Interplay of surface and confinement effects on the molecular relaxation dynamics of nanoconfined poly(methyl methacrylate) chains

The thermally stimulated current (TSC) signatures of the primary (alpha) transition and its precursor, the Johary-Goldstein (beta) relaxation, are used to probe effects of nanoconfinement on the dielectric relaxation dynamics of poly(methyl methacrylate) (PMMA) radically polymerised in situ 50 Angstrom mean pore size silica-gel. Nanoconfinement leads to a broadened and low-temperature-shifted beta band (peaking at T-beta, with DeltaT(beta) = T-beta(conf). - T-beta(bulk) = -15 degreesC for a heating rate of 5 deg/min), signifying the occurrence of faster relaxing moieties compared to the bulk-like PMMA film. Furthermore, both TSCs and differential scanning calorimetry (DSC) estimate a rise of the glass transition temperature for the confined phase (DeltaT(alpha) proportional to DeltaT(g)(DSC) = + 13 degreesC) and an increased width for the corresponding transition signals, relative to the signals in the bulk. Simple free-volume and entropy models seem inadequate to provide a collective description of the above perturbations. The observation of a spatial heterogeneity regarding the relaxation dynamics is discussed in terms of the presence of a motional gradient, with less mobile segments near the interface and more mobile segments in the core, and the interplay of adsorption (e.g., strong physical interactions that slow down molecular mobilities) and confinement effects (e.g., lower entanglements concentration and local density fluctuations that provide regions of increased free space). The results suggest that in the case of high-molecular-weight polymers confined in small-pore systems, adsorption effects have considerable bearing on the glass transition phenomenon whereas confinement primarily influences side-chains' rotational mobilities. The confinement effect is expected to dominate over adsorption for PMMA phases occluded in higher pore sizes and silanised walls.