High-pressure crystallization and melting of polyethylene in n-pentane

The fluid–solid phase separation and crystallization in relatively dilute solutions of polyethylene in n-pentane at high pressures were studied using time- and angle-resolved laser light scattering techniques. Majority of the experiments were carried out by cooling (crystallization) or heating (melting) while holding the pressure constant at selected pressures in the range from 10 to 54 MPa. Crystallizations were also carried out via two other pathways: (1) cooling without pressure adjustment and (2) first crossing the L–L phase boundary via pressure reduction at constant temperature followed by cooling. Crystallization and melting transitions were assessed from the variations of the transmitted light intensity or the scattered light intensity (averaged over all angles) with temperature. Kinetics of phase separation during both the crystallization and the melting were followed by the time evolution of the angular distribution of the scattered light intensities. The kinetics were typical of nucleation and growth processes. From the time evolution of the light scattering patterns the mean particle radii and their evolution were also followed. The polymer particles that form upon crystallization were collected and characterized using a field emission scanning electron microscope (FESEM) and differential scanning calorimetry (DSC). The results show that the PE crystals that form from these high-pressure conditions display a plate-like morphology, which tend to aggregate into ellipsoid-shape structures. DSC studies show that the PE crystals show higher crystallinity compared to the original polyethylene sample and display multiple melting peaks during the first heating scan, which however collapse to a single melting transition peak after the initial heating scan. The crystals that formed from the experiments in which L–L phase boundary was crossed first were found to display two distinct particle groups. These were attributed to crystals that form from polymer-rich and polymer-lean phases that form when L–L phase boundary is crossed.