Segmental relaxation in end-linked poly(dimethylsiloxane) networks

End-linked polydimethysiloxane (PDMS) networks were prepared by end-linking vinyl-terminated PDMS chains with tetrafunctional silanes. Dielectric alpha-relaxation spectra, thermal expansivities, and the (rubbery) elastic modulus were measured for these materials. From the cross-link density at which marked changes occur in the segmental relaxation times and the T-alpha-scaled temperature dependence (fragility), a characteristic length for the alpha-relaxation was determined. The result, 14 Angstrom at T-alpha, is quite close to the length scale associated with constraint of crystallization in similar PDMS networks. Cross-linking also affects the thermal expansion and elastic modulus, although these properties exhibit a smoother variation with cross-link density.