Quasi‐elastic neutron scattering in gels and solutions

Measurements of swelling pressure, neutron spin‐echo scattering, and dynamic light scattering were made in an end‐linked poly(dimethylsiloxane) (PDMS) gel swollen to equilibrium in a good solvent (toluene) and also in the equivalent solution. The macroscopic osmotic modulus is depressed in the gel. Neutron spin echo observations at intermediate and high values of the scattering vector Q reveal that the mobility of the monomers is unaffected by cross‐linking. Elastic neutron scattering at small Q detects non‐uniformities in the polymer concentration distribution, which are absent from the solution. These non‐uniformities play a major role in the dynamic response of the system at lower Q, and and appear to be the cause of the observed reduction in osmotic pressure.