Glass Transition Temperature in Confined Polymers
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Glass Transition Temperature in Confined Polymers Rahmi Ozisik, Tong Liu, and Richard W. Siegel Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY, 12180 ABSTRACT Glass transition temperature of poly(ether imide), PEI, thin films and nanoporous PEI samples was investigated using differential scanning calorimeter. In both of these systems, the glass transition temperature decreased with respect to the bulk value. In the nanoporous system, scanning electron microscope images were used to characterize pore size distribution, and Monte Carlo simulations were performed to calculate the nearest neighbor pore-to-pore distances. Poreto-pore distances and thin film thickness values were used to establish a quantitative analogy between thin films and nanoporous system. INTRODUCTION In 1965, Adam and Gibbs [1] defined cooperative rearranging regions that are domains of several nanometers in size, and are the smallest regions that can undergo a transition to a new configuration independent of neighboring regions. Their work lead to the investigation of glass transition temperature (Tg) in polymers under confined geometries. For example, one approach was to use nanoporous glass with controlled pore size as host and to fill the pores with polymers by capillary wetting. As a result, reduced Tg values and enhanced chain dynamics have been reported [2]. Another system is polymer thin films in which the confining dimension is the film thickness. Since the initial measurement of Tg in polystyrene (PS) thin films supported by a silicon substrate performed by Keddie et al. [3], both substrate-supported and unsupported (freestanding) thin films of different polymers have been studied. Deviations of Tg from the bulk value have been reported when the film thickness decreased to tens to several hundreds of nanometers. It was observed that Tg depends strongly on the interface between the polymer film and the substrate [4-6]. When the interaction between the film and the substrate is weak, Tg of the film has a lower value than the corresponding bulk value. Glass transition temperature of the film is greater than the bulk value when there exists a strong interface between the film and the substrate because of the strong adhesion of polymer chains to the substrate. The influence of the polymer-substrate interaction can be eliminated in freestanding polymer thin films, where only a reduction in Tg is observed [7,8]. In addition to the dependence on polymer-substrate interface, further studies on glass transition behavior in polymer thin films showed complex molecular weight dependence [7-9]. In order to interpret the phenomenon observed in polymer thin films and understand the glass transition behavior, many attempts have been made to generate a quantitative connection between the intrinsic length scale of the cooperative rearranging regions and the length scale at which the confinement effects on Tg are observed [3-5,9]. However, research on glass transition behavior in polymer thin films does not suppor
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