METASTABILITY AND ORDER IN LINEAR, BRANCHED AND COPOLYMERIZED POLYETHYLENES

The melting behaviour of narrow-molar-mass-distribution fractions of l inear polyethylenes during commonly used dynamic cooling and heating p rocedures - due to reorganization processes - shows barely any relatio nship with their crystallization behaviour during normal dynamic cooli ng. When short chain branching (SCB) is introduced, as in for example HDPE and LDPE fractions, the effects become smaller but they do not di sappear. Measurements show that besides known influences such as therm al history and SCB, the chain length also has a considerable influence on the crystallization behaviour. For homogeneous and heterogeneous c opolymers with large variations in comonomer content the crystallizati on behaviour, morphology and melting behaviour were studied in relatio n to the chain macro- and microstructure. In homogeneous ethylene copo lymers, the morphology changes considerably as the comonomer content i ncreases: at low comonomer contents folded-chain crystallization in a lamellar base morphology prevails, with the lamellae usually being org anized into spherulitic superstructures. At higher comonomer contents, the crystallite thickness decreases and the lateral dimensions become smaller, until ultimately there is folded-chain crystallization in a granular base morphology without any organization into superstructures . At the highest comonomer contents, experiments and Monte Carlo simul ations point to a fractal growth type crystallization, where clusters of 'loosely packed ethylene sequences' are formed. Both d.s.c. and rea l-time SAXS yield detailed information for all copolymers, even for th e virtually amorphous copolymers which crystallize and melt just above the glass transition temperature. Even in a homogeneous copolymer, se veral of the above-mentioned morphologies can occur side by side due t o the ethylene sequence length distribution in the polymer. In heterog eneous copolymers such as LLDPE and VLDPE, practically all of the abov e-mentioned morphologies can be found in one and the same sample. (C) 1998 Elsevier Science Ltd. All rights reserved.