Particle-induced network formation in linear PDMS filled with silica

We study the formation of permanent elastomers from linear PDMS chains by solution blending with up to 25 wt% fumed silica. The physical networks are characterized by time-domain multiple-quantum NMR. Based upon dynamic parameters measured for the linear precursor polymer, we develop a reliable strategy for component separation in this complex heterogeneous system, providing information on the amount of monomers involved in network-like material, in elastically inactive yet entangled linear chains, and isotropically mobile chain ends, as well as on the effective network chain length as measured via the average residual dipolar coupling constant. The use of untreated silica leads to permanent networks, for which the NMR results correlate well with macroscopic determinations of the relaxed Young modulus and the degree of swelling. Surface-modified silicas do not lead to percolated network structures, but still lead to the formation of 20-40% network-like material, with effective network chain lengths that depend on the surface functionalization and thus on the nanoparticle dispersion. Characteristic changes in the mobile chain end fraction with temperature, in particular its decrease with increasing degree of filling are interpreted as a consequence of altered contour-length fluctuations. An aging experiment conducted on a sample prepared by melt blending reveals the microscopic changes in the network structure occurring over many months.