Specific heat and dielectric relaxations in ultra-thin polystyrene layers

The effect of thickness on the glass transition dynamics in ultra-thin polystyrene (PS) films (4 nm < L < 60 nm) was studied by thin film ac-calorimetry, dielectric spectroscopy (DRS) and capacitive dilatometry (CD). In all PS-films, it prominent alpha-process was found in both the ac-calorimetric and dielectric response, indicating the existence of cooperative bulk dynamics even in films as thin as 4 nm, Glass transition temperatures (T-g) were obtained from ac-calorimetric data at 40 Hz and from capacitive dilatometry, and reveal it surprising. marginal thickness dependence T-g(L). These results, which confirm recent data by Efremov et al. [Phys, Rev, Lett. 91 (2003)] but oppose many previous observations, is rationalized by differences in film annealing conditions together with the fact that our techniques probe exclusively cooperative dynamics (ac-calorimetry) or allow the effective separation of surface and "bulk"-type mobility (CD). Two other observations, it significant reduction in c(p) towards lower film thickness and the decrease in the contrast of the dilatometric glass transition. support the idea of a layer-like mobility profile consisting of both cooperative "bulk" dynamics and non-cooperative surface mobility. (c) 2005 Elsevier B.V. All rights reserved.