Unusual expansion and contraction in ultrathin glassy polycarbonate films

The thermal expansion behavior of thin polycarbonate (PC) films supported on hydrophilic and hydrophobic silicon substrates is studied with specular X-ray reflectivity. These measurements suggest a suppression of the apparent glass transition temperature (T-g) with decreasing film thickness h on hydrophilic silicon oxide substrates, while no evidence for an h-dependent T-g shift is observed on hydrophobic passivated silicon substrates. More interestingly, we find that when h is comparable to the bulk radius of gyration, the expansion becomes nonmonotonic with temperature T in a reversible manner. Cooling one of these ultrathin PC films well below the apparent Tg gives rise to an apparently negative coefficient of thermal expansion (CTE). This unusual behavior stems from the fact that nominally R-g- thick glassy films exhibit an appreciable isothermal thickening over extended periods of time, with the propensity for this thickening increasing with the depth of cooling into the glassy state; normal thermal contraction upon cooling becomes convoluted with the sub-T-g thickening phenomenon. This is counter-intuitive since deep in the glassy state the molecular mobility is anticipated to be small. The magnitude and rate of the isothermal thickening depend on the ratio of the macromolecular size to the film thickness, suggesting that chain distortion by the film boundaries is associated with this effect. The surface energy of the supporting substrate also influences these unusual behaviors.