Unusual Expansion and Contraction in Ultrathin Glassy Polycarbonate Films Christopher L. Soles,* Jack F. Douglas, Ronald L. Jones, and Wen-li Wu NIST Polymers Division, Gaithersburg, Maryland 20899-8541 Received October 19, 2003; Revised Manuscript Received December 10, 2003 ABSTRACT: 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 (Tg) with decreasing film thickness h on hydrophilic silicon oxide substrates, while no evidence for an h-dependent Tg 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 Rg- 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-Tg 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. Introduction Nanotechnology demands that materials be utilized at increasingly smaller length scales. For polymers this can be problematic since it is generally understood that material properties in confined geometries and at surfaces or interfaces can deviate from their bulk values. For example, in thin polymer films there are well- documented shifts of the glass transition temperature (T g ) as the film thickness approaches the unperturbed dimensions of the macromolecule in bulk. Finite size effects could be of significant impact in fields such as lithography, where crucial properties such as acid diffusivity in a chemically amplified photoresist or polymer viscosity in imprint lithography typically dis- play dramatic changes at T g . Unexpected deviations in various physical properties at increasingly smaller dimensions may very well impede advances in polymer- based nanotechology. T g shifts as a function of thickness are commonly studied in polymer films. Thin film T g ’s are normally inferred from changes in a thermodynamic property (i.e., specific heat, thermal expansion, etc.) probed under nonequilibrium conditions. A time scale is always implicit, depending upon the rate at which the property is probed (i.e., heating rate) and the characteristic frequency of the technique. One classic attribute of polymer glasses is a marked thermal history depen- dence of the physical properties, examples of which include the physical aging and stress relaxation phe- nomena. However, in the thin polymer film literature these long-term effects have been to a large extent ignored. This is disturbing since there are strong indications that the time scales over which a polymer film equilibrates could be quite different in comparison to the bulk. For example, surface forces apparatus measurements on confined liquid films (both polymer and small molecule) demonstrate that viscoelastic time scales in the bulk and molecularly confined films are not the same. 1-4 Surface forces measurements are primarily limited to the liquid state, but it is reasonable to anticipate an extension of this behavior into the glass. In the present paper, we focus on the long-term stability of thin polymer films where the thickness h is comparable to or less than the unperturbed radius of gyration. At these length scales the film thickness should be small enough to induce an in-plane molecular orientation that would seemingly be susceptible to relaxation phenomena. Before discussing our results, it is helpful to briefly summarize preceding relevant observations. Some time ago Reiter 5 annealed thin films of polystyrene (PS) at temperatures (T) below T g and found that the films appreciably thickened with time. At the same time, Orts et al. 6 made the controversial observation that similar sub-R g -thick PS films sup- ported on a silicon (Si) wafer would contract upon heating, prior to dewetting at elevated T. Tolan 7 re- peated these X-ray reflectivity measurements, confirm- ing the negative coefficient of thermal expansion (CTE) seen by Orts et al. 6 More recently, Kanaya et al. 8 confirmed this negative CTE via neutron reflectivity. Reiter investigated these phenomena further and pro- posed a sub-T g elastic dewetting model. 9 In this model, a proliferation and growth of pinholes occurs (below T g ) due to the elastic stresses in the PS film. Conservation of mass dictates that as the pinholes grow in diameter with a concomitant thickening of the material between the holes. The rimless pinholes continue to grow (i.e., thickening the film) until the film becomes unstable and completely dewets the substrate. We will demonstrate that certain parts of the original elastic dewetting mechanism in PS are not general and illustrate several important molecular considerations needed to fully comprehend this curious phenomenon. By studying thin polycarbonate (PC) films, we show that the isothermal thickening is not necessarily a precursor to dewetting. This is already in line with the observa- 2901 Macromolecules 2004, 37, 2901-2908 10.1021/ma035580+ This article not subject to U.S. Copyright. Published 2004 by the American Chemical Society Published on Web 03/24/2004