Thermoelastieity of rubber vulcanizates. I

A new method for evaluating the temperature coefficient of the unperturbed dimensions <r 2 > 0 of a polymer chain from stress‐temperature data was proposed assuming that the parameter C 1 in the MOONEY‐RIVLIN equation (f = (C 1 + C 2 /α)(α−1/α 2 ), where f is the tensile force and α the elongation ratio) has the same form as the factor in the f vs . α relation predicted by the current theory of GAUSSIAN networks. Precise stress‐temperature measurements were performed for two cis ‐1,4‐polybutadiene vulcanizates and one natural rubber vulcanizate, and the results were analyzed by the new equation and also by the familiar FLORY equation. The latter equation led to dln<r 2 > 0 /dT (T is the absolute temperature) which decreased with increasing α, as had been observed for many crosslinked polymers by previous workers. The new equation, on the other hand, gave values which were actually independent of α. These are 0.4 10 −3 (deg. −1 ) for natural rubber and ‐0.4.10 −3 (deg. −1 ) for cis 1,4 polybutadiene. This latter value is sharply contrasted with 0.4.10 −3 (deg. −1 ) reported by CRESPI and FLISI who used the usual FLORY equation, but is consistent, at least in sign, with −1.3 · 10 −3 (deg. −1 ) that has been deduced from recent viscosity data of ABE and FUJITA in a series of chemically similar theta solvents. However, one must wait until the molecular interpretation of the C 1 term in the MOONEY‐RIVLIN equation becomes available, in order to know whether the temperature coefficient of <r 2 > 0 calculated by the proposed method is physically meaningful.