Densification and Depression in Glass Transition Temperature in Polystyrene Thin Films

Abstract

Ellipsometry and X-ray reflectivity were used to characterize the mass density and the glass transition temperature of supported polystyrene (PS) thin films as a function of their thickness. By measuring the critical wave vector (qc) on the plateau of total external reflection, we evidence that PS films get denser in a confined state when the film thickness is below 50 nm. Refractive indices (n) and electron density profiles measurements confirm this statement. The density of a 6 nm (0.4 gyration radius, Rg) thick film is 30% greater than that of a 150 nm (10Rg) film. A depression of 25 °C in glass transition temperature (Tg) was revealed as the film thickness is reduced. In the context of the free volume theory, this result seems to be in apparent contradiction with the fact that thinner films are denser. However, as the thermal expansion of thinner films is found to be greater than the one of thicker films, the increase in free volume is larger for thin films when temperature is raised. Therefore, the free volume reaches a critical value at a lower Tg for thinner films. This critical value corresponds to the onset of large cooperative movements of polymer chains. The link between the densification of ultrathin films and the drop in their Tg is thus reconciled. We finally show that at their respective Tg(h) all films exhibit a critical mass density of about 1.05 g/cm(3) whatever their thickness. The thickness dependent thermal expansion related to the free volume is consequently a key factor to understand the drop in the Tg of ultrathin films.

Stability of Adsorbed Polystyrene Nanolayers on Silicon Substrates

“…This can be explained by the scaling treatment of adsorption kinetics: [25,26] the surface coverage of early arriving chains that lie flat on the surface (i.e., the flattened chains) is a decreasing function of M w (below M w, c ) such that more free surface areas are available for late arriving chains that form bridges jointing up nearby empty sites (i.e., the loosely adsorbed chains). Such an increase in density at the polymer-solid interface is consistent with other experimental [27,28] and simulation results. [29,30] We further advanced the leaching conditions (type of a solvent, leaching time, leaching temperature, etc.…”

“…The choice of the twolayer model was determined by the corresponding FT profile of the XR profile, and the details of the data analysis have been described elsewhere. [2,3,6,33] The densification of the polymer chains in the vicinity of the substrate surface is in line with previous experimental [2][3][4]6,27,28] and simu lation findings. [19,20] The presence of the two-layer formation indicates that the irreversible adsorption of polymer chains on a solid surface is kinetically controlled: the later arriving polymer chains "reel-in" the empty spaces of the flattened layer, forming the outer loosely adsorbed polymer chains on the solid surface.…”

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

“…For instance in ellipsometry, a technique employed to determine the temperature dependence of the thickness of films, normally cooling/heating rates of the order of 0.1 to 30-40 K/min are applied to determine the glass transition behaviour as a function of thickness [5,33,38,41,46,48,91,107,112,113]. Similar rates, or even smaller, are employed in fluorescent spectroscopy [34,93] and positron annihilation lifetime spectroscopy (PALS) [28,102], which both (indirectly and directly, respectively) measure the local free volume, X-ray reflectivity [71,102,113] and capacitive dilatometry (CD) [14,42,65,89]. In the case of conventional DSC, rates of the 30-40 K/min can be achieved [13,14,55,116].…”

Vapor-phase-synthesized fluoroacrylate polymer thin films: thermal stability and structural properties

“…However, such a transition is not evident in the present data. While thin films typically do show a different behavior compared to bulk materials, a film thickness greater than 100 nm (such as the ones used in this study) is often found in agreement with the bulk [25–26]. …”