Nanoscale Inhomogeneities and Thermodynamics of Unfilled Polymer Gels

Received 12 May 2005 ; Accepted 11 August 2005. Dedicated to Professor John L. Stanford on Lhe occasion of his 60th birthday. Address correspondence to Erik Geissler, Laboratoire de SpectromeLrie Physique CNRS UMR 5588, Université J . Fourier de Grenoble, B.P. 87 , 38402, StMartin d'Hères cedex, France. E-mail : [email protected] The description ofthe s tructure ofswollen polymer networks and its relationship with mechanical and the1modynamic properties has been an unresolved problem f ora long tim.e. In spite of a numb er of theoretical approaches, no simple fo nnalism has been found that captures the structural variety of real gels. Here we repon 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 propenies. Experimental data from small angle neutron scattering (SANS), osmotic swelling pressure, and elastic modulus measurements were obtained on polyfluorosil icone (P FSi) gels prepared by different cross-linking processes from precursor chains of monomodal and bimodal distributions. The neutron scattering respollSe of these gels reveals two types of concentration fluctuations, name ly, those originating from nanoscale fro zen-in constraints generated by the cross-links and the timedependenr thermodynamir fluctuations associated with the osmotic properties of the netwo rk chains. The amplitude of the frozen-in concemration. fluctuations deduced from small angle neutron scattering measurements is found to be proponionalro the ratio of the macroscopic elastic shear modulus to the osmotic compression modulus. Th e thermodynamic concentration fluctuations are in agreement with the results of dynamic light scattering and macroscopic osmotic pressure measuremems. The present approach describes the effect of the nanoscale features on the macroscopic propenies of the gel.