Source: Dissertation Abstracts International, Volume: 78-07(E), Section: B.

Advisers: Menachem Elimelech; Chinedum Osuji.

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Membrane-based desalination and aqueous separation processes are crucial for providing clean, freshwater resources to meet our personal, agricultural, and industrial needs. Polyamide thin-film composite (TFC) membranes are the current state-of-the-art technology for these separation processes. While polyamide TFC membranes are high performing, they have intrinsic drawbacks that limit their use in certain applications and hinder our fundamental understanding of membrane transport processes. Three specific challenges to TFC membranes, which are each linked to the polyamide selective layer, are: (1) fouling, (2) the empirical nature of interfacial polymerization, (3) and the permeability-selectivity trade-off. Overcoming these three challenges is essential for further advancing membrane desalination technology and will require innovation to the selective layer material.

This dissertation research aims at addressing the aforementioned challenges to TFC membranes by employing two approaches---nanomaterial surface modification and molecular-level assembly to improve or replace the polyamide selective layer. Nanomaterial surface modification is used to address the challenge of membrane fouling. This work demonstrates that strong, non-depleting, antimicrobial properties can be conferred on the surface of commercial TFC membranes through functionalization of the polyamide selective layer with graphene oxide nanosheets. Graphene oxide functionalization renders the selective layer more hydrophilic without compromising intrinsic permeability or selectivity. Both antimicrobial properties and hydrophilicity are known to delay the onset of membrane biofouling.

Molecular-level assembly is used address the property control limitations of interfacial polymerization. Molecular layer-by-layer (mLbL) assembly of polyamide provides an alternative to the traditional interfacial polymerization process. In this dissertation research a protocol is developed, using mLbL, to create polyamide surfaces with tunable and homogeneous chemical functionality. Surfaces rich in either amine or carboxyl groups are created by intentional selection of terminating monomer and monomer deposition times. This degree of property control is not possible by interfacial polymerization and is important for creating lower fouling propensity membranes and understanding fundamental transport phenomena.

Molecular-level assembly is also used to address the need for highly selective membranes that are not constrained by the permeability-selectivity trade-off of polyamide. This dissertation identifies three polymerizable liquid crystal templates, capable of undergoing alignment by magnetic fields or surface confinement, as potential candidates for high selectivity membrane development. The structure, phase behavior, and alignment criteria of all three templates is examined. Aligned, mechanically robust films fabricated from a liquid crystal template demonstrate a marked increase in ionic conductivity compared to non-aligned films. Current progress in the fabrication of defect-free liquid crystal TFC membranes is discussed.

Overall, the work presented in this dissertation has direct implications for overcoming the intrinsic drawbacks of existing TFC membranes. Novel contributions include: (1) demonstration that graphene oxide functionalization imparts biocidal properties on TFC membranes without impacting membrane transport properties; (2) development of a protocol for fabricating polyamide selective layers with tunable surface functionality; (3) development of a polymerizable liquid crystal template formed by a blend of single-tailed surfactants, polymerizable oil, and water; (4) demonstration of enhanced ionic transport in aligned liquid crystal-templated films compared to non-aligned films; and (5) investigation of the suitability of a liquid crystal template with a leachable core molecule for fabrication of functioning aqueous separation membranes.