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

TLDR

Polymer intercalation into organically‑modified layered silicates is governed by a balance of entropic confinement penalties and energetic polymer–surface interactions, with favorable polymer–surfactant contacts needed to overcome the confinement penalty and enable full layer separation. The lattice model optimizes polymer–surface contacts while reducing apolar interactions to favor intercalation. The mean‑field lattice model predicts three equilibrium states—immiscible, intercalated, and exfoliated—whose free‑energy curves agree with experimental observations.

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 statesimmiscible, intercalated, and exfoliatedall 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.