A RHEOOPTICAL STUDY ON POLYSTYRENE UNDER LARGE TENSILE DEFORMATION AROUND THE GLASS-TRANSITION TEMPERATURE

The tensile stress and the birefringence of polystyrene were measured under elongation at a constant rate up to the elongation ratio of 4 at 115, 105, and 100 degrees C. The tensile stress was separated into tw o component functions, designated with subscripts R and G, through a m odified stress-optical rule, MSOR, considering the effect of finite ex tensibility of the polymer chain on the stress dependence of the stres s-optical coefficient. The R component represents the stress attribute d to the polymer segment orientation and the G component to the twist of the polymer chain. At 115 degrees C, the time dependence of the vis cosity growth function of the two components, eta(ER)(+) and eta(EG)(), can be described in the framework of linear viscoelasticity except for a steep increase of eta(ER)(+) at times longer than 1/(2(epsilon) over dot), where (epsilon) over dot is the rate of elongation. It is w ell-known that this type of steep increase is due to the strong increa se of strain measured under elongation at a constant rate. At lower te mperatures, eta(EG)(+) decreased with increasing strain rate when the rate exceeds 1/1000 of the characteristic relaxation rate of the G com ponent. The steady value of the elongational viscosity, eta(EG)((epsil on) over dot), at various temperatures supported a master curve when e ta(EG)((epsilon) over dot)/eta(EG)(0) is plotted against (epsilon) ove r dot a(TG), where a(TG) is the shift factor for the G component deter mined in dynamic viscoelastic measurements. On the other hand, eta(ER) (+) is always close to the linear viscoelasticity except for the steep rise at t > 1/(2(epsilon) over dot). The relaxation rate of the R com ponent was enhanced in proportion to eta(EG)(0)/eta(EG)((epsilon) over dot) when the G component showed thinning. Thus, MSOR analysis simpli fies the complicated nonlinear viscoelastic response of amorphous poly mers around the glass transition temperature.