This NSF award by the Chemical and Biological Separations program supports work by Professor Nancy Lape at Harvey Mudd College to improve gas separation membranes by tailoring free volume, and therefore gas separation properties, of inorganic/polymer composites.
With increasing environmental awareness and rising energy costs, traditional gas separation methods such as absorption (for CO2/CH4 and CO2/N2 separations) and cryogenic distillation (for N2/O2 separations) are becoming less attractive, while demand is increasing for solvent-free, energy-efficient membrane separation processes. Unfortunately, membrane processes are entirely dependent on the availability of high-performance (sufficiently high permeability and high selectivity) membrane materials that are not currently available. Recent research in nanocomposite films has shown promising gains in this area: the addition of impermeable nanoparticles to an ultra-high free volume polymer can actually increase permeability relative to the pure polymer while maintaining or increasing its selectivity. This runs counter to predictions based on the Maxwell model, long proven for micron-size particles, of permeability decreases upon the addition of impermeable particles. The improvements have been shown to be the result of free volume increases in the polymer, but the mechanism behind this free volume increase, while typically attributed to the formation of interfacial voids, is not understood. Additionally, it is unknown how this permeation enhancement depends on particle size, polymer chain rigidity, and other interfacial effects.
This CAREER award aims to address this deficiency by performing a systematic investigation of the following three factors: (1) primary particle and aggregate size, (2) polymer chain rigidity, and (3) interfacial effects using a four-pronged approach:
(a) Composite membrane formation. A variety of materials and techniques will be used to prepare composite films including a range of particle sizes from nanoscale to microscale to examine the transition between Maxwell-type behavior and permeability enhancement; a range of polymer chain rigidity including rubbery, conventional glassy, and ultra-high free volume glassy polymers; and a range of particle surface treatments to examine interfacial effects. Permeation in previously unstudied membranes will be synthesized and examined using surface-initiated atom-transfer radical polymerization (SI-ATRP) which eliminates interfacial void formation.
(b)Gas permeation tests. Permeation tests on all pure polymer and composite films will be run to determine the permeability and selectivity of various gases.
(c) Characterization. All films will be characterized using positron annihilation lifetime spectroscopy (PALS) and density measurements to examine free volume size and distribution and transmission and scanning electron microscopy (TEM and SEM) to examine particle dispersion.
(d) Molecular Modeling. The molecular-level configuration, interfacial effects, and theoretical free volume of pure polymers and polymer/inorganic nanocomposites will be modeled to illuminate the changes in molecular-level architecture that give rise to changes in free volume. While the literature abounds with molecular modeling (MM) results for pure polymers, nanocomposites are an almost entirely unexamined area for MM.
The main educational goals of this project are centered on two aspects: Course and Educational Method Developments and an Integrated Undergraduate Research and Multi-Tiered Mentoring Program. To address the first aspect, Challenge-Based Instruction Modules will be developed and disseminated for use in courses. Also a new course aimed at chemistry and engineering Harvey Mudd College (HMC) undergraduates in Polymer Chemistry and Engineering is planned. The new course will include experimental modules in the area of the proposed research. The research will be carried out entirely by undergraduate and high school researchers. The undergraduates will benefit from a multi-tiered structured mentoring program in which they are mentored by the PI, alumni, and each other, and will be trained as mentors for high school students. Technical results and the models developed for undergraduate and high school student research and mentoring will be widely disseminated via publications and presentations.
