Small angle x-ray scattering study on role of trapped entanglements in structure of swollen end-linked poly(dimethylsiloxane) networks

Abstract

Amorphous structure of swollen end-linked poly(dimethylsiloxane) (PDMS) network has been investigated by means of small angle x-ray scattering (SAXS) technique as a function of molecular mass of precursor PDMS (M p ) and junction (cross-link) functionality. The M p dependence of the SAXS profiles for tetra-functional PDMS networks shows a striking crossover at around M p ≈M c where M c represents the critical molecular mass to form entanglement couplings in the PDMS melt. These results strongly suggest that structure of swollen end-linked networks significantly depends on whether precursor chains are well entangled or not before end-linking, and that when precursor chain is long enough to form entanglement couplings, trapped entanglements formed in cross-linking (instead of cross-links) play a major role in elementary mesh of the resulting networks. For the networks of M p ≫M c , the correlation length for the network structure is fairly small and comparable to that of dynamic density fluctuation for the solutions of the precursor polymer, and in addition, no appreciable effect of junction functionality on long-range network structure is observed. These observations imply that the contribution of cross-link to static heterogeneity in network structure is screened by trapped entanglement dominant in number.