Random cross-linking in rubbers produces local variations in the elasticity of the network. These variations, whose characteristic size lies in the range 1-100 nm, are revealed when the rubber is swollen in a low molecular weight solvent, owing to the competition between the osmotic pressure of the solvent and the local elastic constraints, which affects the local polymer concentration. Such concentration fluctuations can be measured by small angle X-ray or neutron scattering (SAXS or SANS) as well as by dynamic light scattering.
In filled elastomers, the filler modifies the distribution of the polymer and of the elastic constraints. Swelling these systems in a solvent in which the deuteron/proton ratio can be varied permits the different components in the scattering function of the polymer and of the filler to be separated. Observations on silica particles in a poly(dimethyl siloxane) (PDMS) rubber yield measurements not only of the surface area of the particles but also of the fraction of the surface area occupied by the polymer. Analysis of the dynamic light scattering response of these systems gives confirmation of the validity of the procedure.
Coherent X-ray scattering measurements, combined with dynamic light scattering measurements of the filled uncross-linked polymer melts in the absence of solvent reveal that the structural relaxation process that follows an external mechanical perturbation is a diffusioncontrolled process.