ON THE GROWTH-RATE MAXIMUM IN EXTENDED-CHAIN GROWTH OF CRYSTALS OF LINEAR PARAFFINS

Isothermal crystallization of a pure n-paraffin at temperatures near its melting point results in extended chain crystals whose growth rate-crystallization temperature curve exhibits a maximum and whose melting point appears to be lower than that expected. This behaviour can be explained in terms of polymer crystal nucleation theory with a model in which registration of chain ends is not perfect resulting in a transient layer of cilia on the crystal and to an end surface free energy sigma' for that ciliated surface. The presence of the maximum, however, requires a decreasing nucleation rate with decreasing temperature-contrary to customary expectations. This can be shown to be a consequence of the crystallization of (essentially) fixed length stems: the combination of the relevant nucleation and backward reaction terms thus giving rise to the maximum. The temperature at which the maximum occurs is a function of the degree of imperfection at the end surface.