Dynamic light scattering (DLS) could be measured for end-linked poly(dimethylsiloxane) (PDMS) model networks and ordinary rubber vulcanizates such as dicumyl peroxide (DCP)-cured natural rubber (NR), and the cooperative diffusion constant Dcoop was determined. The validity of the so-called C* theorem was experimentally confirmed by comparison of Dcoop for PDMS solutions with that of swollen PDMS networks. Variations of Dcoop with cross-link density for both PDMS networks and DCP-cured NR approximately corresponded to the prediction expected from both the dynamic scaling law and the C* theorem. The relationship between Z)coop and number-average molecular weight Mn of PDMS prepolymers also nearly obeyed the dynamic scaling law, and the average distance between cross-link sites in swollen PDMS networks at the equilibrium state of swelling, which was estimated from Mn of PDMS prepolymers, was in good agreement with the correlation length ξ calculated from Dcoop- It follows that ξ can be regarded as the mesh size of a swollen network. As a result, an average molecular weight between cross-link sites for PDMS networks determined from the application of the C* theorem, which value was approximately equal to Mn of PDMS prepolymers, was suggested to be reasonable, while the same quantity derived from the conventional Flory-Rehner equation (FR equation) was found to be at least overestimated at (much) lower cross-link densities. These facts imply that the basic assumption of affine deformation in the FR equation is not applicable to such networks. Further, it has become apparent from DLS measurements for swollen DCP-cured NR that trapped entanglements are nearly equivalent to chemical cross-link sites at the equilibrium state of swelling.
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