Effect of meta-carborane on segmental dynamics in a bimodal poly(dimethylsiloxane) network

Bimodal networks of polydimethylsiloxane (PDMS) filled with varying amounts of icosahedral meta-carborane (m-CB) have been developed and characterized by broadband dielectric spectroscopy (BDS) and static ¹H multiple quantum, nuclear magnetic resonance (MQ NMR). Both BDS and MQ NMR showed evidence for a decrease in the polymer chain dynamics. BDS spectra quantified a normal-mode relaxation near 40 Hz at 40°C. The frequency maximum observed for filled samples decreased with increasing m-CB content until contents greater than 5 wt %. The width of the relaxation spectrum increased with the addition of small quantities of filler and decreased with filler contents greater that 5 wt %. Agglomeration effects were observed at loadings greater than 5 wt % as manifest by the onset of low frequency Maxwell - Wagner - Sillars (MWS) processes. The MQ NMR data allowed the characterization of distributions of the residual dipolar couplings, Ω_d and, thus, in the dynamic order parameter, S_b, consistent with the bimodal network architecture expected from the synthesis protocol used. Upon addition of less than 10 wt % m-CB filler, the mean Ω_d for the longer chains increased by 46% and the width of the distribution increased by 33%. The mean Ω_d for the shorter chains increased by much less, indicative of preferential dispersion of the filler particles in the long chain domains of the network structure. We conclude that the mechanism of reinforcement is likely free volume space filling at low loadings transitioning to complex molecular filler and polymer chain interaction phenomena at higher loadings.