Phase separation in polymer blend thin films studied by differential AC chip calorimetry Nicolaas-Alexander Gotzen a , Heiko Huth b , Christoph Schick b , Guy Van Assche a , Carine Neus c , Bruno Van Mele a, * a Vrije Universiteit Brussel, Department of Materials and Chemistry (MACH), Research Unit Physical Chemistry and Polymer Science (FYSC), Pleinlaan 2, B-1050 Brussels, Belgium b Institute of Physics, University of Rostock, Universita ¨tsplatz 3, 18051 Rostock, Federal Republic of Germany c Department of Fundamental Electricity and Instrumentation, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium article info Article history: Received 28 July 2009 Received in revised form 26 October 2009 Accepted 27 October 2009 Available online 4 January 2010 Keywords: Polymer blend Phase separation AC chip calorimetry abstract AC chip calorimetry is used to study the phase separation behavior of 100 nm thin poly(vinyl methyl ether)/poly(styrene) (PVME/PS) blend films. Using the on-chip heaters, very short (10 ms–10 s) temperature jumps into the temperature window of phase separation are applied, simulating laser heating induced patterning. These temperature pulses produce a measurable shift in the glass transition temperature, evidencing phase separation. The effect of pulse length and height on phase separation can be studied. The thus phase separated PVME/PS thin films remix rapidly, in contrast with measurements in bulk. AC chip calorimetry seems to be a more sensitive technique than atomic force microscopy to detect the early stages of phase separation in polymer blend thin films. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Polymer blend thin films are interesting both for their techno- logical applications (e.g. coatings, .), and from an academic point of view. By confining a polymer (blend) in a thin film, both the air- polymer and the polymer–substrate interface can (significantly) influence its physical properties, in comparison with those observed in bulk. For polymer blends, increasing and decreasing cloud point temperatures [1,2] and changes in the phase separation mechanism have been reported [3–9]. One of the applications of polymer blend thin films are the so called ‘smart surfaces’, whose properties change as a result of phase separation by thermal stimuli, e.g. the patterning of polymer blend films by local heating. The idea is to use a polymer blend film, which is stable at the storage/working temperature, but can be annealed in the two- phase region upon heating. For patterning, local heating could, for instance, be obtained using an infrared laser. However, two ques- tions arise; firstly, can there be phase separation at the typically very short irradiation times (ms to ms) and secondly, is the resulting morphology stable? Modulated temperature differential scanning calorimetry (MTDSC) has proven to be a valuable technique for studying phase separation in polymer blends and solutions. MTDSC allows the measurement of accurate glass transition temperatures at low heating rates. Scanning experiments and quasi-isothermal experi- ments enable the monitoring of demixing and remixing kinetics of polymer mixtures by following the apparent heat capacity signal [10–18]. However, the sample mass needs to be rather large (approximately 1 mg), hampering the study of thin films. Chip based nano calorimetry could allow the calorimetric study of phase separation in thin films. In literature, two approaches for nano calorimetry are reported: a quasi-adiabatic approach, as developed by Allen et al. [19–22], and a non-adiabatic one, as developed by Schick et al. [23–29]. In this paper, polymer blend thin films are studied using a non- adiabatic differential AC chip calorimeter. The poly(vinyl methyl ether)/poly(styrene) (PVME/PS) blend system was chosen as a model system. PVME/PS blend thin films have been studied in detail in literature [1–7,30–34] and show a lower critical solution temperature (LCST) behavior, i.e. starting from a homogeneous system, they will phase separate above a certain temperature. The ability of differential AC chip calorimetry to study phase separation in thin PVME/PS blend films is evaluated. The phase separation behavior in bulk is compared with that of 100 nm thin films. Furthermore, the on-chip heater is used to apply heating pulses from the homogeneous to the two-phase region of the state diagram and this for very short durations (10 ms to several seconds). This allows the study of the phase separation kinetics on * Corresponding author. Tel.: þ32 2 629 32 76; fax: þ32 2 629 32 78. E-mail address: bvmele@vub.ac.be (B. Van Mele). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2009.10.062 Polymer 51 (2010) 647–654