The formulation of polymeric nanocomposites via the inclusion of nanoscale filler particles often leads to dramatic improvements in material performance.  The introduction of inorganic nanoparticles creates vast amounts of particle-polymer surface area and particle-polymer interactions, as well as physical confinement effects, can produce substantial changes in the character of the polymer matrix that correlate with the resulting composite properties.  By controlling particle dispersion and the nature of polymer-particle interactions, it is possible to tailor materials to achieve enhanced macroscopic performance for a variety of applications.  In this project, we seek to exploit recent discoveries revealing that the inclusion of nanoscale particles results in marked improvement in the performance of rubbery gas separation membranes, as manifested by significantly increased permeability.  Working with Prof. Benny Freeman at the University of Texas at Austin, we have undertaken a fundamental study to identify the chemical and physical factors that govern nanocomposite morphology in model rubbery and glassy membrane systems and their relation to gas transport properties.  Of particular interest is the influence of particle-polymer interactions and local confinement effects on the dynamics of the polymer matrix, and the potential correlation of polymer relaxation behavior with the resultant permeability and selectivity of the composite membrane.

Initial activities have focused on the preparation and characterization of two model nanocomposite membrane systems based on rubbery and glassy polymers, respectively.  Rubbery nanocomposite networks were prepared by UV photopolymerization of poly(ethylene glycol) diacrylate [PEGDA] in the presence of varying loadings of MgO nanoparticles (nominal particle size of 3 nm).  Polymer networks based on crosslinked PEO have been shown to be effective for the removal of carbon dioxide from light gas mixtures, and we have previously undertaken an extensive study of the dynamics and gas transport properties of these (unfilled) materials as a function of network composition and architecture.  For the nanocomposites, the liquid pre-polymerization mixture was prepared by conventional mixing methods, as well as by planetary mixing.  At higher loadings, particle dispersion was enhanced by the introduction of toluene in selected cases, and the influence of the diluent on the characteristics of the crosslinked network and resulting composites was assessed.  Dynamic mechanical and dielectric studies performed on the PEGDA/MgO nanocomposites revealed the emergence of a second, distinct glass-rubber relaxation event displaced ~ 40°C above Tg of the native network, suggesting the presence of a sub-population of constrained chain segments impeded by their proximity to the nanoparticle surface.  A similar behavior was observed for model glassy nanocomposites prepared via the solution casting of mixtures based on polyetherimide [PEI] and silica nanoparticles.  In the latter case, modified silica particles with varying surface characteristics were introduced, and the emergence of a second Tg at elevated temperature was correlated with the nature of the polymer-particle interaction.  Full characterization of the observed dual Tg behavior, and its relation to nanocomposite morphology, is ongoing.

A key consideration for the interpretation of both the thermomechanical and transport behavior of these materials is the quality of the nanoparticle dispersion.  Bulk density measurements on the PEGDA/MgO nanocomposites have indicated density values at higher MgO loadings that are well below those anticipated based on simple volume additivity.  Comparable trends have been observed for MgO-filled nanocomposites based on 1,2-polybutadiene, as reported by Freeman et al.  The negative deviation in bulk density implies that a significant amount of void volume fraction or enhanced free volume is incorporated into the nanocomposites; in the case of PEGDA/MgO, the degree of void incorporation is a function of the pre-polymerization mixing details.  Preliminary PALS measurements on the PEGDA/MgO material (performed by Dr. Anita Hill at CSIRO, Australia) suggest that the sizescale associated with the incorporated void volume exceeds the resolution limit of the PALS technique (i.e., > 5 nm diameter).  X-ray phase contrast ultramicroscopy studies, also performed at CSIRO, confirmed the presence of a sizeable void fraction, and the dimensions and distribution of the voids presumably have significant ramifications for the observed transport properties.  Activities in the next phase of the project will include detailed evaluation of the morphology obtained in the nanocomposites, as well as a comprehensive study of gas transport.