Frequency Dependence of Nonlinear Viscoelastic Relaxation in Glassy Poly(methyl methacrylate) (PMMA) Subjected to Constant-Speed Stretching

Variation of complex shear modulus of glassy poly(methyl methacrylate) (PMMA) was monitored during uniaxial stretching to investigate the frequency dispersion of strain-induced nonlinear relaxation. With increasing strain εn, the storage modulus G' decreased to a steady value appearing at post-yield range of strain accompanied by a marked increase of the loss modulus G", indicating that glassy structures changed into more unstable ones due to stretching. The variation of the moduli was more remarkable when the timescale of observation, i.e. frequency ƒ of dynamic measurement, was closer to that of deformation, i.e. strain rate εn. Increment of G" as a function of εn was identical independently of εn when observed at a fixed condition of α = εn/ƒ. This observation indicates that β relaxation is not altered by imposition of large strain and that the frequency dispersion of the nonlinear stress relaxation is determined by relative distance from the timescale of deformation and the amount of imposed strain. Relationships between ΔG', decrement of G', and εn were not superposable at a fixed α, because of negative εn dependence of ΔG' in the post-yield regime. This presumably shows that strain aging, structural relaxation of the strain-induced unstable glassy structures under finite stress, occurs during deformation.