Nonlinear Viscoelastic Properties and Change in Entanglement Structure of Linear Polymers 2. Double-step Large Shearing Deformations at Moderately Long Time-scale

Simultaneous measurements of stress relaxation and differential dynamic modulus were made at 268K for nearly monodisperse polybutadiene (Mw=2.2×105, 1,2-structure 70%, Me=3.6×103) and also one having coarse cross-linking (Mc=2.6×104) in double-step large shearing deformations with intermittently superposed small oscillations at a frequency of 0.1Hz, which falls in the middle of the rubbery plateau zone of the polymer. The first and the second strains, γ1 and γ2, were applied with a time interval t1. γ1 was constant 1.0. γ2 was varied from 1.0 to −2.0. t1 was 1000s being several times longer than τk, after which time-strain separability relationship holds in nonlinear relaxation modulus. Relaxation of stress after the second strain did not agree with BKZ prediction at −(3/2) γ1