The role of chain folding in the crystallization kinetics of nucleated polypropylene: An elementary model"

A simple model is proposed to derive the Gibbs potential change associated with the formation of
secondary nuclei in the crystallization of systems consisting of isotactic polypropylene nucleated
with small amounts of indigo. A basic assumption is that the average basal surface tension of critical
nuclei lowers as an effect of chain-nucleant interaction. The actual fold length of individual stem
pairs may be lower than critical due to chain-nucleant encounters. Two limiting cases are
considered, namely, when the fold lengths of adjacent stem pairs are independent of one another
?random fold?, and when they are not ?persistence?. Corresponding expressions of the nucleation
free enthalpy are used to analyze available data of crystallization kinetics. Both scenarios describe
the physical situation well, most likely because under the actual experimental conditions critical
nuclei consist, on average, of one stem pair. The random fold scheme, however, is more informative
as it provides ?at least in principle? a method for determining both the basal and lateral contributions
to surface tension through an analysis of the crystallization kinetics.