Reconsideration of kinetic theories of polymer crystal growth with chain folding

Abstract

When the elementary step in the formation of a surface nucleus is chosen to be the deposition of a full stem, the crystallization path is shown to be, in specific examples, energetically disfavourable. Moreover, the occurrence of a lateral interfacial free energy is, in this context, the sole reason for limitation of fold length. Avoidance of the blow-up problem results from questionable choices of expressions for the affinity of deposition of a stem. Lastly, for PEO oligomers, the thermal dependence of kTlnG(where G is the linear growth rate) is much larger than that of the affinity of deposition of the first stem and a consistent value cannot be found for the lateral interfacial free energy per unit area.

Thus, it appears necessary to recall that the crystallizable entities are molecules made in the liquid state of joined stereoregular segments. The first consideration leads to a tentative interpretation of the PEO data. The second leads to a finer description of the process of molecular deposition, formulated as the result of reversible attachment of successive segments shorter than a stem, while allowing the molecule to fold back at every stage of its deposition. A new mechanism of limitation of fold length results. Such a model allows, in principle, consideration of all the crystallization paths.