ABSTRACT
Surface tension of a polymer melt in a supercritical fluid is a principal factor in determining cell nucleation and growth in microcellular foaming. This chapter introduces recent works on the surface tension measurement of two polymer melts in supercritical fluids under various temperatures and pressures. One is the amorphous polymer, polystyrene (PS), in supercritical CO2 and the other is the crystalline polymer, high density polyethylene (HDPE), in supercritical N2. The surface tension was determined by Axisymmetric Drop Shape Analysis-Profile (ADSA-P). The dependence of the surface tension on temperature, pressure and polymer molecular weight will be discussed. At temperatures above the polymer melting points, the surface tension of PS was found to be similar to that of HDPE; the surface tension decreased with increasing temperature and pressure. Self-consistent field theory (SCFT) calculations were applied to simulate the surface tension of corresponding systems and used to explain the results. Below the melting point, PS solidified and the surface tension did not change any further; but HDPE underwent the process of crystallization, where the surface tension dependence on temperature was different from that above the melting point, and the surface tension decreased with decreasing temperature. It was found that the amount of decrease in surface tension was related to the rate of temperature change and hence the extent of polymer crystallization.
