Contradicting perturbations of the segmental and secondary relaxation dynamics of polymer strands constrained in nanopores

The glass transition and the sub-glassy relaxations in 1 mm bulk-like poly(methyl methacrylate) (PMMA) films and PMMA confined in ∼5 nm silica pores were studied using several variants of thermally stimulated currents (TSCs) spectroscopy applied in the temperature range from −263 to +160 °C. The hindering of the segmental dynamics, demonstrated by the strong high-temperature shift of the α mode, is accompanied by a substantial broadening of both the dielectric and calorimetric signals compared to the bulk phase. Contrastingly, the TSC signatures of local transitions ascribed to -COOCH3 (β mode) and α-CH3 (γ mode) pendant groups shift to lower temperatures. These perturbations appear irrespective of the initiator (benzoyl peroxide or azo-bis-isobutyro-nitrile) used for the in situ free-radical polymerisation process. The data are interpreted in the framework of the balancing of enthalpic and entropic effects of the nanoconstraint, which generate a motional gradient with hindered relaxing units near the attractive interface and sterically or kinetically facilitated rotational motions in the core.