Development of a Layer Model for the Dynamics of Polymer Films

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TECHNICAL SUMMARY:It has long been observed that the glass transition temperature of a large variety of polymer films decreases with decreasing film thickness. The most frequently cited model, the layer model, assumes that a highly mobile layer exists at the free surface of the films and engenders a reduction in the overall dynamics of the films. It is still not known how the surface mobile layer brings about the observed change in the glass transition temperature. In a recent experiment, the temperature dependence of the viscosity of short-chain polystyrene films supported by silicon was measured, and found to display a good consistency with the experimental glass transition temperature. More importantly, the data was found to be fully accountable by a two-layer model assuming the total flow mobility of the film to be the sum of the flow mobility of a surface mobile layer and a bottom, bulk-like layer. In that experiment, only polystyrene films on silicon with a single molecular weight was studied. In this program, a series of experiments will be carried out to explore the validity of the layer model for a wider range of polymer molecular weights and polymer films with different polymer-substrate interactions. Molecular weight is well-known to have a strong effect on the dynamics of polymers. Specifically, it can bring about qualitatively different dynamical behaviors depending on whether the polymer is entangled or not. At the same time, different polymer-substrate interactions have been attributed to be the cause of the increase in the glass transition temperature with decreasing film thickness observed in some polymer film systems. The goal of this program is to develop an encompassing layer model that would allow an effective way of predicting the viscosity and glass transition temperature of a broad range of thin film systems.NON-TECHNICAL SUMMARY:Glass transition temperature is an important physical property of polymers in a myriad of technological applications. It determines when the material softens and eventually loses the ability to hold its shape. Ample evidence accumulated over the past 15 years shows that this property can deviate substantially from that of the bulk when the polymer is made into nanometer thick films. In this program, a physical model will be developed to describe the phenomenon. If successful, the model will enable strategic design of the glass transition temperature of nanometer polymer films, and thereby significantly streamline the adaptation of polymers in nano-scale applications. The major impact of this program would be in the generation of new knowledge and training of personnel in the STEM field. Participants of this program will not only receive training on the state-of-the-art characterization techniques and processing of nanometer polymer films, they will also be exposed to an international environment due to the on-going collaborations between the PI and a number of international groups. Two graduate students will be trained. The PI will also actively recruit undergraduates and high-school students to the program. She has a strong record of engaging junior students in her research.