Polymer Adsorption from Bulk Solution onto Planar Surfaces: Effect of Polymer Flexibility and Surface Attraction in Good Solvent

Abstract

Adsorption of uncharged homopolymers of various flexibilities in good solvent onto planar surfaces at various polymer−surface interaction strengths have been investigated by employing a coarse-grained bead−spring polymer model using simulation techniques. The polymer flexibility ranged from fully flexible to rod-like polymers, and the adsorption strength varied from weak to strong adsorption. Equilibrium adsorption properties were determined by Monte Carlo simulations, and adsorption processes were investigated by Brownian dynamic simulations. In the latter case, the initial systems were composed of a polymer solution and a surface separated by a slab of polymer-free solution. The equilibrium properties of the interfacial systems have been analyzed by monitoring bead and polymer density profiles, number of adsorbed beads and polymers, the components of the radius of gyration perpendicular and parallel to the surface as well as tail, loop, and train statistics. Flexible polymers adsorbed in two layers, and at an increasing surface attraction the number of adsorbed beads and polymers increased and the adsorbed polymers become flatter, whereas rod-like polymers adsorbed in a single and thin layer with a nematic-like order. At increasing polymer stiffness at fixed surface attraction strength, the number of adsorbed beads increased, whereas the number of adsorbed polymers, the polymer extension perpendicular to the surface, and the fraction of beads in tails all displayed nontrivial maxima at similar persistence length. The dynamic analysis showed that the initial adsorption was diffusion controlled, but soon became governed by the probability of a polymer to be captured by the surface attraction. Flexible polymers became flattened after attaching, but their final relaxation mechanism involved an increased perpendicular extension with fewer adsorbed beads and longer tails driven by the surface pressure originating from the surrounding adsorbed polymers. The stiff polymers displayed a much slower final relaxation to their equilibrium state; this relaxation predominately constituting a packing of the rod-like polymers in a 2-dimensional nematic order. Furthermore, we have defined an integration time denoting the adsorption time for adsorbed polymers to become fully integrated into the adsorbed layer. Integration times and residence times of integrated polymers became longer with increasing polymer stiffness and increasing bead−surface attraction.