Effect of Cis-Trans Ratio on the Physical Properties of 1,4 Polybutadienes

The physical properties of vulcanized 1,4 polybutadienes, prepared by heterogeneous catalysis and ranging from 95 per cent cis to nearly 100 per cent trans configuration, were investigated in gum and tread formulations. Comparisons were made at equal effective (physical) degree of crosslinking for both gum- and carbon black-reinforced stocks and for black stocks at equal 300 per cent modulus. In general, as polybutadienes require less sulfur for adequate vulcanization than trans polybutadienes. Vulcanizates of all 1,4 polybutadienes exceeding 15 per cent in cis content are completely rubbery at ordinary temperatures. Polybutadiene of 93 per cent trans content yields tough, leathery, crystalline vulcanizates at 80° F, which become rubbery at moderately elevated temperatures. Without exception the important physical properties change little between 25 and 80 per cent cis content. At both ends of this range the tensile strength of both gum and black stocks rises as a consequence of increasing chain regularity. However, in gum tensile strength not even the highest cis polybutadienes are equivalent to natural or synthetic cis-polyisoprenes. Black stocks of very good tensile strength with elongations ranging from 500 to 700 per cent are obtained with both cis- and trans-polybutadienes. At the crosslinking level for optimum tensile strength, modulus increases with trans content. The cis-polybutadienes have excellent resilience and low hysteresis and maintain their resilience to temperatures as low as −40° F. The latter is true even of the highest as polymers which crystallize at these temperatures. The tendency to crystallize rapidly at low temperatures disappears between 87 and 82 per cent cis content so that polymers of moderately high cis unsaturation have exceptional low-temperature characteristics. These polymers remain completely rubbery down to their brittle point (ca. −85° C). The excellent resilience of cis-polybutadiene is particularly apparent in carbon black-reinforced stocks. Because of the inherently low modulus of these stocks and their low sulfur requirement, these may be vulcanized to rather high degrees of crosslinking. This results in further improvements in resilience and heat build-up with only moderate sacrifices in tensile strength and ultimate elongation. Vulcanizates of 70 to 80 per cent trans-polybutadienes exhibit evidence of crystallinity over a wide range of temperatures and are probably not completely melted at room temperature. Although their resilience is less than that of the high cis-polybutadienes, it is still somewhat better than that of SBR, particularly at low temperatures. The 1,4 polybutadienes are more resistant to oxidative scission than emulsion polybutadiene or SBR and are greatly superior to natural rubber in this respect. The dominant effect in the aging of 1,4 polybutadienes is crosslinking. Because of their low sulfur requirements the cis polybutadienes offer a particular advantage in aging resistance. None of the polybutadienes exhibits ozone resistance comparable to Hevea.