45353-AC7
Investigation of Structure and Dynamics in Novel Nanocomposite Membranes
Polymer nanocomposite materials have been the subject of intense research activity over the last two decades. The introduction of nanoscale structure via the inclusion of nanoparticles, nanoclays, or carbon nanotubes has been correlated with dramatic improvements in material performance as compared to more traditional filled polymers. The dispersion of inorganic nanofillers creates vast amounts of filler-polymer surface area and the resulting filler-polymer interactions, as well as physical confinement effects, can produce substantial changes in the character of the polymer matrix, often leading to new opportunities in terms of material applications. Recently, a series of studies have demonstrated the potential for enhancement of gas separation performance in nanocomposites based on both glassy and rubbery polymers. It was reported that in high free volume glassy polymers, the introduction of nanoparticles increased penetrant diffusion via disruption of polymer chain packing, leading to a subtle increase in local free volume [Merkel et. al, 2002]. For certain rubbery polymer nanocomposite systems, enhancements in permeability have also been realized that reflect a combination of solubility and diffusivity effects, and which correlate strongly with the quality of the nanoparticle dispersion [Matteucci et. al, 2008]. In this project, we have sought to more fully understand these phenomena by examining the influence of inorganic nanoparticles on the static and dynamic properties of model rubbery and glassy polymers. 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 these 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.
Our activities have focused on two model nanocomposite membrane systems: (i) rubbery PEO network nanocomposites prepared via the in-situ polymerization of poly(ethylene glycol) diacrylate [PEGDA] in the presence of magnesium oxide or silica nanoparticles, and (ii) glassy polyetherimide [PEI] nanocomposites formulated by solution processing with modified silica. Previous studies performed in collaboration with Prof. Freeman have demonstrated the viability of XLPEGDA network membranes for the removal of carbon dioxide from light gas mixtures, and we have fully characterized the dynamics and gas transport properties of these (unfilled) materials as a function of network composition and architecture. During the current project period, we have completed the experimental characterization of the static and dynamic relaxation properties of the PEGDA/MgO and PEGDA/silica nanocomposites via dynamic mechanical and dielectric relaxation spectroscopy. The influence of the nanoparticles on the sub-glass and glass-rubber relaxation characteristics of the membranes has been assessed as a function of particle type and loading, and the impact of added dispersion aids (i.e., diluents) on the properties of the polymer matrix was evaluated. Bulk density measurements completed on the PEGDA nanocomposites indicated only minimal negative deviations from volume additivity, suggesting that the large void volumes reported for other (solvent-cast) rubbery nanocomposite systems were not present. Permeability studies showed a decrease in gas transport with increasing particle loading that was consistent with the Maxwell model; modest enhancements in permeability were realized when the crosslinking reaction was conducted in the presence of toluene diluent. For this system, the in-situ polymerization of the PEO network in the presence of the nanoparticles apparently produced a network morphology substantively different from that obtained in previous studies, such that permeability enhancement was not observed.
As a complement to the fundamental relaxation studies performed on the rubbery XLPEGDA nanocomposite membranes, glassy nanocomposites prepared from solution blending of polyetherimide with surface-modified silica nanoparticles have been studied using dynamic mechanical analysis and broadband dielectric spectroscopy. For the PEI materials, the influence of particle-polymer interaction and potential physical confinement has been investigated as a function of particle surface chemistry and bulk particle loading. At moderate to high loadings, the presence of the particles led to the emergence of a second, distinct glass-rubber relaxation event displaced ~ 45°C above Tg of the unfilled polymer, suggesting the presence of a sub-population of constrained chain segments impeded by their proximity to the nanoparticle surface. Initial studies suggest that the attributes of this second relaxation can be correlated with the surface properties of the silica and their relation to particle dispersion. Activities during the final phase of the project (re: no-cost time extension) will focus on a comprehensive characterization of the relaxation behavior of the PEI nanocomposites, in concert with a detailed evaluation of specimen morphology. In addition, transport studies will be completed on selected formulations in an effort to elucidate potential correlations between the static and dynamic properties of the nanocomposites, and their gas separation performance.
