Phase Behavior\nand Self-Assembly of Semiflexible Polymers\nin Poor-Solvent Solutions

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