Molecular modelling of nucleation in polymers.

Abstract

We have investigated the molecular mechanisms of primordial stages of polymer crystallization from solutions using Langevin dynamics simulations and theoretical models. The key feature that distinguishes polymers from small molecules at early stages of nucleation is that, in the case of polymers, a single chain can participate in several nuclei. This results in entropic frustration, leading to spontaneously selected temporary finite structures during nucleation. The experimental observation of finite lamellar thickness (much smaller than extended chain dimension) has been historically attributed to kinetic origins. Our simulations and an exactly solvable model to account for chain entropy show that the finite lamellar thickness is actually the equilibrium result. The growth at the lamellar growth front is found in our simulations to be dominated by chain adsorption and highly cooperative dynamics of all chains. Our results contradict the conventional assumptions and theories of polymer crystallization.