Influence of hydrostatic pressure on rheological properties of polymer melts—A review

Rheological properties of polymer melts are significantly dependent on molecular structure and external parameters. While numerous experimental data of the dependence of the rheological behavior on stress, shear rate, and temperature are available in the literature, much less is known on the effect of hydrostatic pressure. This article reviews the knowledge about the pressure dependence of rheological properties of polymer melts. The different experimental devices for measurements under pressure are described, and the methods of determining the pressure coefficients of viscosity are critically analyzed. Very few investigations of the pressure dependence of viscoelastic properties are addressed. A comparison of the pressure coefficients of commercial polymers obtained by various authors from viscosity measurements with different methods shows the deficits in getting exact data. It can be said, however, that the pressure coefficients increase in the order of high density polyethylene, linear low-density polyethylene, low-density polyethylene, polypropylene, and polystyrene. For amorphous polymers and, particularly, for polystyrene, it is shown how the pressure coefficients decrease with growing temperature. For polyolefins, this dependency is less significant. The free volume concept is discussed with respect to an interpretation of the experimental findings. Results of using the hole fraction theory for describing the pressure dependence of viscosity are presented and critically assessed.