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

Amorphous structure of swollen end-linked poly(dimethylsiloxane) (PDMS) net work 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 junct ion (cross-link) functionality. The M-p dependence of the SAXS profiles for tetra-functional PDMS networks shows a striking crossover at around M(p)ap proximate to M-c where M-c represents the critical molecular mass to form e ntanglement 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 whe n precursor chain is long enough to form entanglement couplings, trapped en tanglements formed in cross-linking (instead of cross-links) play a major r ole in elementary mesh of the resulting networks. For the networks of M(p)m uch greater than M-c, the correlation length for the network structure is f airly small and comparable to that of dynamic density fluctuation for the s olutions of the precursor polymer, and in addition, no appreciable effect o f junction functionality on long-range network structure is observed. These observations imply that the contribution of cross-link to static heterogen eity in network structure is screened by trapped entanglement dominant in n umber. (C) 2000 American Institute of Physics. [S0021-9606(00)51420-2].