Phase stability of weakly crosslinked interpenetrating polymer networks

A phenomenological theory is formulated for chemically quenched binary interpenetrating polymer networks, considering both simultaneously crosslinked networks and sequentially crosslinked networks, as well as pseudointerpenetrating networks (where only one component is crosslinked and the other is a linear polymer). We construct free energy functionals for these systems, starting from the classical Flory theory of rubber elasticity, and study their properties as a function of the volume fraction φ of one component and of the Flory-Huggins interaction parameter v. It is suggested that in the limit of small crosslink density the contribution to the free energy due to the mutual entanglement of the networks is small and can be neglected, provided that the spatially homogeneous phase is thermodynamically stable. The stability limit (spinodals) of this phase for the various cases are derived, and also predictions are made for the initial growth of inhomogeneous modes in the unstable regions of the phase diagrams, within the framework of the linearized theory of spinodal decomposition. We also discuss how the predictions of this theory could be checked experimentally.