Effect of Molecular Weight on the Surface Tension of Polystyrene Melt in Supercritical Nitrogen

Abstract

This paper presents experimental results on the effect of molecular weight on the surface tension of polystyrene melt in supercritical nitrogen. The surface tension was determined by the axisymmetric drop shape analysis-profile (ADSA-P) method, for which a high-pressure and high-temperature cell was used to form pendant drops of the polystyrene melt. For two monodisperse polystyrenes of Mw ∼ 100 000 and 400 000 and one polydisperse polystyrene, a linear relationship was found between surface tension and temperature and between surface tension and pressure within a temperature range of 170−210 °C and a pressure range of 500−2000 psi. With an increase in pressure or temperature, the surface tension of all three polystyrenes decreases. Monodisperse polystyrene of a higher molecular weight has a higher surface tension by 6−9 mJ/m2 under all experimental conditions. The surface tension dependence on temperature and on pressure is more significant for the higher molecular weight polystyrene. For the polydisperse polystyrene, high surface tension values seem to be related predominantly to its high molecular weight portion of polystyrene molecules. An empirical equation, the Mecleod relation, was used to relate surface tension with the density difference between the polymer and supercritical nitrogen satisfactorily.