Crystallization of Ethylene-Octene Copolymers at High Cooling Rates

Abstract

Quiescent crystallization is usually separated into its component parts of primary nucleation, linear spherulite (or lamellar) growth and secondary crystallization. In this chapter, only the linear spherulite growth rates are considered. Regime theory, which describes the linear spherulite growth in flexible polymers, comprises two separate processes. The first process is the deposition of secondary nuclei on the growth face, usually denoted as occurring at a rate “i,” the second process being the subsequent growth along the face at the niches formed by the secondary nuclei, often referred to as the rate of surface spreading, and denoted by the rate “g.” The relative rates of these two processes determine the regime at which the crystallization occurs. Increasing branch content shifts regimes from the I–II transition to II–III transition and reduces the growth rate significantly. When studies are extended to rapid cooling experiments, in which the polymer generates its own pseudo-isothermal crystallization temperature, the applicable existing regime III region is continued for a restricted range of temperature. When very high supercoolings are approached, a new form of crystallization occurs, in which the comonomer units are no longer excluded, and molecular weight dependencies appear to be eliminated. The copolymers become indistinguishable from one another based on their spherulite growth rates.