LAMELLAR THICKENING OF POLYETHYLENE UNDER HIGH-PRESSURE

Single crystal mat (SCM) samples of polyethylene (PE) were prepared fr om dilute solution of p-xylen, then they were annealed at pressures of 200 and 500 MPa. Lamellar thickness of the original and annealed SCM samples was measured by small-angle X-ray scattering method. Orientati on of the molecular chain in those SCM samples was investigated by wid e-angle X-ray diffraction pattern. From these X-ray measurements, anne aling temperature dependence of the lamellar thickness, i.e., lamellar thickening, under high pressure was obtained. Melting process of the SCM samples was also investigated at 200 and 500 MPa by high pressure differential thermal analysis. Then correspondence between the lamella r thickening and the melting process was studied. The lamellar thickne ss increases markedly with approaching to the melting temperature of t he orthorhombic crystal even in the high pressure region where the hig h pressure phase (hexagonal phase) appears. The annealing temperature dependence curve of the lamellar thickness at 200 MPa can be superimpo sed on the curve at 500 MPa by shifting the curve along the temperatur e scale by 47 K. Large scale lamellar thickening occurs in the orthorh ombic crystal phase in the high pressure region. The formation process of extended-chain crystal is discussed. (C) 1997 John Wiley & Sons, I nc.