Intellectual Merit. Micro-, meso-, and macroporous polymers greatly benefit the fields of tissue engineering, photovoltaics, microelectronics, hydrogen storage, and gas separations. Porous polymers are often produced through polymerization-induced phase separation (PIPS), thermal-induced phase separation (TIPS), or other phase-inversion techniques. This study represents a departure from traditional solution- or melt-phase methods. The goal is to develop vapor deposition polymerization (VDP) as a controllable, environmentally sound technique to producing porous and micro-structured polymers. Reactive monomers and non-reactive species (porogens) will be co-deposited onto a cooled substrate to force phase separation and arrest kinetically trapped micro- and nano-scale microstructures. The concept will be studied via two avenues: i) radical chain-growth polymerization of amorphous polymers and ii) condensation polymerization of rigid-rod, high performance polymers. For both cases, reactant and porogen concentrations will be systematically varied with time to create depth-dependent morphologies and densities. This will enable new routes to custom-designing asymmetric membranes. A second goal is to mechanistically relate observed morphologies to process conditions and to models of phase behavior. To achieve this, cross-sectional analysis of as-deposited films will be studied using optical and electron microscopy techniques.
Initial experiments have focused on poly(methyl methacrylate) as a system to study free-radical growth of polymers in the presence of a porogen. Results indicate that films can be grown from the vapor phase in a controlled and repeatable manner. When a low-molar mass porogen is introduced, phase separatio occurs. To study condensation polymerization, a second low pressure (~10-6 Torr) VDP reactor has been reconfigured for co-deposition of polyimide precursors with thermally-degradable porogens. Similar experiments will be conducted using p-type phthalocyanine dyes (instead of a porogen) to fabricate and test polymer-stabilized photovoltaics.
Broader Impacts. The research has strong connections to potential applications. Specifically, VDP of microporous rigid-rod polymers will provide new routes to 1) all-organic gas separation membranes and hydrogen storage materials, 2) low-K dielectric materials that are compatible with current trends in microelectronics processing, and 3) to polymer-stabilized organic photovoltaics. It is expected that future research programs in at least one of these directions will spawn from this study. Research will also lead to an improved understanding of bulk polymerization, phase separation, and vitrification occurring in micron-thick polymer films. The study will ultimately improve the ability to control micron-scale membrane features and their one dimensional spatial distribution. The integration of research and education will be through curriculum development of hands-on application projects and special topics in undergraduate and graduate-level courses. Community outreach activities include organizing professional development, Green Engineering, lectures for local high school teachers. These lectures will highlight environmental issues and solutions, and provide new teaching tools to bridge the gap between real-life problems and traditional textbook learning.
