NONLINEAR STRESS RELAXATION OF POLYISOBUTYLENE IN SIMPLE EXTENSION

In the reported experimental study, relaxation of stress in simple extension of polyisobutylene has been measured at stretch ratios ( lambda ) up to about 2. Data at 3, 25, and 50 degree C were reduced to 25 degree C. At any instant of reduced time, the dependence of stress on lambda could be described by the Mooney-Rivlin equation; the coefficients C//1 and C//2 were obtained from two linear plotting procedures. At short times, C//1 decreased rapidly while C//2 remained essentially constant. Tentative interpretations in terms of motions of entanglements are discussed. Similar measurements on a sample of polyisobutylene which had been heated with vulcanizing ingredients and was very slightly crosslinked gave higher values of C//2 but lower values of C//1.