Viscoelastic properties of 1,2‐polybutadiene—comparison with natural rubber and other elastomers

Abstract

AbstractViscoelastic properties of uncrosslinked 1,2‐polybutadiene (91.5% vinyl, 7.0% cis, 1.5% trans, number‐average molecular weight 99,000) were studied by dynamic shear measurements between 0.15 and 600 cps (torsion pendulum and Fitzgerald transducer) and shear creep measurements over time periods up to 3.7 × 104 sec., in the temperature rang from 5 to 50°C. More limited dynamic measurements were made on a sample of unvulcanized natural rubber with number‐average molecular weight 350,000 at frequencies from 0.4 to 400 cps and temperatures from 13 to 48°C. All data were reduced to 25°C. by shift factors calculated from equations of the WLF form with the following coefficients: 1,2‐polybutadiene, c1 = 6.23, c2 = 72.5; natural rubber, c1 = 5.94, c2 = 151.6. In the transition zone, the relative positions of the loss tangent curves on the logarithmic frequency scale for these and other rubbers (1,4‐polybutadiene with 50% trans configuration; styrene–butadiene rubber with 23.5% styrene content; and polyisobutylene) provided relative measures of local segment mobility. At 25°C., these ranged over a factor of 3700 with 1,2‐polybutadiene and polyisobutylene the lowest and 1,4‐polybutadiene the highest. When the frequency scale of each rubber was reduced to a temperature 100°C. above its glass transition temperature, however, the loss tangent curves for all except polyisobutylene were nearly coincident; the latter still showed a lower mobility by a factor of about 1/800. The terminal relaxation time and steady‐state compliance for the 1,2‐polybutadiene calculated from the Rouse theory were larger than those observed experimentally. The level of compliance corresponding to the entanglement network of 1,2‐polybutadiene, JeN, was calculated by integration over the loss compliance, J″, to be 1.62 × 10−7 cm.2/dyne; integration over G″ to obtain the corresponding modulus gave reasonable agreement. From such JeN, values, the average number of chain atoms between entanglement points, jZe, was estimated as follows: 1,2‐polybutadiene, 132; natural rubber, 360; 1,4‐polybutadiene, 110; styrene–butadiene rubber, 186; polyisobutylene, 320. Values of jZe were also estimated from the minimum in the loss tangent and compared with those reported from the molecular weight dependence of viscosity. The three sources were in generally good agreement.