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, a prominent α-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 (Tg) were obtained from ac-calorimetric data at 40 Hz and from capacitive dilatometry, and reveal a surprising, marginal thickness dependence Tg(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, a significant reduction in cp 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.