Microkinetics of lamellar crystallization in a long chain polymer

Abstract

The growth kinetics of single crystal lamellae grown from polyisoprene melts have been determined by thin film electron microscopy which also reveals the relevant microstructure. The growth rate of a continuous lamellar crystal is independent of the length of the crystal and (in the absence of crosslinks) depends only upon temperature and the chemical composition of the polymer. The temperature dependence is closely similar to that observed for bulk crystallization and spherulite growth, with a maximum crystallization rate around — 24 °C. The growth rate at a given temperature is extremely sensitive to the chemical composition of the polyisoprene, being for a polymer containing 10% trans -polyisoprene one thousandth of that for pure cis -polyisoprene. Between these extremes the logarithm of the growth rate decreases linearly with increasing trans content. A similar effect is produced by the introduction of chemical crosslinks instead of trans units. These effects are attributed to a reduction in the probability of formation of the ‘secondary nucleus’, whose size can be calculated from the experimental data. The secondary nucleus is found to correspond to three molecular folds and this agrees well with an independent estimate obtained, by using thermodynamic theory, from the dependence of lamellar thickness on temperature.