Nonlinear relaxation of stress and birefringence in simple extension of 1,2-polybutadiene

Relaxation of stress and birefringence in simple extension has been studied for two samples of 1,2-polybutadiene with 95% and 88% vinyl content and weight-average molecular weight 1.9 and 2.9 × 10⁵, respectively. The extension ratio, λ, ranged from 1.14 to 2.08, temperatures from 0 to 15°C, and times, reduced to 0°C, up to 3 × 10⁵ sec. The stress-optical coefficient C was negative and positive, respectively, for the two samples, the difference being attributable to opposite signs and very different magnitudes of the contributions of the 1,2 and 1,4 moieties to the birefringence. For each polymer, C was independent of time but increased (algebraically) with temperature. For one polymer a very minor dependence of C on λ was observed. At any instant of time, the dependence of both stress and birefringence on λ could be described by equations of the Mooney–Rivlin form with coefficients C₁,C₂ and B₁,B₂, respectively. At short times the contributions of the C₁ and C₂ terms to the stress and of the B₁ and B₂ terms to the birefringence are roughly equal. With increasing time, C₁ and B₁ decrease gradually while C₂ and B₂ remain constant over several decades in time. Finally, C₂ and B₂ decrease rather rapidly. A tentative interpretation of these phenomena in terms of motions of entanglements is given.