Thesis (Ph. D.)--University of Massachusetts Amherst, 2009. / Includes bibliographical references (p. 98-107). Print copy also available.

Thesis (MSc)--University of Stellenbosch, 2006. / Bibliography. Also available via the Internet.

Polymer-Based membranes play a critical role in several industrially important gas separation processes, e.g., carbon dioxide removal from natural gas. However, an intrinsic trade-off between membrane flux (characterized by its permeability) and selectivity to one gas over the other has limited their effectiveness in practical environments. While some incremental success has been obtained by empirically developing new polymer chemistries, the best hopes for transformative improvements may require novel architectures employing predictive structure/property relationships. In this work, we develop a novel hybrid membrane construct comprised of inorganic nanoparticles grafted with polymer chains to form grafted nanoparticles. We find that the grafting architecture almost exclusively results in enhanced gas transport properties, in contrast with those expected from conventional predictions. These enhancements, found to be a result of elevated diffusion constants, are broadly tunable with the grafted chain length and leads to order of magnitude increases in gas permeability. We conjecture that the grafted polymer chains serve to impart added free volume to the composite material, which manifests itself as enhanced gas diffusion relative to the pure polymer. Indeed, multiple experimental and simulation probes verify this picture, and indicate that the free volume increases are a result of the grafted chains adopting anisotropic conformations to fill space. Building off of this finding, we systematically study the effects of the nanoparticle core size and chain grafting density, and find that both the chain length where the maximum permeability occurs, as well as the extent of the enhancement, varies depending on the relative sizes of the chains and the nanoparticle. A thorough structural analysis of the grafted nanoparticles in dilute solution as well as bulk samples indicate that the relation between the measured polymer brush height and the chain length undergoes a transition at intermediate chain lengths, similar to the observed gas permeability enhancements. Using a simple scaling approach, we show that this transition is related to the crossover from a concentrated polymer brush with higher order scaling to a semi-dilute brush where the chains are more ideal. We hypothesize that this impenetrable concentrated brush phase is the source of the added free volume, and that this effect is diminished when the grafted chains are longer than the transition point and the penetrable, semi-dilute polymer brush begins to dominate gas diffusion. When cast in the framework of free volume theories, this prediction accurately captures the trends in gas diffusion; the result is a unique structure/property relation that can be used to design optimal membrane materials. We expand on these constructs to probe other grafted nanoparticle-based architectures incorporating free polymer chains and advanced chemistries to further manipulate the gas transport properties of these mixed-matrix materials. The addition of free chains with judiciously chosen molecular weights and loadings gives a nearly independent means to tune membrane selectivity, which when combined with the intrinsic permeability increases in the matrix-free grafted nanoparticles results in superior materials that can exceed the current performance Upper Bound. We relate this result to the spacial distribution of the free chains throughout the grafted polymer corona, and how this affects the distribution of the free volume in the material as it selectively cuts off larger gas molecules. We further leverage this universal grafting platform by grafting polymer chains with novel chemistries to design membranes with record-setting selectivities while also increasing permeability by nearly two orders of magnitude. We conclude that grafted nanoparticle constructs allow for precise and predictive control of gas transport properties through a new structure/property relation, and serve as a novel material design platform with the potential to function as high performance gas separation technologies.

No description available.

Thesis (M.S.)--Ohio University, November, 1985. / Title from PDF t.p.

Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2006. / Tolbert, Laren, Committee Member ; Perahia, Dorva, Committee Member ; Perry, Joseph, Committee Member ; Collard, David, Committee Member ; Bunz, Uwe, Committee Chair.

Thesis (MSc)--University of Stellenbosch, 2006. / Bibliography. Also available via the Internet.

M.S.

This research focuses on the utilization of two living polymerization methods, anionic and group transfer, for the synthesis of well defined end functional materials and graft copolymers. Group transfer polymerization was utilized to synthesize acrylic terminal poly(methyl methacrylate) (PMMA) macromonomers of controlled molecular weight and narrow molecular weight distribution. A systematic series of PMMA macromonomers were copolymerized with 2-ethylhexyl acrylate to afford poly(2-ethylhexyl-g-methyl methacrylate) copolymers. These copolymers were synthesized in high yields with a high degree of incorporation of the PMMA macromonomer. These graft copolymers showed little or no phase mixing between the two components as evidenced by differential scanning calorimetry. It was determined by optimization studies that the reaction was complete within 50 hours at 65° C. Increases in initiator concentration surprisingly did not significantly effect homopolymerizations of 2-ethylhexyl acrylate with respect to molecular weight while the efficiency of incorporation of the macromonomer into the graft copolymers increased. End functional hydrogenated poly(butadiene) (HPBd) materials and HPBd-containing graft copolymers were synthesized using anionic polymerization methods. These materials were tested for their ability to act as adhesion promoters between poly(propylene)/EPDM and cycloaliphatic polyurethane coatings. Hydroxyl or carboxyl end functional materials were synthesized where molecular weight and chain microstructure were syStematically varied. Effective adhesion was achieved when the molecular weight of the polymer was approximately 20 kg/mol, the polymer was hydrogenated, contained 90 mole percent or greater 1,2- content in the poly(butadiene) precursor, and contained a functional end group, which may be either hydroxyl or carboxyl. To increase the concentration of functional groups over the above materials graft copolymers were utilized. Acrylic terminal HPBd macromonomers were synthesized and copolymerized with butyl acrylate in combination with either N,N-dimethylacrylamide (DMAA), 2-hydroxyethyl methacrylate (HEMA), methacrylic acid (MA), or t-butyl methacrylate (TBMA). Systematic compositions of these graft copolymers were synthesized and tested for adhesion. The acidic containing graft copolymers provided the most positive adhesion results. In one case the hydroxyl containing material also gave positive adhesion results. The DMAA containing materials failed in all cases. The TBMA route allowed for greater control over the composition of the acidic graft copolymers. / Ph. D.

Polymer based membranes play a key role in several industrially important gas separation technologies, e.g., removing CO2 from natural gas, with enormous economic and environmental impact. In this thesis, we develop a novel hybrid membrane construct comprised entirely of inorganic nanoparticles grafted with polymer chains. For all graft architectures studied, the permeability of several small gases and condensable solvents are higher in GNP membranes than the neat polymer analogs. More interestingly, the matrix-free GNPs displayed a non-monotonic peak in gas permeability as a function of grafted chain molecular weight, M_n, at a fixed grafting density, σ. Furthermore, in contrast to neat polymer membranes, which suffer from degraded performance over time due to chain densification and “aging”, the performance of GNP membranes is preserved for months to years. We show that these enhancements are not limited to a single polymer, thus we suggest that this grafting mechanism may be an option to improve permeability in polymer membranes in general. We conjecture the grafted polymer chains must stretch to fill the interstitial voids in the NP “lattice”, as such voids would be free-energetically unfavorable due to the relatively high surface tension of the polymer melt. Since this stretching leads to an unfavorable chain conformational entropy, we expect a decrease in the polymer density, which we verify experimentally as well as through molecular dynamics simulations. When a penetrant molecule is placed in these regions of highest distortion, the chains can assume more favored, undistorted conformations. This in turn creates a driving force for further penetrant uptake. Therefore, we systematically study the structure and dynamics of matrix-free GNP materials at various chain grafting densities and a wide range of graft molecular weight. Small angle scattering experiments reveal that the core nanoparticle spacing systematically increases with increasing molecular weight but the overall morphology remains amorphous and isotropic. Whereas previous studies1 have found the brush height in matrix-free GNPs scales as the degree of polymerization 〖~N〗^0.5, we find that the brush height in our systems scales 〖~N〗^0.7, indicating the chains are indeed highly stretched. Moreover, studies of the structural evolution upon swelling with solvent show that the brush is fully wetted and the solvent distribution is homogeneous within the film. Additionally, we systematically probe the dynamics of matrix-free GNP systems over broad length and time scales using linear and non-linear mechanical rheology, and broadband dielectric spectroscopy. The linear viscoelastic response shows that while the polymeric signal (e.g. glassy and Rouse dynamics) is equivalent for a range of graft chain lengths, the terminal flow of these materials is slowed by several decades compared to the neat melts of corresponding molecular weight. The low frequency (long time) response shows that below a critical molecular weight, these systems transition from polymeric to that of a colloidal system. To understand this behavior, a scaling theory is developed to describe the polymer brush conformation, which reveals that at this transition point the grafted particles behave as a system of packed “rigid” spheres. We note that the transition point coincides with the maximum observed in the transport behavior, and that the reduced system mobility may be responsible for the reduced aging effects. On the other hand, secondary relaxations for GNPs at this transition molecular weight are found to be faster than the neat polymer of corresponding molecular weight, which is attributed to a lower effective polymer density found in these samples. Therefore, the critical question underpinning this work is: how do the structure and dynamics influence and/or result from increased free volume in matrix-free grafted nanoparticle materials? We conclude that matrix-free grafted nanoparticle constructs allow for precise control of structure-property relationships over multiple length scales, and serve as a novel materials design platform with the potential to function as high performance gas separation technologies.