Dielectric characterization of poly(methyl methacrylate) geometrically confined into mesoporous SiO2 glasses

Characteristics of the dielectric relaxation mechanisms (gamma, beta, beta ', alpha and rho) in poly(methyl methacrylate) (PMMA) and hybrids of PMMA p olymerized into 5 nm SiO2 pore matrices were studied by means of the therma lly stimulated depolarization currents (TSD) technique, applied in the rang e 10-460 K. Low frequency relaxations (10 mu Hz-10 mHz) were investigated b y measuring the isothermal discharging current, with the loss factor epsilo n"(f) determined using a new development of the Hamon method. Compared to p ure PMMA, the hybrids presented a 14 to 18 degrees reduction of the TSD bet a -relaxation maximum (T-beta) and a drastic high temperature shift of the prominent syndiotactic alpha -peak. The average energy barrier for dipole ( re)orientation (W) slightly decreases for both the alpha and beta relaxatio ns. The latter observations, as well as the time evolution of the TSD spect ra, are discussed in terms of the variation of the initiator (azobisisobuty ronitrile) content and the effects of polymerization in spatial confinement s (e.g. reduced monomer-to-polymer conversion at high initiator loadings an d interaction effects). The shifts reflect the presence of several antagoni stic mechanisms controlling the molecular dynamics of the polymeric phase. The extent of the polymer-SiO2 hydrogen bonding interaction induces an incr ease of the energy barrier for the activation of the motions of the carboxy methyl lateral groups (beta -process) and parts of the main chain (alpha -p rocess). On the other hand, the reduction of the chain entanglements (due t o the pore-directed propagation of polymerization) loosens several steric h indrances on the rotational motion of the side group, explaining the accomp anying decrease of W.