Free-energy landscape of polymer-crystal polymorphism

TL;DR

This study maps the complex free-energy landscape of syndiotactic polystyrene, revealing the stability and kinetic traps of its polymorphs through advanced simulations and quantum calculations, aiding understanding of polymer polymorphism.

Contribution

First comprehensive mapping of the free-energy landscape of a crystalline polymer using multiscale simulations and quantum calculations, elucidating polymorph stability and kinetic traps.

Findings

Beta polymorph is more stable at standard conditions.

The alpha polymorph can act as a kinetic trap.

The combined approach effectively uncovers complex free-energy landscapes.

Abstract

Polymorphism rationalizes how processing can control the final structure of a material. The rugged free-energy landscape and exceedingly slow kinetics in the solid state have so far hampered computational investigations. We report for the first time the free-energy landscape of a polymorphic crystalline polymer, syndiotactic polystyrene. Coarse-grained metadynamics simulations allow us to efficiently sample the landscape at large. The free-energy difference between the two main polymorphs, $\alpha$ and $\beta$, is further investigated by quantum-chemical calculations. The two methods are in line with experimental observations: they predict $\beta$ as the more stable polymorph at standard conditions. Critically, the free-energy landscape suggests how the $\alpha$ polymorph may lead to experimentally observed kinetic traps. The combination of multiscale modeling, enhanced sampling, and quantum-chemical calculations offers an appealing strategy to uncover complex free-energy landscapes with polymorphic behavior.