Nonlinear Stress Relaxation of Polyisobutylene in Simple Extension

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Abstract
Relaxation of stress in simple extension of polyisobutylene (viscosity‐average molecular weight 2.3×10 6 ) has been measured at stretch ratios (λ) up to about 2. Data at 3, 25, and 50°C were reduced to 25°C. At any instant of reduced time, the dependence of stress on λ 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. Two to three logarithmic decades of time later, C 2 decreased. Thus, the terminal zone of viscoelastic behavior corresponds solely to the C 2 contribution. This result confirms measurements by Arai and Niinomi (Kogyo Kagaku Zasshi, 74, 2525 (1971)) on polyisoprene and styrene‐butadiene rubber and measurements by Noordermeer and Ferry (J. Polym. Sci., (1976), in press) on 1,2‐polybutadiene. 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 .

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