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

Abstract

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 Dc, while the transition from the isotropic phase to the two-dimensional nematic state is suggested to persist down to ultrathin films.

Capillary Nematization of Semiflexible Polymers

“…For both the snapshot of the local order parameter orientation, Figure , and the large value , S ( z ) > 0.7, of the local order throughout the slit, Figure , suggest that one encounters a well ordered nematic state: the order is clearly more pronounced than in a corresponding bulk system at this density. This is clearly an effect due to “capillary nematization,” as previously studied by SCFT as well as by simulations of lattice models of semiflexible polymers …”

“…Unfortunately, our data do not give a clear picture of the location of the critical point of capillary nematization, Figure . We recall that one expects a critical slit thickness , and a first order capillary nematization transition from the isotropic phase to the nematic phase only occurs if . When we increase the density in this case, we should then observe a linear increase of the average nematic order parameter S from S I ( L z ) at to S N ( L z ) at , reflecting the discontinuous jump from S I ( L z ) to S N ( z ) at the chemical potential , provided one studies the problem in the grand‐canonical ensemble.…”

“…Of course, it is nevertheless interesting to compare the present treatment to the lattice model study presented in refs . in more detail.…”

“…This “capillary nematization,” well known for hard rod fluids (e.g., ref and by lattice model Monte Carlo methods . However, as stated above, lattice Monte Carlo models do not include effects related to the deflection length, and SCFT approach does not describe the monomer packing effects near the walls.…”

“…In the context of such applications, the interaction of the semiflexible macromolecules with confining walls clearly is of great importance. Thus, the effect of confining walls on the semiflexible polymers in solutions and melts has been studied theoretically by various computational and analytical approaches . However, this work suffers from certain limitations.…”