Pressure effects in polymer melt rheology

Abstract

Abstract Wide ranges of pressure and temperature are encountered in polymer processing operations, as, for example, in injection molding. While the temperature dependence of viscosity has been widely studied, the pressure dependence has not. The present work focuses on the measurement of the melt viscosity of polystyrene at high pressures (up to 124 MPa or 18,000 psi) and high shear rates (1–100 s −1 ) at 180°C. The apparatus was a capillary rheometer with the downstream chamber being held at a high back pressure by means of a needle valve. The data so obtained were combined with zero shear viscosity data from the literature; and then correlated with a shear‐dependent rheological model of the authors, using a shift factor suggested by Utracki (based on the Simha–Somcynsky equation of state). The final correlation calls for making both the elastic modulus and the time constant dependent on pressure, with the modulus being the dominant factor at high shear rates.