| Summary: | Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2009. === Vita. === Includes bibliographical references (p. 158-184). === The operating cost of a membrane filtration system is generally determined by two major factors: the permeability of the membrane to water, and the lifetime of the membrane. Both of these are strongly affected by the chemical structure and surface properties of the membrane. Hence, the development of novel membrane materials that improve these two properties would make membrane treatment of water streams cheaper. One of the most important reasons for low permeability and short membrane life is fouling, which makes it one of the most important challenges faced in membrane operations, especially in processes where the feed has high concentrations of biomolecules, such as wastewater treatment, and in food and biochemical industries. In this thesis, the self-organization of amphiphilic comb copolymers is employed to develop improved membranes for aqueous filtration. The use of self-assembling copolymers leads to the desired properties (surface chemistry, selectivity) without additional processing steps. One aspect of this thesis focuses on the development of size-selective nanofiltration (NF) membranes that can fractionate small molecules by size through the microphase separation of the amphiphilic comb copolymers. This size scale corresponds to a "missing link" in the separations currently offered by commercial membranes. Such membranes formed by coating a porous support membrane with the comb copolymer poly(vinylidene fluoride)-graft-poly(ethylene oxide methacrylate) (PVDF-g-POEM) were first introduced by Akthakul et al. (Macromolecules 37 (2004) 7663-7668). The microphase separation of the comb copolymer results in the formation of interconnected effective "nanochannels" of the hydrophilic poly(ethylene oxide) (PEO) side-chains, which allow water permeability and size selectivity. This thesis includes work that characterizes the fouling resistance of these membranes in more detail, including their performance in the presence of various foulants as well as in the context of membrane bioreactor (MBR) operation for wastewater treatment. === (cont.) In each of these cases, PVDF-gPOEM thin film composite (TFC) NF membranes were shown to resist irreversible fouling completely, recovering their initial flux upon cleaning with water. The mechanism of this exceptional resistance to adsorptive fouling was attributed to net steric repulsion forces between the PEO brush formed on the membrane surface and the foulant molecule, based on interaction force measurements. The ability to fine-tune the pore size of these membranes by feed properties such as temperature, pressure, and ionic strength was also studied in this thesis. The degree of swelling of the PEO chains in the feed determines the effective diameter of the nanochannels, and the variations in selectivity and water flux can be related with the phase diagram of PEO/water mixtures as predicted. The combination of these properties make PVDF-g-POEM TFC NF membranes very promising for the food and pharmaceutical industries, where different and fine-tuned separations are needed at different stages of the production process, and feeds have large concentrations of biomolecules that lead to severe fouling. Another objective of this study was to extend the size-selective NF membranes to different copolymer chemistries. Such membranes were prepared and characterized from the comb copolymer polyacrylonitrile-graft-poly(ethylene oxide) (PAN-g-PEO). This copolymer is synthesized using free radical copolymerization, a method with simpler scale-up and good control over copolymer composition, both of which posed difficulties in the use of PVDF-g-POEM. PAN-g-PEO TFC NF membranes also showed high fluxes, size-based selectivity, and complete resistance to irreversible fouling. Amphiphilic comb copolymers with PEO side-chains have also been used to impart fouling resistance to ultrafiltration (UF) membranes. The comb copolymer is added to the casting solution during the manufacture of the membrane by phase inversion, and segregates to the polymer surface during coagulation to form a fouling-resistant PEO brush on the polymer/water interface (Hester et al., Macromolecules 32 (1999) 16431650). === (cont.) This thesis introduces PAN-based UF membranes prepared by this method, using PAN-g-PEO as an additive. Such membranes were shown to exhibit significantly enhanced flux. Furthermore, PAN/PAN-g-PEO blend UF membranes resist irreversible fouling completely, recovering their initial flux completely upon a water rinse or backwash. This property is exceptional, and has not been reported for any other polymeric porous membrane system, to the author's knowledge. The fouling resistance of these membranes arises from a steric repulsion of foulant molecules by the PEO brush. The performance of these membranes in the context of the filtration of oily wastewaters, a process severely impacted by fouling, has also been demonstrated in this thesis. PAN/PAN-g-PEO blend UF membranes promise to cut costs and energy use significantly in several UF applications limited by fouling, including municipal and industrial wastewater treatment, MBRs, and separations in the food and pharmaceutical industries. Overall, this thesis was instrumental in extending, developing and understanding the use of the self-organization of amphiphilic comb copolymers in the manufacture of better filtration membranes, and bringing this method closer to industrial application. This approach can be extended to design different comb copolymer chemistries for applications such as heavy metal removal, affinity filtration, and desalination. === by Ayse Asatekin Alexiou. === Ph.D.
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