Effect of Sample Preparation on the Glass-Transition of Thin Polystyrene Films Z. FAKHRAAI, 1 J. S. SHARP, 2 J. A. FORREST 1 1 Department of Physics and Guelph–Waterloo Physics Institute, University of Waterloo, 200 University Ave. W., Waterloo, Ontario, Canada, N2L 3G1 2 School of Physics and Astronomy, University of Nottingham, University Park, Nottingham, NG7 2RD, U.K. Received 1 June 2004; revised 12 August 2004; accepted 15 August 2004 DOI: 10.1002/polb.20281 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT We have investigated the effect of sample preparation on the glass-transi- tion temperature (T g ) of thin films of polystyrene (PS). By preparing and measuring the glass-transition temperature T g of multilayered polymer films, we are able to assess the contribution of the spincoating process to the reduced T g values often reported for thin PS films. We find that it is possible to determine a T g even on the first heating cycle, and that by the third heating cycle (a total annealing time of 15 min at T = 393 K) the T g value has reached a steady state. By comparing multilayered versus single layered films we find that the whole T g depends only on the total film thickness, and not on the thickness of the individual layers. These results strongly suggest that the spincasting process does not contribute significantly to T g reductions in thin polymer films. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4503– 4507, 2004 INTRODUCTION There has been a significant amount of interest in recent years in the physical properties of very thin polymer films. 1–3 In particular, in a number of cases the properties of thin films display signif- icant deviations from that of the bulk material, or even that of much thicker films. Of particular interest is the glass transition temperature (T g ). For many different polymers, measurements of the T g in thin films have revealed large differ- ences from the bulk value. Of these polymers, the most well studied material is atactic polystyrene (PS). Since the original study by Keddie et al. 4 showing thin film T g values lower than that of the bulk, there have been many studies aimed at quantifying this behavior. 5 A recurring concern about these and other studies of thin polymer films involves the effect of sample preparation. 6, 7 Such films are typically spin cast from dilute so- lutions of polymer in a good solvent (such as tol- uene in the case of PS) and then annealed above the bulk T g value for a time usually ranging from a few hours to a few tens of hours. During the process of spincasting, the chains may be in con- figurations not typical of those in equilibrium melts. In addition, even though the films are an- nealed, in many cases (depending on the sub- strate on which they are prepared) they are very different from the equilibrium state of a fully dewetted film. This concern is often dismissed, although there is not yet any proof that it is valid to do so. Although a definitive explanation to the observed T g reductions has not yet emerged, there is compelling evidence that it is related to the presence of the free surface. 8 One way to distin- guish between an effect due to the free surface Correspondence to: J.A. Forrest. (E-mail: jforrest@ uwaterloo.ca) Journal of Polymer Science: Part B: Polymer Physics, Vol. 42, 4503– 4507 (2004) © 2004 Wiley Periodicals, Inc. 4503