Biomedical subjects
Publications and source records attributed to Clifford E Woodward.
We develop a density functional theory for infinitely long flexible and semiflexible polymers, as an appropriate simplification of the corresponding treatment of finite chains. We demonstrate that very long chains sometimes are required to reproduce even qualitatively correct limiting behaviors.
According to a recently developed density functional theory, athermal polymer solutions, in which the solvent particles are smaller than the monomers, may undergo a bulk fluid-fluid phase separation, driven by excluded volume effects. In recent work, we showed that an inert surface immersed in the dilute polymer phase can, in principle, be wetted by the condensed phase. However, we show here that the "prewetting transition" we assumed in our earlier studies is in fact a different type of surface transition. Rather than completely wet the surface at coexistence, the condensed phase layer which forms in the presence of the dilute bulk remains globally stable (and is finite in width) even as the bulk coexistence conditions are approached. Hence, the adsorbed phase inhibits complete wetting of the surface by the dilute phase. The surface transition is first order for the systems we study here and, for longer polymers, the surface phase coexistence line meets the bulk coexistence curve nontangentially to give rise to a lower transition point. For short polymers, we find that the surface transition can occur for a supercritical bulk. We develop a simple one-component thermal model, which displays analogous behavior at an adsorbing surface and provides us with some insight into the qualitative mechanisms responsible.
Coexistence conditions for prewetting and layering at a hard surface in additive hard sphere polymer solutions, where the solvent particles are smaller than the monomers, have been calculated by density functional methods. Various chain lengths and pressures have been investigated. An unexpected finding is that prewetting in these systems may proceed below the bulk critical pressure. We rationalize this behavior in terms of local properties of the pressure tensor. For longer chains, a different behavior is observed where the systems display a lower wetting pressure, i.e., a low pressure bound for surface wetting.
We assess the accuracy of a density functional theory for athermal polymer solutions, consisting of solvent particles with a smaller radius than that of the monomers. The monomer and solvent density profiles in a slit bound by hard, flat, and inert surfaces are compared with those obtained by a Metropolis Monte Carlo simulation. At the relatively high density at which the comparison is performed, there are considerable packing effects at the walls. The density functional theory introduces a simple weight function to describe nonlocal correlations in the fluid. A recent study of surface forces in polymer solutions used a different weighting scheme to that proposed in this article, leading to less accurate results. The implications of the conclusions of that study are discussed.