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 (alpha) 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 ((beta) mode) and (alpha)-CH3 ((gamma) 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 freeradical 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.