Phase behaviour and self-assembly of semiflexible polymers in poor-solvent solutions

TL;DR

This study combines Langevin dynamics and Monte Carlo simulations to explore the phase behavior and self-assembly of semiflexible polymers in poor solvents, revealing complex transitions and structures influenced by chain stiffness and temperature.

Contribution

It provides a comprehensive phase diagram for semiflexible polymers in poor solvents, linking microscopic parameters to macroscopic structures and transitions, including novel twisted and toroidal crystal formations.

Findings

Identified coil-globule and globule-crystal transitions in single chains.

Discovered formation of twisted rod and toroid crystals depending on stiffness.

Observed phase transitions from gas to globule and nematic bundle in multi-chain systems.

Abstract

Using Langevin dynamics complemented by Wang-Landau Monte Carlo simulations, we study the phase behavior of single and multiple semiflexible polymer chains in solution under poor-solvent conditions. In the case of a single chain, we obtain the full phase diagram in the temperature-bending rigidity (stiffness) plane and we provide connections with a classical mean field result on a lattice as well as with past results on the same model. At low bending rigidity and upon cooling, we find a second order coil-globule transition, followed by a subsequent first order globule-crystal transition at lower temperatures. The obtained crystals have the shape of a twisted rod whose length increases with the increase of the stiffness of the chain. Above a critical value of the stiffness, we also find a direct first order globule-crystal transition, with the crystal having the form of a twisted toroid. Close to the triple point, we find a region with isoenergetic structures with frequent switching from rods to toroids, with the toroid eventually becoming the only observed stable phase at higher stiffness. The model is then extended to many thermally equilibrated chains in a box and the analogous phase diagram is deduced where the chains are observed to first fold into a globule bundle at low stiffness upon cooling, and then rearrange into a nematic bundle via a nucleation process involving an isotropic-nematic transition. As in the single chain counterpart, above a critical stiffness the chains are observed to undergo a direct transition from a gas of isotropically distributed chains to a nematic bundle as the temperature decreases in agreement with recent suggestions from mean field theory. The consequences of these findings for self-assembly of biopolymers in solutions are discussed.