Can Thickness and Interfacial Interactions Univocally Determine the Behavior of Polymers Confined at the Nanoscale?

Abstract

The behavior of polymers confined in ultrathin films (thickness < 200 nm) can sensitively differ from that observed in macroscopic samples. Based on the simple arguments of finite size and interfacial effects, film thickness, and surface interactions should be sufficient to univocally determine the deviation from bulk behavior. However, recent models suggest that a third key parameter, namely, the interfacial free volume, should also be considered. We describe a novel methodology that quantifies the volume available for structural relaxation at the interface between a thin polymer layer and its supporting substrate. Experiments performed at different annealing conditions verified that the shift in glass transition temperature, measured in thin films upon confinement, is proportional to the degree of adsorption and, thus, to the interfacial free volume.

Cooling Rate Dependent Glass Transition in Thin Polymer Films and in Bulk

“…This is because the specific heat jump is mostly affected by the presence of a layer adsorbed on the substrate. For PS with M w ¼ 85,000 g/mol and annealed well above bulk T g , the adsorbed layer was estimated to be of the order of several nanometers [75,76]. The decrease of the specific heat for supported thin PS films was shown by several authors [55,65].…”

“…This is likely due to the fact that films deposited on a soft substrate, in the case of the work of Gao et al [57], Krytox oil, exhibit weak degree of adsorption. As mentioned, this constitutes a crucial parameter in determining the magnitude of T g deviations [75][76][77].…”

“…Later the effect of the interfacial interaction polymer/substrate on the T g was highlighted by several studies [35,48,77,84,90,94,107,120] and summarized in a review [77]. Interestingly, at a given interfacial interaction polymer/substrate, the film T g can be tuned by annealing at temperatures considerably larger than the polymer bulk T g [75,76]. An intimate relation with polymer adsorption was shown in these studies and the relevant magnitude for what concerns T g depression was found to be the amount of free interface, that is, that portion of the interface where, for kinetics and conformational reasons, the polymer could not adsorb.…”

“…And, vice-versa, recovering equilibrium on heating through the T g implies diffusion of volume holes from the external world into the glass. In thin polymer films supported on a substrate, this is the upper (free) surface plus the portion of polymer/support interface exhibiting no adsorption [15,73,76]. The merit of the model is that it predicts higher efficiency of maintaining equilibrium without requiring an acceleration of the rate of spontaneous fluctuations.…”

Effects of surface modification and thermal annealing on the interfacial dynamics in core–shell nanocomposites based on silica and adsorbed PDMS

“…Parallel to the study of the interfacial layer in polymer nanocomposites, various studies of polymers adsorbed thermally or/and chemically on solid surfaces [19,20,22,23], as well as of thin polymer films [24], have demonstrated similarities between polymer nanocomposites and thin polymer films in thermal stability, polymer chain conformations and dynamics. Core-shell based nanocomposites [25,26] form a next class of interest in the same direction, where the polymer is adsorbed in multiple layers (shells) onto the nanoparticles (core).…”