Nanoscale Inhomogeneities and Thermodynamics of Unfilled Polymer Gels

Abstract

The description of the structure of swollen polymer networks and its relationship with mechanical and thermodynamic properties has been an unresolved problem for a long time. In spite of a number of theoretical approaches, no simple formalism has been found that captures the structural variety of real gels. Here we report a new approach to describe inhomogeneities in the nanometer size range that develop in the course of the cross‐linking process, and their relationship with the macroscopic elastic properties. Experimental data from small angle neutron scattering (SANS), osmotic swelling pressure, and elastic modulus measurements were obtained on polyfluorosilicone (PFSi) gels prepared by different cross‐linking processes from precursor chains of monomodal and bimodal distributions. The neutron scattering response of these gels reveals two types of concentration fluctuations, namely, those originating from nanoscale frozen‐in constraints generated by the cross‐links and the time‐dependent thermodynamic fluctuations associated with the osmotic properties of the network chains. The amplitude of the frozen‐in concentration fluctuations deduced from small angle neutron scattering measurements is found to be proportional to the ratio of the macroscopic elastic shear modulus to the osmotic compression modulus. The thermodynamic concentration fluctuations are in agreement with the results of dynamic light scattering and macroscopic osmotic pressure measurements. The present approach describes the effect of the nanoscale features on the macroscopic properties of the gel. Dedicated to Professor John L. Stanford on the occasion of his 60th birthday.