Metal Oxide Growth on Polymer Surfaces and within Polymer Volumes: Applications in Novel Membrane Materials

The stresses facing the water-energy nexus in coming decades demand advances in membrane materials research for improved separation performance, energy-efficiency, and new functionality. Water and solute interactions with porous membrane surfaces are complex and are governed by a wide-range of phenomena over length and time scales. Polymers have been used for decades to make membranes relevant to many challenging applications at large production volumes. Inorganic metal oxides have many interesting functional properties, but wholly inorganic membranes are expensive, and integrating them into membranes remains a challenge. In this dissertation, atomic layer deposition (ALD) and sequential infiltration synthesis (SIS) are explored for their use in the modification of polymeric membranes and in the fabrication of novel isoporous structures. Fundamental insights into the interactions between the metalorganic molecules used in these deposition techniques with the chemical functionalities in polymers are described, yielding first reports of novel metal-oxide/polymer hybrid materials. This work begins by exploring the ALD of hydrophilic aluminum oxide onto highly porous hydrophobic polypropylene membranes to create dual-faced Janus membranes. The lack of reactive sites for ALD on these hydrophobic polymers yields a gradient deposit of Al2O3 through the membrane interior. Controlling the vapor exposure dose enables precision placement of the wetting transition within the membrane, yielding materials suitable for rapid gas bubble aeration. Next, the dynamics of SIS of aluminum oxide into polyethersulfone is studied using in situ optical techniques. In situ fourier transform infrared spectroscopy (FTIR) and spectroscopic ellipsometry probe the chemical and physical interactions of trimethyl aluminum (TMA) with this polymer which is ubiquitous in ultrafiltration membranes. These analyses reveal the mechanism by which TMA forms reversible associations with polyethersulfone and compares them to existing studies of other polymer systems. In the next chapter, in situ FTIR and density functional theory are used to comprehensively compare the behavior of TMA to its Group 13 analogues trimethyl indium and trimethyl gallium. While the three show qualitatively similar interactions with polymethyl methacrylate (PMMA), the effective diffusivity of TMA is shown to be substantially lower than its heavier counterparts due to the stronger adduct that it forms with carbonyl. The implications are that lower temperatures and purge times are needed to successfully grow conductive indium oxide via SIS. These experimental chapters are followed by a computational analysis involving the influence of pore size distribution in membranes along considerations energy efficiency and separation performance. This analysis yields a convenient metric to determine under what conditions pursuing an isoporous material as – one with uniform pore size distribution – should be justified. Lastly, ongoing experimental work in the development of metal oxide isoporous membranes is detailed. Using block copolymer (BCP) self-assembly, a thin film consisting of a PMMA matrix and hexagonally close-packed cylinders of polystyrene is used to template the growth of metal oxides via SIS. The metal oxides directly template the PMMA domain while leaving void polystyrene pores. The processing of these films and their integration into isoporous membranes is discussed, with an outlook to future studies