The major objective of this research is a systematic experimental exploration of hydrophilic materials that can be applied as coating materials for conventional ultrafiltration (UF) membranes to improve their fouling resistance against organic components. This objective is achieved by developing new, fouling-reducing membrane coatings and applying these coatings to conventional UF membranes, which can provide unprecedented reduction in membrane fouling and marked improvements in membrane lifetime. Novel polymeric materials are synthesized via free-radical photopolymerization of mixtures containing poly(ethylene glycol) diacrylate (PEGDA), photoinitiator, and water. PEGDA chain length (n=10-45, where n is the average number of ethylene oxide units in the PEGDA molecule) and water content in the prepolymerization mixture (0-80 wt.%) were varied. Crosslinked PEGDA (XLPEGDA) exhibited high water permeability and good fouling resistance to oil/water mixtures. Water permeability increased strongly with increasing the water content in the prepolymerization mixture. Specifically, for XLPEGDA prepared with PEGDA (n=13), water permeability increased from 0.6 to 150 L um/(m2 h bar) as prepolymerization water content increased from 0 to 80 wt.%. Water permeability also increased with increasing PEGDA chain length. Moreover, water permeability exhibits a strong correlation with equilibrium water uptake. However, solute rejection, probed using poly(ethylene glycol)s of well defined molar mass, decreased with increasing prepolymerization water content and increasing PEGDA chain length. That is, there is a tradeoff between water permeability and separation properties: Materials with high water permeability typically exhibit low solute rejections, and vice versa. The fouling resistance of XLPEGDA materials was characterized via contact angle measurements and static protein adhesion experiments. From these results, XLPEGDA surfaces are more hydrophilic in samples prepared at higher prepolymerization water content or with longer PEGDA chains, and the more hydrophilic surfaces generally exhibit less BSA accumulation. These materials were applied to polysulfone (PSF) UF membranes to form coatings on the surface of the PSF membranes. Oil/water crossflow filtration experiments showed that the coated PSF membranes had water flux values 400% higher than that of an uncoated PSF membrane after 24 h of operation, and the coated membranes had higher organic rejection than the uncoated membranes. / text

Adenosine triphosphate (ATP) is the major carrier of chemical energy in cells. The diffusion of ATP in hydrogels, which have a structural resemblance to the natural extracellular matrix, is therefore of great importance to understand many biological processes. In continuation of our recent studies of ATP diffusion in poly(ethylene glycol) diacrylate (PEG-DA) hydrogels by pulsed field gradient nuclear magnetic resonance (PFG-NMR), we present precise diffusion measurements of ATP in aqueous solutions of PEG-DA polymers, which are not cross-linked to a three-dimensional network. The dependence of the ATP diffusion on the polymer volume fraction in the hydrogels, φ, was found to be consistent with the predictions of a modified obstruction model or the free volume theory in combination with the sieving behavior of the polymer chains. The present measurements of ATP diffusion in aqueous solutions of the polymers revealed that the diffusion coefficient is determined by φ only, regardless of whether the polymers are cross-linked or not. These results seem to be inconsistent with the free volume model, according to which voids are formed by a statistical redistribution of surrounding molecules, which is expected to occur more frequently in the case of not cross-linked polymers. The present results indicate that ATP diffusion takes place only in the aqueous regions of the systems, with the volume fraction of the polymers, including a solvating water layer, being blocked for the ATP molecules. The solvating water layer increases the effective volume of the polymers by 66%. This modified obstruction model is most appropriate to correctly describe the ATP diffusion in PEG-DA hydrogels.

This dissertation focuses on improving the chromatographic efficiency of polymeric organic monoliths by characterizing and optimizing the bed morphology. In-situ characterization techniques such as capillary flow porometry (CFP), 3-dimensional scanning electron microscopy (3D SEM) and conductivity measurements were developed and implemented to quantitatively characterize the morphology of poly(ethylene glycol) diacrylate (PEGDA) monoliths. The CFP measurements for monoliths prepared by the same procedure in capillaries with different diameters (i.e., 75, 150, and 250 μm) clearly showed a change in average through-pore size with capillary diameter, thus, certifying the need for in-situ measurement techniques. Serial sectioning and imaging of PEGDA monoliths using 3D SEM gave quantitative information about the average pore size, porosity, radial heterogeneity and tortuosity of the monolith. Chromatographic efficiency was better for a monolith with smaller average pore size (i.e., 5.23 μm), porosity (i.e., 0.49), radial heterogeneity (i.e., 0.20) and tortuosity (i.e., 1.50) compared to another monolith with values of 5.90 μm, 0.59, 0.50 and 2.34, respectively. Other than providing information about monolith morphology, these techniques also aided in identifying factors governing morphological changes, such as capillary diameter, polymerization method, physical/chemical properties of the pre-polymer constituents and weight proportion of the same. A statistical model was developed for optimizing the weight proportion of pre-polymer constituents from their physical/chemical properties for improved chromatographic efficiency. Fabricated PEGDA columns were used for liquid chromatography of small molecules such as phenols, hydroxyl benzoic acids, and alkyl parabens. The chromatographic retention mechanism was determined to be principally reversed-phase (RP) with additional hydrogen bonding between the polar groups of the analytes and the ethylene oxide groups embedded in the monolith structure. The chromatographic efficiency measured for a non-retained compound (uracil) was 186,000 plates/m when corrected for injector dead volume. High resolution gradient separations of selected pharmaceutical compounds and phenylurea herbicides were achieved in less than 18 min. Column preparation was highly reproducible, with relative standard deviation (RSD) values less than 2.1%, based on retention times of the phenol standards (3 different columns). A further improvement in chromatographic performance was achieved for monoliths fabricated using a different polymerization method, i.e., living free-radical polymerization (LFRP). The columns gave an unprecedented column performance of 238, 000 plates/m for a non-retained compound under RP conditions.

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Membrane technology has been used in water purification for decades. However, membrane fouling remains a limiting factor. One way to control fouling is through surface modification. Several studies report that increasing surface hydrophilicity can reduce membrane fouling. Surface modification via physical coating (i.e., thin-film composite membrane) was explored in this research to prevent membrane fouling. Before making thin-film composite membranes, it was important to study structure/property relations in a series of potential coating materials. This research aims to contribute to a better fundamental understanding of the structure/property relations which govern water transport, rejection of model foulants (i.e., emulsified oil droplet or protein), and fouling characteristics in hydrogels based on poly(ethylene glycol) diacrylate (PEGDA) and N-vinyl-2-pyrrolidone (NVP). Crosslinked poly(ethylene glycol) (PEG) free-standing films were prepared by UV-induced photopolymerization of PEGDA crosslinker in the presence of varying amounts of water or monofunctional poly(ethylene glycol) acrylate (PEGA). The crosslinked PEGDA films exhibited polymerization induced phase separation (PIPS) when the water content of the prepolymerization mixture was greater than 60 wt%. Visible light absorbance measurements, water uptake, water permeability, and salt kinetic desorption experiments were used to characterize the structure of these phase-separated, crosslinked hydrogels. The films with PIPS exhibited a porous morphology in cryogenic scanning electron microscope (CryoSEM) studies. Dead-end filtration experiments using deionized water and bovine serum albumin (BSA) solutions were performed to explore the fundamental transport and fouling properties of these materials. The total flux of pure water through the films after prior exposure to BSA solution was nearly equal to that of the as-prepared material, indicating that these PEGDA films resist fouling by BSA under the conditions studied. Crosslinked NVP free-standing films were prepared by UV-induced photopolymerization in the presence of water, with NVP as the monomer and N,N’-methylenebisacrylamide (MBAA) as the crosslinker. A series of crosslinked films were polymerized at various prepolymerization water contents, NVP/MBAA ratios and at various levels of UV light intensity in the polymerization. Like PEGDA, the NVP films also underwent phase-separation during polymerization. The influence of monomer/ crosslinker ratio, prepolymerization water content, and UV intensities on membrane morphology and water transport was characterized with CryoSEM, bio-atomic force microscope (Bio-AFM) and dead-end filtration. Molecular weight cutoff (MWCO) measurements were used to characterize the sieving property of crosslinked NVP films polymerized at different UV intensities. UV intensity was found to have an impact on the interconnectivity of crosslinked membranes. Finally, tests of fouling resistance to protein solution (bovine serum albumin) and oily water emulsion were performed. The NVP crosslinked films had good protein and oily water fouling resistance. Overall, both crosslinked PEGDA and NVP films exhibit fouling resistance to oily water emulsions or protein solution. NVP films had more porous structure and higher water permeability than did PEGDA films, while the more compact structure of PEGDA films led to better rejection of model foulants (e.g., protein) than in NVP films. Based on different applications (e.g., oil/water separation, protein filtration), different coating materials must be chosen according to the membrane morphology, transport property, and rejection of model foulants to achieve the highest water flux and foulant rejection in membranes used for water purification. / text

Cross-linked poly(ethylene oxide) (XLPEO) based on poly(ethylene glycol) diacrylate (PEGDA) is an amorphous rubbery material with potential applications for carbon dioxide removal from mixtures with light gases such as methane, hydrogen, oxygen and nitrogen. Changing the polymer network structure of XLPEO through copolymerization has previously been shown to influence gas transport properties, which correlated with fractional free volume according to the Cohen-Turnbull model. This project explores strategic modifications of the cross-linked polymer structure and their effect on the chemical, physical and gas transport properties with an aim to develop rational, molecular-based design rules for tailoring separation performance. Experimental results from calorimetric and dynamic thermal analysis studies are presented, along with pure gas permeability and solubility obtained at 35°C. Incorporation of dangling side chains by copolymerization of PEGDA with methoxy-terminated poly(ethylene glycol) methyl ether acrylate, n=8 (PEGMEA) was previously shown to be effective in increasing fractional free volume of XLPEO through the opening of local free volume elements, which in turn increased CO₂ permeability. Through a comparative study ofshort chain analogs to these co-monomers, incorporation of an ethoxy-terminated co-monomer was shown to be more effective than a comparable methoxy-terminated co-monomer in increasing gas permeability. For instance, copolymerization of PEGDA with 71 wt% ethoxy-terminated diethylene glycol ethyl ether acrylate increased CO₂ permeability from 110 barrer to 320 barrer. Gas permeability increase was not observed when hydroxy or phenoxy-terminated pendants were introduced, which was attributed to reduction in chain mobility due to increased inter-chain chemical interactions or steric restrictions, respectively. Based on these results, incorporation of a co-monomer containing a bulky non-polar terminal group, tris-(trimethylsiloxy)silyl, was examined in order to further increase gas permeability. Addition of 80 wt% TRIS-A co-monomer increased CO₂ permeability of cross-linked PEGDA to 800 barrer. However, the resulting changes in chemical character of the copolymer reduced CO₂/light gas selectivity, even as gas permeability increased. The effect of incorporating a bulky, stiff functional group in the cross-linker chain was studied using cross-linked bisphenol-A ethoxylate diacrylate, which showed 40% increase in permeability compared to cross-linked PEGDA. This study affirmed the importance of polymer chain interaction, in addition to free volume, in determining the gas transport properties of the polymer. / text