Equilibrium properties of polymer solutions near surfaces: Monte Carlo simulations.

다국어 초록 (Multilingual Abstract)

Knowledge of conformational features of polymer solutions at interfaces is a prerequisite for understanding and controlling important technological processes such as colloid stabilization, biopolymer adsorption, corrosion inhibition, lubrication, adhesion, membrane separations and aggregation-induced separations.
In this exposition, some examples are shown of the unique contributions that a carefully designed simulation study can make towards an improved interpretation of experimental and theoretical work. One section is devoted towards the improvement of an existing Monte Carlo algorithm to enable better tests of theoretical results. Large-scale end-swapping configurational bias moves provide the best chance of simulating high chain lengths and concentrations.
The main focus of this paper is on polymers adsorbing from a good solvent onto a solid surface that has a weak (reversible) attraction on monomers. We analyze the concentration gradient that develops when the interface is in equilibrium with a bulk solution, and the force induced by adsorbed chains when subject to confinement.
Strong evidence is presented supporting the scaling prediction for long chains that the loop and overall concentration decay as <italic>z</italic><super> −4/3</super> when adsorption occurs from semi-dilute or dilute athermal solutions. Mean-field theories predict a concentration decay that is too rapid at all chain lengths, consequently they underestimate adsorbed amount and layer thickness. Simulations provide the only quantitative description of finite-length random walks with excluded volume subject to adsorption conditions.
Adsorbed chains under compression exhibit lower pressures than predicted by mean-field theory as a result of its lower adsorption capacity. Even when interpreted with respect scaled variables, quantitative discrepancies persist in the compression of saturated/semidilute layers. A different comparison shows that the simulation results presented have substantial experimental implications. The compression of two adsorbed layers against each other is quantitatively similar to the compression of a single layer, when the surfaces bear high adsorbed amounts.
A study also is made of the concentration gradient and confinement energy of a single chain between athermal walls. It is found that good agreement exists with theoretical predictions based on the “magnetic analogy” for a universal correlation between compression force and the depletion of segments near the walls.
Finally several remarks are made concerning further research on problems including polymer depletion at interfaces and adsorption of polymers from a bidisperse distribution of molecular weight.