Crystallization of Polyethylene from Melt with Lowered Chain Entanglements

A method of obtaining polymer with markedly decreased entanglements concentration in melt, via high-pressure crystallization of high-density polyethylene (HDPE), is elaborated. It is shown, by calculations and experiments, that melting of chain-extended crystals obtained in this process provides a chain-disentangled melt for a period of 20−30 min. The disentanglement is tested by means of spherulite growth rate measurements in a regime II melt crystallization. Spherulites grow faster from partially disentangled melt than from melt with normal concentration of entanglements: the growth rate is increased by 25−45%, and the conversion rate is also markedly higher. The difference is augmenting with decreasing undercooling consistently with conclusions made from nucleation theory including reptation and decaying with increasing time of melt annealing at 160 °C before crystallization, which is a result of entanglements reconstitution. The crystallization behavior in initially chain disentangled samples subjected to 25−30 min melt annealing is typical of entangled polyethylene, which indicates a complete entanglement restoration. The activation energy for reptation, determined from these data, is approximately 25 kJ/mol. Differences in overall isothermal crystallization kinetics of chain entangled and chain disentangled samples are considerable at moderate undercooling. The nucleation density during crystallization from disentangled melt is reduced in consequence of desorption of chains from heterogeneities surfaces during prior high-pressure crystallization of the samples. Melt annealing causes readsorption and restoration of normal nucleation density. The morphology of samples crystallized from a chain-disentangled melt is significantly different than those crystallized from a chain-entangled melt while the crystal thicknesses are similar.