New approach to the mechanism of oligomer crystallization

Abstract

A new mechanism for crystal growth of oligomers has been proposed. In order to grow, the crystals must have well ordered facets. This has been taken into account by assuming that their thickness is larger than C/(T*–T_c), where C is an appropriate constant, T* a reference temperature and T_c the temperature of crystallization. Application of this relation to crystals which have thermodynamic size is justified. These crystals grow by crystallographic attachment of sufficiently long molecules in a process exactly comparable to the growth of a crystal of an alk-1-ene (known from the measurement of the absolute entropy of the crystals). From these premises, the growth rate of poly(ethylene oxide) lamellae is predicted with surprising success. The segregation of the molecules according to their molecular weight and the very sensitive effect of seeding is also predicted. Application of the model to the crystallization of polyethylene fractions is discussed.