Shear Direction Dependence of Yield Stress Growth in Glassy Polycarbonate Stress-Relaxed under Post-Yield Shear Strain

To study the mechanism of nonlinear viscoelastic behavior of glassy polymers, glassy polycarbonate was subjected to shear deformation performed with multistep switching of shear rate ͘γ, including switching to ͘γ = 0 (stress relaxation), to ͘γ = −͘γ (reversal of direction) as well as to ͘γ = +͘ γ. Birefringence was measured along with shear stress to obtain glassy and rubbery components of stress, τG and τR, by means of modified stress-optical rule (MSOR). When strained after stress relaxation, τG showed a yield point which grew with increasing time elapsed under stress relaxation, independently of mechanical histories before stress relaxation. The growth of yield point was faster in specimens deformed to direction where relaxed stress turned to increase than opposite direction. This observation indicates that the plastic flow of glass is achieved when the absolute value of the increase or decrease in glassy stress τG from the beginning of deformation after stress relaxation comes to a certain value corresponding to deformation condition, i.e., shear rate in this case. Thus, it is concluded that the mechanical rejuvenation, the origin of acceleration of viscoelastic relaxation due to deformation, is a consequence of rotations of molecular segments around chain axis.