Rheological Properties of Molten Polymers Part II Creep Function of Commercial Polystyrene

This paper describes an experimental technique for determining viscoelastic constants for molten polymers by tensile creep experiments. The constants determined were retardation times, shear compliances, zero-shear flow viscosity, and shift factors. Data are given in terms of a generalized Voift model. Polymers evaluated were several commercial polystyrenes. Experimental data indicate that the creep curve obtained at different temperature do not exactly shift according to the time-temperature superposition principle. A possible explanation of the discrepancy is proposed in the paper. Shift factors as obtained by creep experiments are compared with calculated values obtained by the WLF equation. Agreement is poor. However, the general expression of WLF type is applicable in correlating shift factors as a function of temperature. New sets of constants are obtained for polystyrene due to viscoelastic deformation is compared with the activation energy due to viscous flow deformation. For temperatures greater than , the values obtained by the two type of deformation are equal, however, for temdue to viscoelastic deformation is much greater than viscous deformation. The activation energy obtained for viscoelastic deformation at lower temperatures agrees with values by retraction studies and birefringence measurements.