Metastability and order in linear, branched and copolymerized polyethylenes

Abstract

The melting behaviour of narrow-molar-mass-distribution fractions of linear polyethylenes during commonly used dynamic cooling and heating procedures – due to reorganization processes – shows barely any relationship with their crystallization behaviour during normal dynamic cooling. When short chain branching (SCB) is introduced, as in for example HDPE and LDPE fractions, the effects become smaller but they do not disappear. Measurements show that besides known influences such as thermal history and SCB, the chain length also has a considerable influence on the crystallization behaviour. For homogeneous and heterogeneous copolymers with large variations in comonomer content the crystallization behaviour, morphology and melting behaviour were studied in relation to the chain macro- and microstructure. In homogeneous ethylene copolymers, the morphology changes considerably as the comonomer content increases: at low comonomer contents folded-chain crystallization in a lamellar base morphology prevails, with the lamellae usually being organized 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 simulations point to a fractal growth type crystallization, where clusters of `loosely packed ethylene sequences' are formed. Both d.s.c. and real-time SAXS yield detailed information for all copolymers, even for the virtually amorphous copolymers which crystallize and melt just above the glass transition temperature. Even in a homogeneous copolymer, several of the above-mentioned morphologies can occur side by side due to the ethylene sequence length distribution in the polymer. In heterogeneous copolymers such as LLDPE and VLDPE, practically all of the above-mentioned morphologies can be found in one and the same sample.