Reversible negative thermal expansion of polymer films

Abstract

A basic understanding of the properties of thin polymer films is of fundamental importance for developing applications in nanotechnology. Results of energy and angle dispersive x-ray reflectivity measurements on polymer thin films as a function of temperature exhibit reversible negative thermal expansion below the glass transition temperature T(g). Above T(g), the thickness expansion becomes almost equal to the expected bulk volume expansion. These results could be explained on the basis of evolution of disorder with temperature at the interfaces, chain entanglement and associated entropy changes.

Structural relaxation of spin-cast glassy polymer thin films as a possible factor in dewetting

“…a decrease of 1.7 nm in a 148 nm film) even when annealed above T g and cooled to room temperature again. This finding is consistent with the results of other workers [3,26,27,28] who have all seen typically a decrease of only 2 or 3 Å in films as thick as 30 nm. Since both PMMA and PS have comparable glass transition temperatures and bulk moduli, this result suggests that there are some other important factors in determining the structural relaxation after quenching.…”

“…In studies in the literature that consider the dewetting of PS, the possible effects of structural relaxation are not apparently very relevant. (Incidentally, the literature often refers to negative expansivity [27,28] in thin films. Such decreases in film thickness resulting from structural relaxation are seen at a constant temperature also.…”

Glass Transition of Ultrathin Sugar Films Probed by X-Ray Reflectivity

“…As is well known, the volumes of conventional materials shrink with lowering temperature and its accompanying entropy decrease, but certain materials expand as the temperature reduces. The latter phenomenon, called negative thermal expansion (NTE) [17,31], was apparently exhibited by the sugar nano-films during the cooling process.…”

“…The importance of these findings in the pharmaceutical field cannot be overstated. Specular X-ray reflectivity (XRR) is a unique and powerful analytical method (Figure 2) that is frequently applied to condensed soft-matter films, including glass transition studies of ultrathin polymer films [16][17][18][19][20], but which has not been applied in amorphous sugar studies. XRR can evaluate the layered structure of a material, such as the film thickness, electron density, surface roughness, and interfacial width [21,22].…”

Slow Relaxation of Spin-Cast Poly(methyl methacrylate) Confined in Thin Films

“…The technique must be fast enough to detect thickness changes occurring over a few minutes, while it must also be stable enough to observe changes occurring over several days. Neutron [11] and X-ray [12] reflection have been shown to have sufficient resolution to probe small dimensional changes in thin films. They are thus appropriate techniques for this type of experiment.…”