Surface tension of high density polyethylene (HDPE) in supercritical nitrogen: Effect of polymer crystallization

Surface tension of a polymer melt in a supercritical fluid is a principal factor in determining cell nucleation and growth in microcellular foaming. This work focuses on the surface tension of a crystalline polymer, high density polyethylene (HDPE), in supercritical nitrogen under various temperatures and pressures. The surface tension was determined by Axisymmetric Drop Shape Analysis-Profile (ADSA-P). The dependence of the surface tension on temperature and pressure, at temperatures above the HDPE melting point, ∼125 °C, was found to be similar to that of the amorphous polymer polystyrene (PS) in supercritical CO2, previously reported, i.e., the surface tension decreased with increasing temperature and gas content. Below 125 °C and above 100 °C, HDPE underwent the process of crystallization, where the surface tension dependence on temperature was different from that above the melting point, and decreased with decreasing temperature. Differential scanning calorimetry (DSC) characterization of the polymer was carried out to reveal the process of HDPE crystallization and relate this to surface tension behavior. It was found that the amount of the decrease in surface tension was related to the rate of temperature change and hence the extent of polymer crystallization.