Surface Tension Measurement of High Density Polyethylene and Its Clay Nanocomposites in Supercritical Nitrogen

Abstract

Surface tension of a polymer melt in a supercritical fluid is a principal factor in determining cell nucleation and growth in polymer microcellular foaming. Previous work has presented the surface tension of the amorphous polymer, polystyrene (PS), in supercritical CO2 determined by Axisymmetric Drop Shape Analysis-Profile (ADSA-P), together with theoretical calculations for a corresponding system. The dependences of the surface tension on temperature, pressure and polymer molecular weight were discussed and the physical mechanisms for three main experimental trends were explained using Self Consistent Field Theory (SCFT). This thesis introduces recent work on the surface tension measurement of the crystalline polymer, high density polyethylene (HDPE), in supercritical N2 under various temperatures and pressures. The surface tension was determined by ADSA-P and the results were compared with those of the amorphous polymer PS. 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 PS; that is, the surface tension decreased with increasing temperature and pressure. Below 125°C and above 100°C, HDPE underwent a process of crystallization, where the surface tension dependence on temperature was different from that above the melting point, i.e., 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 the 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. In the second part of the thesis, surface tension dependences on temperature, pressure and clay concentrations were studied for HDPE nano-clay composites (HNC) and compared with pure HDPE. It was found the trends with temperature and pressure were the same with PS in CO2 and HDPE in N2; that is, the surface tension decreased with increasing temperature and pressure. In all nanocomposite samples, the surface tension decreased compared with pure HDPE. This could be a good explanation for the better polymer foaming quality with the addition of clay in the polymer. A minimum surface tension was found with the sample at ~3% concentration of clay. The degree of crystallinity of HNC was analyzed by Differential Scanning Calorimetry (DSC) at different clay concentrations. A minimumz crystallinity was also found at the clay concentration of 3%. The possible relationship between surface tension and polymer crystallinity was discussed.