High pressure crystallization of ultra-high molecular weight polyethylene

Abstract

The morphology of pressure-crystallized ultra-high molecular weight (2.5×106) polyethylene samples was studied by scanning electron microscopy. The DSC and small angle X-ray analyses were also performed. High pressure crystallization (HPC) (at a cooling rate of 2 K min−1 from the melt at 590 MPa), of an as-polymerized powder sample gives a fibrous band in morphology. The fibrous band is formed by the parallel arrangement of long fibrils whose diameters are less than about 500 Å. The width of the fibrous band is about 0.2 μm. The HPC of a bulk sample which was kneaded in the molten state at atmospheric pressure gives three types in its morphology: Isolated fibril, textile structure and band structure. The textile structure is the network superstructure of the fibrils, and the band structure is also formed by the parallel arrangement of the fibrils. The parallel arrangement of the fibrils induced by the pressure in the molten state occurs prior to the growth of extended-chain crystals (ECC). The heat of fusion of the pressure-crystallized powder sample is much lower than that of an ECC sample of medium molecular weight PE. Though the lamellar thickness of powder and bulk samples increases by HPC, that of the pressure-crystallized bulk is not so large, and its distribution is wide. © 1987 The Society of Polymer Science, Japan.