Self-consistent field theory for polymer adsorption: Molecular volume effects

Abstract

In this work, we develop a self-consistent field model for homopolymer adsorption as a special case of a general theoretical formulation reported previously (H. J. Ploehn, Macromolecules, 27 (1994) 1617). The model treats polymer molecules as chains of continuously-distributed, idealized segments, thereby avoiding the artificiality of lattice discretization. The continuum formulation properly accounts for polymer stiffness by allowing polymer segments and solvent molecules to have differing molar volumes. We find that the polymer molecular weight, solvent quality, and surface adsorption energy influence the polymer density distribution and adsorbed amount in ways that are consistent with experimental observations. Varying the ratio of solvent and polymer molar volumes alters the polymer adsorbed amount by changing the balance of local mixing enthalpy and entropy. Using no adjustable parameters, the predicted variation of adsorbed amount with polymer molecular weight agrees well with some published experimental results. We observe unsatisfactory agreement when both the polymer molecular weight and its solution concentration are high.