Orientational ordering transitions of semiflexible polymers in thin films: A Monte Carlo simulation

Abstract

Athermal solutions (from dilute to concentrated) of semiflexible macromolecules confined in a film of thickness D between two hard walls are studied by means of grand-canonical lattice Monte Carlo simulation using the bond fluctuation model. This system exhibits two phase transitions as a function of the thickness of the film and polymer volume fraction. One of them is the bulk isotropic-nematic first-order transition, which ends in a critical point on decreasing the film thickness. The chemical potential at this transition decreases with decreasing film thickness ("capillary nematization"). The other transition is a continuous (or very weakly first-order) transition in the layers adjacent to the hard planar walls from the disordered phase, where the bond vectors of the macromolecules show local ordering (i.e., "preferential orientation" along the x or y axes of the simple cubic lattice, but no long-range orientational order occurs), to a quasi-two-dimensional nematic phase (with the director at each wall being oriented along either the x or y axis), while the bulk of the film is still disordered. When the chemical potential or monomer density increase, respectively, the thickness of these surface-induced nematic layers grows, causing the disappearance of the disordered region in the center of the film.

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.…”