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 similar to 5 nm silica pores were studied using several variants of thermally stimulated currents (TSCs) spectroscopy applied in the temperature range from -263 to +160 degrees 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 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. (c) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.