Elastomers and Rubber Elasticity - American Chemical Society

4

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 18, 2016 | http://pubs.acs.org Publication Date: July 19, 1982 | doi: 10.1021/bk-1982-0193.ch004

Synthesis and Properties of Diene Elastomers With Controlled Structure I. G. HARGIS, R. A . LIVIGNI, and S. L . A G G A R W A L The General Tire & Rubber Company, Research Division, Akron, OH 44305

Using organolithiums or complexes of diorganomagnesiums with trialkylaluminums, i n combination with specific barium salts, we are able to control the trans-1,4/cis-1,4 ratio i n butadiene-styrene copolymers, while still maintaining a low vinyl content. The important reaction variables for doing this are the barium/metal alkyl ratio, polymerization temperature, and i n i t i a t o r con centration. For SBR's prepared with Ba-Mg-Al initiators in comparison to barium modified alkyllithiums, the trans-1,4 content i s higher (87%vs.79%)and the vinyl content i s lower (3%vs.8%).Moreover, the Ba-Mg-Al i n i t i a t o r system has the characteristics of a " l i v i n g polymerization" providing control of average molecular weight, molecular weight distribu tion, and molecular architecture. The crys t a l l i n e melting temperature of these trans-1,4 polybutadienes can be decreased to near room temperature by adjustment of the trans-1,4 content and/or the incorporation of styrene. Their crystallization behavior compares with natural rubber, as follows: (1) their rate of crystallization i s more rapid; and (2) their amount of c r y s t a l l i n i t y i s temperature dependent, but considerably less strain dependent. These experimental rubbers have excellent green strength and building tack.

During the last two decades, a number of diene homopolymers and copolymers have been developed to fill the diverse elastomer needs in the production of t i r e s . The earliest developments were mainly concerned with the preparation of stereoregular cis-1,4-polyisoprene, as a substitute for natural rubber, using 0097-6156/82/0193-0073$07.75/0 © 1982 American Chemical Society

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


74

ELASTOMERS

AND

RUBBER

ELASTICITY

Ziegler-Natta catalysts (_1) and organolithium compounds (2). This was soon followed by the synthesis of high cis-1,4 polybutadiene using Ziegler-Natta catalysts Ç 3 ) . More recently, a number of different copolymer structures have been prepared from butadiene and styrene, using modified organolithiums as polymerization i n i t i a t o r s (4). Organolithium i n i t i a t e d polymerizations have gained prominence because stereocontrol i s combined with excellent polymerization rates, and the absence of a chain termination reaction f a c i l i t a t e s control of molecular weights and molecular weight distributions (5) . This paper w i l l describe the ways i n which the present needs i n t i r e rubbers can be met by preparing butadienestyrene solution polymers of controlled structure. This appears especially realizable today, as a result of two new classes of organo-alkaline earth polymerization i n i t i a t o r s , Ba-Li and Ba-Mg-Al, which provide unique steric control of the trans-1,4/cis-l,4 ratio of polybutadiene segments (_6). Although A l f i n rubbers (7) contain trans contents at 70-757, their v i n y l contents are much greater (20-25%) than i s characteristic of the new rubbers reported here. Experimental Materials. Barium t-butoxide-hydroxide was prepared by reacting barium metal with a mixture of t-butanol and water (10 mole % water) i n l i q u i d monomethylamine, as shown i n Figure 1. n-Butyllithium (Foote Mineral Co., 15 wt.7 i n nhexane) was diluted with dry cyclohexane to the desired concentration. MAGALA-7.6E i s a trade name for a complex of n-dibutylmagnesium and triethylaluminum (Mg/Al molar ratio of 7.6) supplied by Texas Alkyls as a 10 wt.7 solution i n nheptane. MAGALA-7.6E was diluted with dry cyclohexane prior to i t s use. n-Hexane ( P h i l l i p s Petroleum Co., 997«) was dried by passing i t through Linde 5A molecular sieves. Butadiene ( P h i l l i p s Petroleum Co., 99 mol %) was purified by passing i t through Linde 13X molecular sieves. Styrene (Gulf O i l Chemical Co.) was vacuum d i s t i l l e d from a small quantity of (nbutyl)(sec-butyl) magnesium. 0

0

Polymerizations. The polymerizations were carried out i n an argon atmosphere i n capped glass bottles f i t t e d with a neoprene rubber gasket inner l i n e r . In charging the polymerizations, the order of addition of materials was solvent f i r s t , then metal a l k y l s , next the barium s a l t , and f i n a l l y the monomer(s). The amount of metal a l k y l charged was s u f f i c i e n t to t i t r a t e the acidic impurities present i n the solvent and polymerization bottle, plus the calculated amount for i n i t i a tion of polymerizations. The mole r a t i o of barium to metal alkyl(s) was based on the moles of total a l k a l i n i t y of barium to the moles of carbon-metal assayed. Unless otherwise stated,


4.

Diene Elastomers with Controlled Structure

HARGIS E T A L .

the mole ratios of barium salt to BuLi or Bu Mg, denoted as B a / L i or Ba /Mg , were 0 . 5 or 0 . 2 , respectively. The percent conversion was calculated from a measurement of the total solids after removal of solvent and unreacted monomers by vacuum d i s t i l l a t i o n . a

s +

+

2+

s+

Polymer Characterization. The copolymer composition and polybutadiene microstructure were obtained from infrared analysis and checked for certain copolymers using C NMR. The microstructure values determined from IR and C NMR were found to be essentially i d e n t i c a l . The t r a n s - 1 , 4 and vinyl content were determined using the 9 6 7 cm" and 9 0 5 cm" i n f r a red absorption bands, respectively. I n t r i n s i c v i s c o s i t i e s were determined i n toluene at 3 0 ° C . Gel permeation chromatograms (GPC) were obtained using a Waters Gel Permeation Chromatograph. Solutions at wt.7o were injected onto columns at a flow rate of 1 ml/minute. The instrument oven and the d i f f e r e n t i a l r e f T a c t o m e t e r were at 5 0 ° C . The column set configuration used, as designated by Waters Associates, was 1 0 A + 1 0 Â + 1 0 A + ΙΟ !.


1 3

1 3

1

1

1

e

B

4

3

Polymer C r y s t a l l i n i t y . Thermal transitions were obtained by D i f f e r e n t i a l Thermal Analysis (DTA) and D i f f e r e n t i a l Scan ning Calorimetry (DSC) using a heating rate of 20°C/minute. Crystalline melting temperatures were determined from the position of the endothermic peak(s) present i n the curve, obtained after rapidly cooling the sample from room temperature to - 1 5 0 ° C . X-ray d i f f r a c t i o n patterns were obtained from films cured with 17o dicumyl peroxide i n the absence of f i l l e r s . A l l the experiments were carried out at room temperature using CuKo/ radiation and a nickel f i l t e r using f l a t plate photography. Stress-optical measurements were made with a conventional apparatus. The l i g h t source was a mercury lamp. Polymers were cured i n the absence of reinforcing f i l l e r s with a sulfur based recipe. Polymer Properties. Green strength data were obtained from stress-strain measurements on unvulcanized polymers with an Instron tester at room temperature. The crosshead speed was 5 0 . 8 cm/minute. Sample specimens were prepared by press molding tensile sheets at 1 2 1 ° C for 5 minutes with a ram force of 1 1 3 6 0 kg. Tack strength was measured using the Monsanto Tel-Tak machine. The test specimens were compounded polymers pressed between Mylar f i l m at 1 0 0 ° C . Two cm χ cm die-cut sample strips were placed at right angles to each other and retained i n special sample clamps. A fixed load, 0 . 2 2 1 MPa was then applied for specified contact times ranging from 6 seconds to 6 minutes. The samples were pulled apart at a con stant separation rate of 2 . 5 4 cm/minute. The test was run at room temperature. 0 . 6 4

5 . 0 8



76

ELASTOMERS

AND

RUBBER

ELASTICITY

There are a number of specific requirements which must be met for any t i r e rubber candidate, as shown i n Figure 2 . I n the production of radial t i r e s , building tack and green strength are very important, and these performance properties are achieved by the use of NR. These properties are not readily attainable from synthetic elastomers prepared from low cost monomers. We have focused our studies, therefore, on copolymers of butadiene and styrene prepared with two alkaline earth based catalyst systems: barium t-butoxide-hydroxide/BuLi (Ba-Li), and barium tbutoxide-hydroxide/Bu Mg/Et3Al (Ba-Mg-Al). We have found them to be extremely useful i n controlling stereoregularity of diene polymerizations and i n providing SBR's with high tack and green strength. s

1

High Trans BR s by Ba-Li Catalyst Systems Preparation and Properties of Barium Salt. The catalyst used to prepare this new class of c r y s t a l l i z i n g polybutadienes consists of a barium t-butoxide-hydroxide salt i n combination with an organolithium ( _ 8 , 9 ^ _ 1 0 ) . Rather specific preparative techniques must be used i n forming this barium s a l t , as shown i n Figure 1 . The use of an amine solvent provided quantitative conversion of the metal to barium s a l t s . The molecular structure of polybutadiene prepared with BuLi and barium salts i s greatly dependent on the presence of specific amounts of water and t-butanol used i n the barium s a l t formation. The data i n Figure 3 demonstrate that the greatest effect i s obtained as the hydroxide content of the solution phase of the barium salt increased from 0 to 7 . 4 mole °L Ba(OH) . This particular s a l t results from a H 0-t-butanol mixture containing 2 . 5 mole 7 » water. The amount of t r a n s - 1 , 4 increased from 6 3 7 o to 7 6 7 , and accompanying this change the i n t r i n s i c v i s c o s i t y increased from 1 . 6 0 to 5 . 2 2 . The polymers were g e l free. The most important polymerization variables on which the molecular structure of polybutadienes prepared with Ba-Li catalysts depends are described as follows. a

a

0

Mole Ratio of Barium t-Butoxide-Hydroxide to n-BuLi. Figure 4 shows that the amount of t r a n s - 1 , 4 structure i s increased from 5 5 7 o , which i s the t r a n s - 1 , 4 content of a polybutadiene prepared with BuLi alone i n nonpolar solvents, to a maximum of about 8 0 7 t r a n s - 1 , 4 content for polybutadienes prepared at a mole r a t i o of barium s a l t to BuLi equal to 0 . 5 . This barium s a l t contains about 9 mole 7« hydroxide ion. I t i s important to note that the v i n y l content i s not affected by an increase i n the mole r a t i o of Ba/Li from 0 to 0 . 5 . However, a further increase i n the mole ratio above 0 . 5 results i n a decrease i n the t r a n s - 1 , 4 content and a corresponding increase i n the v i n y l content. o



HARGis E T A L .


Ba t-BUTOXIDE-HYDROXIDE IS D I S S O L V E D

Figure 1.

SALT

IN B E N Z E N E A T

50°C

Preparation of Ba t-butoxide-hydroxide salt for polymerization catalyst.

1. P R O C E S S I B I L I T Y 2. T A C K A N D G R E E N

STRENGTH

3. SKID A N D A B R A S I O N

RESISTANCE

4. T H E R M A L A N D C H E M I C A L 5. LOW

HEAT

STABILITY

BUILD-UP

6. LOW C U T G R O W T H

RATE

7. C O S T :

MONOMERS

Figure 2.

LOW

COST

Requirements for tire rubbers.


ELASTOMERS

78

BARIUM SALT MOL % H 0 IN tertBUTANOL + H 0 2


2

AND RUBBER

ELASTICITY

% DIENE STRUCTURE

MOL % HYDROXIDE* IN Ba[(t-BuO) . (OH) j 2

x

x

[η]· " 1

2

T

R

A

N

S

V

,

N

Y

L

d

l

/

8

0

0

63

10

1.60

2.5

7.4

76

7

5.22

5.0

8.5

76

7

6.67

7.5

8.9

79

7

5.63

10.0

12.5

76

7

5.51

76

6

8.05

48

10

2.10

15.0 100

0

5

* IN SOL P H A S E

Figure 3.

Effect of hydroxide content of Ba t-butoxide-hydroxide on the molecu lar structure of polybutadiene prepared in toluene at 30°C.

90 h

03 Ba

Figure 4.

2 +

/Li

+

ΙΌ

M O L E RATIO

Polybutadiene microstructure versus the mole ratio of Ba t-butoxidehydroxide to n-BuLi. Conditions: solvent, toluene; 30°C.


4.

HARGis

E T AL.


79

Catalyst Concentration. The concentration of the catalyst has a marked effect on the trans-1,4 content of polybutadienes prepared with BuLi and barium t-butoxide-hydroxide i n toluene at 3 0 ° C , as shown i n Figure The trans-1,4 content increases with a decrease i n the molar ratio of the i n i t i a l butadiene to BuLi concentration [(M)/(BuLi)]. The trans-1,4 content approaches a l i m i t i n g value of about for polybutadienes prepared with large amounts of this catalyst. 5 .


8 0 7

o

Polymerization Temperature. The stereoregularity of polybutadienes prepared with the BuLi-barium t-butoxide-hydroxide catalyst i n toluene i s exceedingly temperature dependent. Figure 6 compares the trans-1,4 dependence for polybutadiene prepared with BuLi, alone, and with the BuLi-barium t-butoxide-hydroxide complex i n toluene (the molar ratio of the i n i t i a l butadiene to BuLi concentration was 5 0 0 ) . The upper curve demonstrates that the percent trans content increased rapidly from to trans-1,4 as the temperature decreased from 75°C to 2 2 ° C . From 22°C to 5°C, the microstructure does not change. The increase in trans-1,4 content occurred with a decrease i n cis-1,4 content, the amount of vinyl unsaturation remaining at For the polybutadienes prepared using BuLi alone, there i s only a very s l i g h t increase i n the trans-1,4 content as the polymerization temperature i s decreased. The amount of both low and high molecular weight polymer produced, as a function of polymerization temperature, can be seen i n Figure 7 . In this Figure, the MWD*s of polybutadienes prepared with barium t-butoxide-hydroxide and BuLi i n toluene at 30°C and 5°C are compared. Although both polymers show a broad MWD, the fraction of low molecular weight polymer present in the polybutadiene prepared at 5°C i s greatly decreased. 6 2 ?

0

807

5 - 8 7 o .

Variation of Styrene Content with Extent of Conversion. Figure 8 gives the relationship between copolymer composition and the extent of conversion for copolymers of butadiene and styrene wt.T, styrene) prepared i n toluene, at 3 0 ° C , with n-BuLi and barium salts of t-butanol and water. For comparison purposes, the copolymer composition curve i s shown for the reaction i n i t i a t e d using n-BuLi alone. Copolymerization using n-BuLi results i n very l i t t l e incorporation of styrene into the copolymer chain u n t i l about conversion, after which the styrene content increases very rapidly. In contrast, copolymerization using the barium salts and n-BuLi results i n an increased incorporation of styrene at the same extents of conversion. Thus, this polymerization catalyst yields more random copolymers with styrene, while s t i l l maintaining a low vinyl content and high trans-1,4 content. ( 2 5

757o

Chain Termination i n Ba-Li Polymerizations. Another important feature of these butadiene based rubbers, prepared


ELASTOMERS

RUBBER

ELASTICITY

79h

UJ


AND

H 5

«

1 1.00

1 2.00

I 3.00

[(BUTADIENE) /(BuU)0]x

10"

0

I 4.00

3

Figure 5. Variation of trans-2,4 content with the molar ratio of monomer to initi ator. Conditions: solvent, toluene; catalyst, Ba[(t-BuO) (OH) ] and n-BuLi. 2x

2 Ο if)

80

Ba[(t-Bu0)

75

n-BuLi

2 x

IN

x

(0H) ]AND x

TOLUENE

70

styrene, at extension ratios. The percent c r y s t a l l i n i t y that develops i s temperature dependent, there being an increase i n the amount of c r y s t a l l i n i t y with a decrease i n temperature. However, the amount of c r y s t a l l i n i t y that develops i s essentially independent of s t r a i n . The amount of c r y s t a l l i n i t y that develops at room temperature, regardless of the level of s t r a i n , i s extremely small ( 9 ) . Figure 2 0 shows the percent c r y s t a l l i n i t y as a function of temperature and extension ratio for high trans SBR styrene, trans) prepared with a Ba-Mg-Al catalyst. A 7 5 7 «

1 4

3

(227o

8 7 7 »




HARGis E T A L .

Figure 17. X-ray diffraction patterns for high txms-SBR (15% styrene, 85% trans).


94


ELASTOMERS A N D R U B B E R ELASTICITY

λ

% TRANS

% STY.

75

14

3.75 "

A

•

3.10

•

2.50

>-

ο %A

• • V

-40

Figure 19.

-20 0 20 40 TEMPERATURE (°C)

60

Percent crystallinity of high tram-SBR prepared with Ba-Li as a func tion of temperature and extension ratio.




HARGIS E T A L .

STYRENE

TEMPERATURE C O Figure 20. Percent crystallinity of high txms-SBR prepared with Ba-Mg-Al as function of temperature and extension ratio.


96

ELASTOMERS

AND RUBBER

ELASTICITY

comparison of Figures 18, 19, and 20 shows that the amount of c r y s t a l l i n i t y developed i n both Ba-Li and Ba-Mg-Al high trans SBR s i s temperature sensitive, but not as strain sensitive as that of NR. The most significant difference i n the c r y s t a l l i z a tion behavior of the two types of SBR's i s that the higher trans-1,4 content (and increased styrene content to adjust the c r y s t a l l i n e melt temperature to near 25°C) leads to an increase i n the dependence of c r y s t a l l i n i t y on strain and an increase i n the degree of c r y s t a l l i n i t y at room temperature. That i s , the Ba-Mg-Al catalyzed styrene-butadiene copolymer compared to the Ba-Li SBR shows a marked improvement i n both s t r a i n s e n s i t i v i t y and i n the amount of strain induced c r y s t a l l i n i t y . In observing the time dependent changes i n birefringence and stress-optical c o e f f i c i e n t , for elongated samples at 25°C, i t was found that the rate of c r y s t a l l i z a t i o n of high trans SBR s was very much faster, some 10 times more rapid, than that for NR (_8). This i s consistent with the reported rates of isothermal c r y s t a l l i z a t i o n for NR (2.5 hours at -26°C) and for 807, trans-1,4 polybutadiene (0.3 hours at -3°C) i n the relaxed state (12). The main conclusions of the strain induced c r y s t a l l i z a t i o n behavior of high trans polybutadiene based rubber and natural rubber are: (1) the rate of c r y s t a l l i z a t i o n i s extremely rapid compared to that of NR; (2) the amount of strain induced cryst a l l i z a t i o n i s small compared to that of NR, especially at room temperature; and (3) for the high trans SBR's relative to NR, c r y s t a l l i z a t i o n i s more sensitive to temperature at low extension r a t i o s , and c r y s t a l l i z a t i o n i s less sensitive to s t r a i n .


1

1

Solution SBR of Controlled Structure for Tire Carcass Compounds Two additional properties that may depend on the strain induced c r y s t a l l i z a t i o n behavior of NR are green strength and building tack. A comparison of the performance of the experimental high trans SBR's with NR was, therefore, carried out. Green Strength. Green strength i s a measure of the elastumeric behavior of an uncured (no chemical cross-links) rubber. Natural rubber i s an outstanding example of a rubber having good green strength. The c r i t e r i a that we have used, which are similar to those of Briggs et a l . (13), i n evaluating this property are (1) the value of the slope i n the stress-strain curve for unvulcanized rubber between 100-3007, elongation, and (2) the nominal stress at 3007, elongation. Figure 21 shows the comparison of the stress-strain curves of high trans SBR prepared with Ba-Mg-Al and containing 157, styrene and 857, trans-1,4 content versus NR and an emulsion SBR. These rubbers were compounded with 45 phr HAF carbon black. As evidenced by a positive slope and a high stress value for 3007, elongation, the high trans SBR has high green strength, as does NR. In



4. HARGis E T A L .

97


S B R 1500 S L O P E OF ' S T R E S S ' V S ELONGATION ( 3 0 0 - 1 0 0 % )

( l

" S T R E S S " AT 3 0 0 % ELONGATION

HTSBR

. ~'

M 1

58

NR M

'

{

125

)

80

NR 200

BREAK

*150|-

^D^ ' 8

•

, ^ Τ

X

1

1

NO BREAK

c/> —.π-"·

CO UJ

>•

0

H îooh CO

5 0 Hi • • • " " • ^

t

100

BREAK^"SBR.ISOO

200

300 PERCENT

Figure 21.

400

500

600 700

ELONGATION

Green strength comparison.


HTSBR

ELASTOMERS A N D

98

R U B B E R ELASTICITY

contrast, a typical emulsion SBR has a negative slope and a very low stress at elongation—a consequence of the property of unvulcanized SBR to neck down on elongation.


3 0 0 7 ,

Tack Strength. Tack strength i s defined as the force required to separate two uncured polymer surfaces after they have been brought into contact. The l i m i t i n g tack strength of a rubber i s necessarily i t s green strength, or the force required for i t s cohesive f a i l u r e . Although high green strength i s necessary, i t i s , by i t s e l f , i n s u f f i c i e n t to insure good tack. High tack strength i s an especially desirable property i n the fabrication of rubber a r t i c l e s , especially those having a complex geometry, prior to vulcanization. This property i n t i r e rubbers i s needed so that the unvulcanized t i r e , i . e . green t i r e , w i l l hold together before curing. A comparison of tack values for uncured, black f i l l e d ( 4 5 phr HAF carbon black) compounds of NR versus high trans SBR ( 1 5 7 o styrene, 8 5 7 , trans) i s shown i n Figure 2 2 . These rubbers are equivalent to those used for the green strength measurements. The tack strength values of high trans SBR and NR are nearly the same for the various measured contact times. I t i s significant that both rubbers have comparable tack strength for low contact times ( s i x seconds). This behavior i s often referred to as "quick-grab". Concluding Remarks Although the exact nature of the active center i n polymerizations of butadiene with these Ba-Mg-Al catalysts i s not known, we believe that the preference for t r a n s - 1 , 4 addition is a direct consequence of two aspects of this polymerization system, namely: ( 1 ) the formation of a specific organobarium structure i n a highly complexed state with Mg and Al species, and ( 2 ) the association of the polybutadiene chain end with a dipositive barium counterion which i s highly electropositive. The u t i l i t y of solution SBR as tread rubbers has become generally accepted i n the t i r e industry. As we have shown, high t r a n s - 1 , 4 c r y s t a l l i z i n g SBR's are excellent candidates for other parts of the t i r e as well. The outstanding green strength and building tack of high trans containing SBR's certainly j u s t i f y their consideration as t i r e rubbers of the future. Acknowledgments As i s the case for most research accomplishments of today, this work required the combined efforts of many people. The authors are indebted to these people and are grateful for the contributions that they have made. A special note of thanks to J . A. Wilson for h i s experimental assistance and to R. E.




HARGis E T A L .


100

ELASTOMERS

AND RUBBER

ELASTICITY

Bingham, R. R. Durst, and H. J . Fabris f o r their contributions i n the development of the Ba-Mg-Al polymerization catalyst.


Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.

9.

10. 11. 12. 13.

Horne, S. E. Jr. Rubber Chem. & Technology 1980, 53, G71. Stavely, F. W.; and coworkers. Ind. Eng. Chem. 1956, 48, 778. Horne, S. E. Jr. Rubber Chem. & Technology 1980, 53, G74. Hsieh, H. L . ; Glaze, W. H. Rubber Chem. & Technology 1970, 43, 52. Morton, M.; Fetters, L. J. Rubber Chem. & Technology 1975, 48, 359. U. S. Patents 3,992,561 (1976); 4,020,115; 4,033,900; 4,048,427 (1977); and 4,297,240 (1981) to The General Tire & Rubber Company. Newberg, R. G.; Greenberg, H. Rubber Chem. & Technology 1970, 43, 333. Livigni, R. Α.; Hargis, I. G.; Aggarwal, S. L. "Structure and Properties of Rubbers for Tires and New Developments for Crystallizing Butadiene Rubbers"; Paper presented at I n t ' l . Rubber Conf., Kiev, U.S.S.R., 1978. Aggarwal, S. L.; Hargis, I. G.; Livigni, R. A. "A New Class of Crystallizing Butadiene Elastomers: Morphology and Properties. II."; Paper presented at Int'l. Symp. on Macromol. Chem., Tashkent, U.S.S.R., 1978. Aggarwal, S. L.; Hargis, I. G.; Livigni, R. A. Polymer Preprints 1981, 22(2), 134. Stein, R. S. "Rheology"; Eirich, F. R., Ed.; Academic Press, New York and London, 1969; Vol. V, p. 279. Mandelkem, L. "Crystallization of Polymers"; McGraw Hill, New York, 1964; p. 264. Briggs, G. J.; Holmes, J. M.; Wei, Y. K. Elastomerics 1979, 111(8), 30.

R E C E I V E D March 1,

1982.


Elastomers and Rubber Elasticity - American Chemical Society

Recommend Documents