Lattice Model of Polymer Melt Intercalation in Organically-Modified Layered Silicates

Abstract

A mean-field, lattice-based model of polymer melt intercalation in organically-modified mica-type silicates (OLS) has been developed. In general, an interplay of entropic and energetic factors determines the outcome of polymer intercalation. Free energy curves and their dependence on energetic and entropic factors suggest three possible equilibrium states immiscible, intercalated, and exfoliated all of which have been experimentally observed. The entropic penalty of polymer confinement may be compensated for by the increased conformational freedom of the surfactant chain as the layers separate. When the total entropy change is small, small changes in the system's internal energy will determine if intercalation is thermodynamically possible. Complete layer separation, though, depends on the establishment of very favorable polymer−OLS interactions to overcome the penalty of polymer confinement. For alkylammonium-modified layered silicates, a favorable energy change is accentuated by maximizing the magnitude and number of favorable polymer−surface interactions while minimizing the magnitude and number of unfavorable apolar interactions between the polymer and the functionalizing alkyl surfactants.