Polymer Communication Structural relaxation and dynamic fragility of freely standing polymer films Simone Napolitano * , Michael Wübbenhorst Katholieke Universiteit Leuven, Laboratory for Acoustics and Thermal Physics, Department of Physics and Astronomy, Celestijnenlaan 200D, B-3001 Leuven, Belgium article info Article history: Received 12 August 2010 Received in revised form 20 September 2010 Accepted 22 September 2010 Available online 29 September 2010 Keywords: Ultrathin polymer films Freely standing films Glass transition abstract In addition to a tremendous reduction in the glass transition temperature, dielectric spectra of freely standing films reveal two other intriguing features: a temperature dependent asymmetric broadening of the structural relaxation peak towards lower temperatures and a reduction of the dynamic fragility down to the monomer limit. We verified that this experimental evidence is a manifestation of a gradient of glass transition temperatures across the film thickness induced by an enhanced molecular mobility at the two free surfaces of the membrane. As a direct implication of the peculiar features just described, the properties of freely suspended membranes neither correspond to those in bulk nor to a simplified scenario where the structural relaxation peak is merely shifted towards lower temperatures. Ó 2010 Elsevier Ltd. All rights reserved. The excellent processability and the broadly-tunable perme- ability of macromolecular materials have stimulated a large-scale development of polymer membranes for applications ranging from ultrafiltration to separation of gases and smart packaging [1]. Due to the continuing miniaturization of devices, the production and characterization of thinner membranes are pushed down to the nanometer scale [2e4]. Compared to inorganic materials (failing as sensing elements already in the sub-millimeter range due to an excessive rigidity), polymers, owing to lower processing costs, intrinsical ductility and lower stiffness, are more suitable candi- dates for future applications of nanomembranes in fields like biomedicine and microelectronics [5]. Although such applications are promising, a more accurate understanding of the physical properties of nanomembranes is a prerequisite for their future improvements. Valuable contributions to this issue come from the ongoing debate on dynamics of ultrathin polymer films, for a Review see Refs. [6e8]. In particular, from purely geometrical considerations, freely standing films, showing depressions of the glass transition temperature (T g ) by more than 70 K [9], are appropriate benchmark systems to test properties of freely sus- pended polymer nanomembranes. Furthermore, unveiling the molecular origins of the relaxation processes taking place in the presence of free surfaces and their penetration depth is crucial for the comprehension of the behavior of soft matter at the nanoscale and has a huge impact on the development of ultrastable glasses [10]. These investigations are also inline with deGennes’s suggestion to perform experiments that probe the distribution in glass transition temperatures [11] within thin polymer films rather than determining just their mean value. Although a large number of investigations focused on the properties of nanometer-thick layers supported on solid substrates, just a limited number of experi- mental studies has been devoted to the characterization of freely standing films [12e17], due to experimental conditions requiring greater efforts (e.g. longer sample preparation, lower signal/noise ratios, difficulties to apply electric fields and to couple to mechanical stresses). In this Communication, we show that, in addition to a tremen- dous reduction in the glass transition temperature [9,16,17], dielec- tric spectra of freely standing films reveal more intriguing features not present in supported and capped films [18e20]: a temperature- dependent asymmetric broadening of the structural (a-)peak towards lower temperatures and a reduction of the dynamic fragility down to the monomer limit. We compare our experimental results with other work on thin films supporting a strong gradient of T g ’s extending inside the film for a few hundreds of nanometers. Freely standing membranes of atactic polystyrene (M w ¼ 932,000 g/ mol, M w /M n ¼ 1.2, R g ¼ 28 nm) were obtained by spincoating dilute solutions of the polymer in chloroform onto freshly cleaved mica sheets. Polymer layers were subsequently floated on ultrapure water and finally allowed to dry in air for several days before being finally transferred and suspended onto interdigitated comb electrodes (IDE) (Xensor Integration). To reduce thermal and mechanical stresses, IDE structures were preheated at T g BULK þ 10 K during the transfer of the membrane. Differently to the conventional methods used to investi- gate ultrathin films, IDE structures permits to apply electric fields to * Corresponding author. E-mail address: simone.napolitano@fys.kuleuven.be (S. Napolitano). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.09.060 Polymer 51 (2010) 5309e5312