Thermal and quenched fluctuations of polymer concentration in poly(dimethylsiloxane) gels

Using neutron and light scattering techniques, we studied, as a function of swelling degree in a good solvent, the fluctuations of polymer concentration in highly cross-linked gels prepared in the bulk. Using the method proposed by Pusey and Van Megen, we separated the total scattering intensity into the contributions arising from thermal and quenched fluctuations. The former are equivalent to those measured in an entangled polymer solution with same concentration while the latter are responsible for the excess of intensity scattered from swollen gels and for the typical anisotropic intensity patterns measured an elongated gels. The relaxation of the thermal fluctuations is still governed by the cooperative diffusion of all the chains in the gel, and the associated characteristic length is the screening length for excluded and hydrodynamic interactions. We propose a new picture for fluctuations of concentration in polymer gels and discuss it in relation with recent models.