On High-Pressure Crystallization And The Characterization Of Linear Low-Density Polyethylenes

High pressure crystallization has been used to compare three linear low-density polyethylenes because of its potential for forming lamellae whose thicknesses are equal to the inter-branch separation. Major differences are revealed between one polymer prepared using a metallocene catalyst and the other two which have been generated by conventional Ziegler-Natta synthesis. The broad distribution of inter-branch lengths in the latter two polymers and the narrow spectrum of the former are revealed both by temperature rising elution fractionation (TREF) thermograms and the melting endotherms of the respective polymers after high pressure crystallization. The metallocene-catalysed polymer shows unprecedented behaviour in that its lamellar thickness is invariant. In consequence, it crystallizes at high pressure entirely as orthorhombic lamellae; the hexagonal phase is not formed.