Phase Behavior and Self-Assembly of Semiflexible Polymers in Poor-Solvent Solutions

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 a 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 the self-assembly of biopolymers in solutions are discussed.