Crystallization thermodynamics and kinetics in semicrystalline diblock copolymers

The crystallization of a series of diblock copolymers containing poly(ethylene) (PE) has been studied using simultaneous small-angle and wide-angle X-ray scattering (SAXS/WAXS). The PE crystalline domain thickness, obtained from a correlation function analysis of the SAXS data, was found for all samples to be independent of quench temperature because it is set by the branching density of the PE. In contrast to the PE crystalline domain thickness, the lamellar domain spacing of the homopolymers and block copolymers is found to decrease for deeper quenches, reflecting an increased degree of crystallinity of the PE. The equilibrium melting temperature of a PE homopolymer (M-w = 23 000) prepared by hydrogenating poly(1,4-butadiene) is found to be significantly lower than that for linear PE due to chain folding induced by the short chain branches. The crystallization kinetics following quenches to different temperatures below the PE melting temperature have been analysed using the Avrami equation. For symmetric diblocks containing a rubbery amorphous block, a growth exponent n = 3 is found in the initial growth regime. A diblock containing glassy poly(vinylcyclohexane) (PVCH) amorphous blocks was also found to have n = 3, but the onset of crystallization was supressed to lower temperatures in the PVCH-containing sample, indicating that grafting the PE chains to glassy walls significantly retards the crystallization process. Comparisons of crystallization rates and growth mechanisms are made with high and low molecular weight PE homopolymers. The characteristic time for crystallization, extracted from the Avrami equation, was shown to follow Arrhenius behaviour as a function of temperature. (C) 1997 Elsevier Science Ltd. All rights reserved.