Wall-induced orientational order in athermal semidilute solutions of semiflexible polymers: Monte Carlo simulations of a lattice model

An athermal solution of semiflexible macromolecules with excluded volume interactions has been studied at various concentrations (dilute, semidilute, and concentrated solutions) in a film of thickness D between two hard walls by grand canonical Monte Carlo simulations of the bond fluctuation lattice model. Analyzing profiles of orientational order parameters across the film, we find that for thick films two phase transitions occur at chemical potentials of the polymers (or polymer densities, respectively) where the bulk polymer solution still is in the disordered isotropic phase. At rather small polymer densities, polymers accumulate at the walls due to an entropic attraction and undergo a transition to two-dimensional nematic order. Due to the properties of the lattice model, this order has Ising character, and the simulation results seem to be compatible with a second-order transition. Increasing the polymer density, nematically ordered “wetting” layers form at both walls; the increase of thickness of these layers is compatible with a logarithmic divergence when the chemical potential of the isotropic–nematic transition in the bulk is approached. In a system of finite width, D, between the walls, this leads to capillary nematization, exhibiting a reduction of the transition chemical potential inversely proportional to D. This transition exists only if D exceeds some critical value D_c, while the transition from the isotropic phase to the two-dimensional nematic state is suggested to persist down to ultrathin films.