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

Abstract

Characteristics of the dielectric relaxation mechanisms (γ, β, β’, α and ρ) in poly(methyl methacrylate) (PMMA) and hybrids of PMMA polymerized into 5 nm SiO₂ pore matrices were studied by means of the thermally stimulated depolarization currents (TSD) technique, applied in the range 10–460 K. Low frequency relaxations (10 μHz-10 mHz) were investigated by measuring the isothermal discharging current, with the loss factor ϵ”(f) determined using a new development of the Hamon method. Compared to pure PMMA, the hybrids presented a 14 to 18 degrees reduction of the TSD β-relaxation maximum (T_β) and a drastic high temperature shift of the prominent syndiotactic α-peak. The average energy barrier for dipole (re)orientation (W) slightly decreases for both the α and β relaxations. The latter observations, as well as the time evolution of the TSD spectra, are discussed in terms of the variation of the initiator (azobisisobutyronitrile) content and the effects of polymerization in spatial confinements (e.g. reduced monomer-to-polymer conversion at high initiator loadings and interaction effects). The shifts reflect the presence of several antagonistic mechanisms controlling the molecular dynamics of the polymeric phase. The extent of the polymer-SiO₂ hydrogen bonding interaction induces an increase of the energy barrier for the activation of the motions of the carboxymethyl lateral groups (β-process) and parts of the main chain (α-process). On the other hand, the reduction of the chain entanglements (due to the pore-directed propagation of polymerization) loosens several steric hindrances on the rotational motion of the side group, explaining the accompanying decrease of W.

Keywords:

• Polymeric-inorganic hybrids
• Poly(methyl methacrylate)
• Hydrogen bond
• Glass transition
• Thermally stimulated depolarization
• Isothermal discharge
• Dielectric relaxation