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

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Abstract
This paper describes an experimental technique for determining visocelastic 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 Voigt model. Polymers evaluated were several commercial polystyrenes. Experimental data indicate that the creep curves obtained at different temperatures 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 samples. Activation energy of polystyrene due to viscoelastic deformation is compared with the activation energy due to viscous flow deformation. For temperatures greater than 120 °C, the values obtained by the two types of deformation are equal, however, for temperatures lower than 120 °C, the activation energy due 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.

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