Onset of self-assembly

TL;DR

This paper develops a theoretical model for polymer entanglement using an extended Cahn-Hilliard functional, predicting a crossover to entanglement characterized by critical slowing down and increased tensile modulus, fitted to experimental data.

Contribution

It introduces a novel gauge-theory-based functional to describe polymer entanglement and predicts the crossover behavior beyond Gaussian approximation.

Findings

Crossover to entanglement marked by critical slowing down.

Tensile modulus increases significantly during crossover.

Model fitted to experimental data on polystyrene melts.

Abstract

We have developed a theory of polymer entanglement using an extended Cahn-Hilliard functional, with two extra terms. One is a nonlocal attractive term, operating over mesoscales, which is interpreted as giving rise to entanglement, and the other a local repulsive term indicative of excluded volume interactions. This functional can be derived using notions from gauge theory. We go beyond the Gaussian approximation, to the one-loop level, to show that the system exhibits a crossover to a state of entanglement as the average chain length between points of entanglement decreases. This crossover is marked by critical slowing down, as the effective diffusion constant goes to zero. We have also computed the tensile modulus of the system, and we find a corresponding crossover to a regime of high modulus. The single parameter in our theory is obtained by fitting to available experimental data on polystyrene melts of various chain lengths. Extrapolation of this fit yields a model for the cross-over to entanglement. The need for additional experiments detailing the cross-over to the entangled state is pointed out.