Thickness Dependence of Free-Standing Thin Films GE ´ RALD GUE ´ RIN, ROBERT E. PRUD’HOMME Centre de recherche en sciences et inge ´nierie des macromole ´cules, De ´partement de chimie, Universite ´ Laval, Sainte-Foy, Que ´bec G1K 7P4, Canada Received 28 February 2006; revised 10 September 2006; accepted 14 September 2006 DOI: 10.1002/polb.20995 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Below a critical thickness, of about 60 nm, the glass transition tempera- ture of polystyrene (PS) films decreases with film thickness, as demonstrated using free-standing films. A geometrical model is developed here describing this phenom- enon in the case of ideal (Gaussian) chains. This model, which can be considered as an application of the free volume model, assumes that the decrease of the glass tran- sition temperature from thick to ultrathin films is due to the modification of the interpenetration between neighboring chains. The theoretical curve deduced from the model is in excellent agreement with the PS experimental results, without using any adjustable parameters. From these results, it can be concluded that new chain motions, usually buried in bulk samples, are expressed by the presence of the sur- face. V V C 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 10–17, 2007 Keywords: glass transition; polystyrene; thin films INTRODUCTION Even if surface properties of polymers are most important, for example, in the area of adhesion and for the development of biomaterials, it is still debated whether molecular motions at the polymer surface are the same as in the bulk; in other words, if there is a surface glass transition temperature different from the bulk glass tran- sition temperature (T g ). The development of highly sensitive techniques such as near-edge-X- ray absorption fine structure (NEXAFS) spec- troscopy and scanning force microscopy gives the possibility to study directly the surface of thick polymeric materials. However, the results are still conflicting, as high- lighted by the following examples concerning poly- styrene (PS) films. With NEXAFS spectroscopy, Liu et al. 1 did not detect any change of mobility of the polymer chains at the surface, while Tanaka et al. 2,3 found, by scanning force microscopy, a decrease in T g of at least 20 K, which was confirmed by X-ray photoelectron spectroscopy. 4 These results were assigned to an increase in the polymer chain mobil- ity but, more recently, the same authors observed 5 that the a-transition peak of high molecular weight polymer chains located at the surface, even if slightly lower than that in the bulk, does not depend on the film thickness, contrary to the b-relaxation peak, which decreases with the film thickness. Techniques such as ellipsometry, 6,7 X-ray re- flectivity, 8,9 or positron annihilation spectros- copy 10,11 allow the study of films having thick- nesses of a few nanometers. In such experi- ments, the glass transition has shown a strong dependence on the film thickness and on the na- ture of the substrate: 12 the presence of attrac- tive interfacial interactions at the polymer–sub- strate interface leads to an increase of T g , 13 while repulsive interfacial interactions induce a Correspondence to: R. E. Prud’homme, Department of Chemistry, University of Montre ´al, C.P. 6128, Succursale Centre-ville, Montre ´al, Que ´bec H3C 3J7, Canada (E-mail: re.prudhomme@umontreal.ca) Journal of Polymer Science: Part B: Polymer Physics, Vol. 45, 10–17 (2007) V V C 2006 Wiley Periodicals, Inc. 10