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

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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<γ2<0 for the uncross-linked sample, but it agreed at any γ2 for the corss-linked sample. At −(3/2) γ1<γ2<0, the uncross-linked sample showed irreversible change in the height of rubbery plateau, while the cross-linked sample showed reversible change. Such the differences between the two samples are discussed in terms of the two origins for nonlinear viscoelasticity of well entangled polymers, chain anisotropy and disentanglement.
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