A. Şerbescu, K. Saalwächter.
Particle-induced network formation in
linear PDMS filled with silica.
Polymer 50, 5434-5442 (2009).
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.