Frequency Dependence of Nonlinear Viscoelastic Relaxation in Glassy Epoxy Network Subjected to Constant-Speed Stretching

Abstract

Frequency f and strain rate ̇ε_n dependences of linear dynamic shear modulus were measured for glassy epoxy network during uniaxial stretching processes. With increasing strain ε_n, the storage shear modulus G´ slightly decreased to a steady value appearing at post-yield strain-hardening range of strain. The loss shear modulus G˝ markedly increased in the same strain range, where G´ decreased, and then leveled off. These variations of G´ and G˝ indicated that the glassy structure in the epoxy network changed into more unstable ones by stretching. When compared at a fixed condition of α= ̇ε_n/f, the functional relation between strain-induced increment of G˝ and ε_n was identical independently of ̇ε_n of stretching. Thus, frequency dispersion of the nonlinear relaxation was found to be determined only by the relative distance from the timescale of deformation and the amount of imposed strain. Whereas the decrement of G´ at a fixed α was not superposable when plotted against ε_n, because of (ε_n ) ̇ dependence of their steady values. The variation of G´ was affected not only by destabilization of glassy structure due to deformation. The observed f and ̇ε_n dependences of G´-ε_n and G˝-ε_n relations for glassy epoxy network during stretching were qualitatively the same as those observed for poly(methyl methacrylate) (PMMA). Thus, the dependence on ε_n, f and ̇ε_n of the nonlinear relaxation under constant-speed deformation conditions reported here is presumably universal for glassy polymers. The strain-induced variation of G´ and G˝ was smaller for epoxy network compared with PMMA stretched exactly at the identical condition. This result indicates that glassy structures in the epoxy network before stretching are more unstable because of constraint arising from crosslinked molecular structures.