The overarching goals of this work are to further characterize the diffusion behavior of polymer ultra-thin films and polymers in confined geometries, elucidate the length scales and magnitudes of such behavior, and to develop a self consistent and comprehensive fundamental understanding of the mechanisms that are responsible for the observed phenomena. Preliminary results by the PIs from both experiment and molecular simulation suggest a novel model based on changes in both polymer chain mobility and free volume distribution that can explain all of the observed glass transition, coefficient of thermal expansion, diffusion, and proton conductivity results in a consistent manner. Combined, these two concerted mechanisms can lead to the rich diversity of polymer thin film behavior observed thus far.
A unique team has been assembled to integrate experimental characterization with molecular simulation in order to elucidate the origin of these thin film phenomena. Professor Clifford Henderson directs the experimental studies and has extensive experience in polymer characterization and polymer thin films. Professor Peter Ludovice has extensive expertise in molecular simulation and l supervises the modeling work.
The intellectual merits of the proposal can broadly be defined as: (1) characterizing the diffusion behavior of ultra-thin polymer films and (2) developing a fundamental model to explain the physiochemical properties of polymer ultra-thin films and polymers in confined geometries. The broader impacts of this activity include: (1) providing guidance on and opportunities for overcoming some of the roadblocks in microlithography to benefit the microelectronics industry, (2) providing guidance on methods to enhance polymer membrane performance for fuel cells and gas separations, (3) educating undergraduate and graduate students in a manner that synergistically blends modeling and experiment, and (4) enhancing minority and secondary school education in science and engineering.
Background: Currently there are significant knowledge gaps in fundamentally understanding the thermodynamic and transport properties of polymer thin films and polymers in confined systems (e.g. composite membranes), and the dependence of these properties on polymer type, interface type, film thickness, and preparation conditions. Polymers in thin film and confined geometry configurations are a critical element today in a variety of applications including semiconductor manufacturing, biomedical and tissue engineering, industrial gas separations, and fuel cells. The lack of a fundamental understanding of the physiochemical properties of polymer thin films poses a roadblock to the rational design of improved materials and processes for these applications. Recent polymer film studies have shown that a wide variety of polymer properties deviate from bulk behavior as the film thickness decreases below some critical thickness that varies depending on the property of interest. No single theory proposed thus far can adequately explain all of the observed thin film behavior, and in many cases the observed changes in film properties appear to be inconsistent with one another. For example, it has been observed that the glass transition temperature (Tg) of supported polymer thin films can decrease when the film is coated on a weakly interacting substrate and current explanations of this are based on increases in polymer chain mobility near the film surfaces. On the other hand, recent experiments by the PIs have shown that the diffusion coefficient of small penetrant molecules in such supported thin films decreases dramatically as the film thickness decreases. The simple chain mobility argument does not explain this and would in fact predict an opposite behavior. Recent measurements by the PIs also suggest that the proton conductivity of ultra-thin polymer films does not exhibit reductions similar to the gaseous penetrant behavior as film thickness decreases.
