The Effects of Temperature and Pressure on the Dynamic Longitudinal Volume Viscosity of Two Model Polymers

A method of studying dynamic longitudinal volume viscosity (17*) at high pressure using a modified Instron capillary rheometer is described. Two model polymers were chosen as examples: an amorphous polysulfone and a semicrystalline high-density polyethylene. Dynamic longitudinal volume viscosity was measured at a fixed pressure using a continuous temperature sweep. The pressure ranged up to 2000 bar, while temperatures were swept through the liquid to solid transition for each of the materials. The effects of frequency (from 0.0125 to 0.125 Hz) and strain (0,06 to 0.24%) on tjt were also investigated. In both polymer systems, |?7*[and 171 increased as the pressure was increased or the frequency was decreased. In spite of the small strains employed, the viscosity was found to decrease as the strain was increased. The temperature-dependent 17t varied with the nature of the transitions. In polysulfone, |i7*| decreased sharply with temperature above the T_g. However, in polyethylene, a positive dependence of |i7*| on temperature, limited to only a small temperature range above T_m, was observed. Under all conditions, |tj*| varied only one decade, a much smaller change than that found for shear viscosity.