THERMAL AND QUENCHED FLUCTUATIONS OF POLYMER CONCENTRATION IN POLY(DIMETHYLSILOXANE) GELS

Using neutron and light scattering techniques, we studied, as a functi on of swelling degree in a good solvent, the fluctuations of polymer c oncentration in highly cross-linked gels prepared in the bulk. Using t he method proposed by Pusey and Van Megen, we separated the total scat tering intensity into the contributions arising from thermal and quenc hed fluctuations. The former are equivalent to those measured in an en tangled polymer solution with same concentration while the latter are responsible for the excess of intensity scattered from swollen gels an d 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 assoc iated characteristic length is the screening length for excluded and h ydrodynamic interactions. We propose a new picture for fluctuations of concentration in polymer gels and discuss it in relation with recent models.