Quantitative analysis of chiral transitions and its implication for non-classical crystallization in isotactic polypropylene

Abstract

Molecular dynamics simulations were used to quantify chiral transitions in an isolated isotactic polypropylene (iPP) chain on a nucleating agent substrate during the early stages of ordering. The effects of temperature, dihedral rigidity, and epitaxy of a single iPP chain placed on α-iPP, β-iPP, and TPDT surfaces were investigated. Using the adaptive biasing force (ABF) method, potentials of mean force (PMF) were computed for iPP segments to explain the selective stabilization of helical segments on various epitaxies. Whereas, chiral selection of helices from the extended-coil conformations was found to be more efficient, the selection of helices from coiled segments was poor. For α-iPP and β-iPP substrates, the stability of the extended-coils and formation of helices from extended-coils were positively correlated with each other. In contrast, a negatively correlation was found for the TPDT substrate, showing that a helix existed as a competitive conformation with the extended-coils when selective stabilization was absent. These observations support the validity of multi-stage nucleation routes during polymer crystallization, Further, they show that selective stabilization of helices by an epitaxy plays a vital role in the dynamics of multi-stage nucleation.