, which r e p r e s e n t s a photograph taken by t r a n s m i s s i o n e l e c t r o n microscopy o f an u l t r a - t h i n f i l m o f t h i s type o f polymer, s t a i n e d by osmium t e t r o x i d e . The white domains r e p r e s e n t the hydrogenated 1,4-polybutadiene end b l o c k s , and these have dimensions s i m i l a r t o those found f o r p o l y s t y r e n e domains i n s t y r e n e - d i e n e styrene t r i b l o c k copolymers ( 8 ). T h i s type o f e l e c t r o n microscopy c o u l d not be used f o r the H2-BBB polymers, s i n c e OsO^ s t a i n i n g was not a p p l i c a b l e . However, the c o m p a t i b i l i t y o f t h e two b l o c k s i n the l a t t e r was demonstrated by m i x i n g s o l u t i o n s o f t h e r e s p e c t i v e hydrogenated homopolymers o f s i m i l a r molecular weight and c a s t i n g c l e a r f i l m s . 2

2




Figure 3.

Photomicrograph of H -l ,4-poly butadiene.

Figure 4.

Photomicrograph of polyethylene (Dow 991).

2



MORTON E T A L .

Figure 5.


z

Photomicrograph of H -BBB-60-54, (M.W. χ 10 is 54-72-54). 2

3

Figure 6. Photomicrograph of H -BBB-30-55, (M.W. χ 10' is 55-257-55). 2



108


Figure 7.

Figure 8.

3

Photomicrograph of H -BBB-60-19, (M.W. χ 10 is 19-25-19). 2

Transmission electron microphotograph of H -BIB-34, (M.W. X 10'' is 46-190-46). 2


MORTON

5.


ET AL.

109

Thermal A n a l y s i s . D i f f e r e n t i a l Scan C a l o r i m e t r y was c a r r i e d out w i t h the DuPont 990 Thermal Analyzer a t a 5-10°C scan r a t e . F i g u r e s 9 and 10 show the type o f DSC Thermograms obtained on samples before and a f t e r s t r e t c h i n g them t o the breaking p o i n t . Three p o i n t s a r e evident from these f i g u r e s : a ) the endotherm a t about 107°C. i n both f i g u r e s i n d i c a t e s a c r y s t a l m e l t i n g p o i n t corresponding almost e x a c t l y t o that found f o r Dow p o l y e t h y l e n e 991; b ) t h e t e n s i l e t e s t caused a s m a l l i n c r e a s e i n both the c r y s t a l l i n i t y and the m e l t i n g p o i n t ; and c ) the H -BBB polymer e x h i b i t s a s m a l l broad endotherm peaking a t about -10°C, which apparently disappears during s t r e t c h i n g . I t i s suggested t h a t t h i s endotherm i s due t o some tendency toward c r y s t a l l i z a t i o n o f the hydrogenated 45% 1,2-polybutadiene center b l o c k , because o f the occurrence o f some l o n g e r sequences o f p o l y e t h y l e n e u n i t s (9_).


2

U n i a x i a l T e n s i l e P r o p e r t i e s . The s t r e s s - s t r a i n curves f o r the H -BIB and H -BBB polymers a r e shown i n F i g u r e s 11 and 12, r e s p e c t i v e l y . As expected, these curves appear t o be a f u n c t i o n of the end-block content ("hard phase") i n both cases. However, i n a d d i t i o n , F i g u r e 12 i l l u s t r a t e s c l e a r l y the d e l e t e r i o u s e f f e c t o f the low molecular weight end b l o c k s on the t e n s i l e s t r e n g t h (H -BBB-60-19 v s . H -BBB-60-54, and H -BBB-30-18 v s . H -BBB-30-55). This agrees w i t h the morphology shown i n F i g u r e s 5-7, where c r y s t a l formation was shown t o depend on the molecular weight o f the end b l o c k . I t a l s o agrees w i t h p r i o r data i n the l i t e r a t u r e ( l ) . Apparently, an end-block molecular weight o f about 50,000 i s ~ r e q u i r e d f o r h i g h s t r e n g t h i n these polymers. A c t u a l l y , o f a l l the polymers d e s c r i b e d here, o n l y the ones w i t h end-block contents o f about 30% q u a l i f y as t h e r m o p l a s t i c elastomers, as defined by reasonably good recovery from s t r a i n . F i g u r e s 13 and 14 i l l u s t r a t e t h i s p o i n t by showing t h e amount o f t e n s i l e set obtained a f t e r s t r e t c h i n g t o v a r i o u s degrees o f s t r a i n . I t i s obvious that those polymers having more than about 30% endb l o c k content show u n u s u a l l y h i g h unrecovered deformations ( i . e . , " c o l d drawing"). Even at 30% end-block, t h e t e n s i l e set reaches a value o f 100% f o r the H -BIB polymers a t a s t r a i n r a t i o o f 9 and f o r the H -BBB polymers a t a s t r a i n r a t i o o f 6. This value i s much higher than those obtained f o r analogous styrene-butadienestyrene t r i b l o c k s ( β), and i n d i c a t e s t h a t t h e c r y s t a l l i n e domains apparently s u f f e r g r e a t e r d i s t o r t i o n s than the amorphous p o l y styrene domains. I t should be mentioned, o f course, t h a t these d i s l o c a t i o n s are not "permanent", i n t h a t a h i g h degree o f recovery can be obtained by annealing t h e samples a t 90°C. o r higher* Strength-Temperature R e l a t i o n s . One o f t h e key p r o p e r t i e s o f t h e r m o p l a s t i c elastomers i s t h e i r r e s i s t a n c e t o e l e v a t e d temperatures. Figures 15 and 16 show the e f f e c t o f temperature on the t e n s i l e s t r e n g t h o f the two types o f b l o c k copolymers. 2

2

2

2

2

2

2

2




110


ET AL.



MORTON


111


112


4000

ι

2

'

»

I

4

I

I

I

6 STRAIN

8

1

1

1

10

L_

12

Figure 11. Stress-strain behavior of H -BIB. Key: A, H -BIB-34; B1B-41; 0,H -BIB-53. 2

2

O, H 2

2

50

0

2

4 6 STRAIN

8

!0

Figure 12. Stress-strain behavior of H -BBB. Key: A , H -BBB-60-19; V , H BBB-30-18; Q, H -BBB-60-54; · , H -BBB-30-55. 2

2

2

2


2

MORTON ET A L .



5.

13$ l N 3 0 d 3 d


113

114



STRAIN Figure 14.

Tensile set of H -BBB as a function of strain. Key: Δ, H -BBB-3055; O, H -BBB-60-54. 2

2

2


MORTON ET A L .



5.


115

116

ELASTOMERS A N D

R U B B E R ELASTICITY

F i g u r e 15 shows t h a t the H -BIB polymers s u f f e r a d r a s t i c drop i n s t r e n g t h , c o n s i d e r a b l y more than t h a t of an analogous styreneisoprene-styrene t r i b l o c k (8_), Thus, at 60°C, the H -BIB-34 shows a s t r e n g t h of o n l y about 5 MPa (50 kg. cm" ), which represents a reasonable s t r e n g t h . The strength-temperature curve o f a s i m i l a r H -BBB t r i b l o c k i s shown i n F i g u r e 16, and seems to i n d i c a t e b e t t e r p r o p e r t i e s (^10 MPa at 6 0 ° C ) . In f a c t i t behaves s i m i l a r l y t o the segmented p o l y e s t e r elastomer, H y t r e l 4056, except t h a t i t s strength drops f a s t e r as the temperature approaches 100°C. This i s , of course, not s u r p r i s i n g , s i n c e the H y t r e l polymer i s s t a t e d to have a c r y s t a l m e l t i n g p o i n t of about 150°C. In g e n e r a l , however, these r a p i d drops i n s t r e n g t h w i t h i n c r e a s i n g temperature c o r r e l a t e w i t h the r a p i d drop i n dynamic modulus found (1(3, 11) f o r other segmented, c r y s t a l l i n e b l o c k copolymers. In t h i s connection, an i n t e r e s t i n g r e l a t i o n i s shown i n F i g u r e 17, where both the t e n s i l e s t r e n g t h and the r e l a t i v e c r y s t a l l i n i t y (from DSC d a t a ) are p l o t t e d a g a i n s t temperature, f o r the H -BBB-30-55 t r i b l o c k . I t can be r e a d i l y seen t h a t , at 80°C, where the s t r e n g t h has dropped to 10% of i t s o r i g i n a l v a l u e , the c r y s t a l l i n e content i s s t i l l over 80% of i t s i n i t i a l value. 2

2

2


2

2

Solvent R e s i s t a n c e . One of the d i s t i n c t advantages of a c r y s t a l l i n e thermoplastic elastomer over an amorphous one should be i t s s u p e r i o r solvent r e s i s t a n c e , s i n c e the l a t t e r types are g e n e r a l l y s o l u b l e . Table I I I shows the s w e l l i n g behavior of the H -BIB t r i b l o c k s i n toluene at 25°C I t can be seen that the maximum s w e l l i n g obtained was i n the case of the H -BIB-34, which had the lowest end-block content. Furthermore, the e q u i l i b r i u m s w e l l i n g r a t i o of 3.26 obtained f o r t h i s polymer i s c o n s i d e r a b l y l e s s than the value o f 5 o r 6 g e n e r a l l y e x h i b i t e d by a w e l l v u l c a n i z e d n a t u r a l rubber. Two other f e a t u r e s are notable i n Table I I I . The s w e l l i n g values f o r the compression molded samples (CM) run c o n s i s t e n t l y l e s s than those f o r the s o l v e n t - c a s t f i l m s . Apparently there i s more "entrapment" of the amorphous p o r t i o n s w i t h i n the c r y s t a l l i t e s during and a f t e r the molding. Secondly, the degradation experienced by the polymer hydrogenated at 69°C. (see Table I I ) i n s t e a d of 52°C. i s c l e a r l y demonstrated by the h i g h e r s w e l l i n g value. 2

2


MORTON E T A L .


5.

Figure 17.


117

Effect of temperature on crystallinity and tensile strength of H -BBB30-55. 2

Table I I I S w e l l i n g o f H -BIB Polymers (Toluene -25°C) 2

Polymer

Swelling V o l . Ratio FC *

CM *

H -BIB-34

3.26

H2-BIB-4I

2.13

H -BIB-53

1.82

1.80

H -BIB-34 ( 6 9 ) * * *

4.12

2.75

2

2

2

2.84 —

M ** c

FC

CM

6800

4500

2200 1800

1800

13,000

*FC = F i l m cast from s o l v e n t , CM = compression molded **Mc = M o l . wt. between " c r o s s l i n k s " , c a l c u l a t e d using Χχ = 0.43 0.05 v f o r p o l y i s o p r e n e . ***Polymer hydrogenated a t 69 C i n s t e a d o f 52 C. +

2


3900

118

ELASTOMERS

A N D RUBBER

ELASTICITY

Acknowledgement This work was supported i n part by Grant No. DMR78-09024 from the National Science Foundation, and by a grant from the Shell Development Co. Literature Cited


1.

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