Control of size and charge selectivity in amphiphilic graft copolymer nanofiltration membranes

Abstract

The throughput and efficiency of membrane separations make polymer filtration membranes an important resource for the pharmaceutical, food and wastewater treatment industries. Nanofiltration (NF) membranes fill an important niche between nonporous reverse osmosis membranes, which have comprehensive solute rejection and low solvent permeability, and porous sieving ultrafiltration membranes. However, challenges in NF membrane design remain outstanding. At the effective pore size of NF membranes (~0.5 nm-2 nm), both electrostatic and steric factors determine membrane selectivity. Most NF membranes are charged under a wide range of environmental conditions and thus preferentially exclude charged solutes. This charge selectivity precludes separation of molecules based solely on size. An additional limitation of NF membranes is the tendency to foul by adsorption of feed components. The purpose of this thesis is to demonstrate control of membrane selectivity in fouling resistant membranes via manipulation of the chemistry of a specific copolymer system, polyacrylonitrile (PAN)-based poly(ethylene oxide) (PEO) graft polymers. Previous work with amphiphilic graft copolymers as membrane materials has included PANg- PEO with an average graft length of 9 (PAN-g-PEO9). PAN-g-PEO9 was shown to have excellent fouling resistance as an antifouling additive in porous ultrafiltration membranes and as a dense selective layer coated onto a support base membrane-a thin-film composite (TFC) NF membrane. The comb morphology of the polymer imposes high interfacial area on the microphase-separated domains, resulting in a bicontinuous structure consisting of a glassy PAN matrix interpenetrated by PEO-filled "nanochannels" that can act as vias for water and small solutes (with a size cutoff of ~0.8 nm). It also presents a PEO brush on the comb surface which acts as a steric barrier to resist irreversible fouling of the membranes. The understanding from previous work on PEO comb NF membranes is that the pore size is determined by the nanochannel's size, i.e. the PEO domain size. Because the graft characteristics (spacing and length) of comb copolymers determine the domain size, it was expected that varying the graft length would allow broad, precise control of the size cutoff of the TFC membranes, a concept demonstrated previously with amphiphilic graft copolymer NF membranes of poly(vinylidene fluoride)-graft-poly(oxyethylene methacrylate) (PVDF-g-POEM). The first aim of this thesis was to tailor the retention properties of PAN-g-PEO TFC NF membranes by modifying the chemistry to tune the electrostatic and steric properties sufficiently to enable complex separations, particularly of solutes with high fouling potential. Comb copolymers incorporating ~40 weight % PEO with side chains varying from 5-40 EO units were synthesized by free radical methods and compared as selective-layer coatings on PAN UF membranes. 3 Membranes incorporating combs with 9 EO units or more were shown to resist irreversible fouling when challenged by a model protein feed solution (bovine serum albumin) for 24 hours. Fouling resistance was found to be compromised, however, upon exposure to acid (pH 2) solution, used to simulate chemical cleaning procedures in industry. Thickness-normalized permeabilities of these PAN-g-PEOn NF membranes exceeded those of commercially available NF membranes by approximately an order of magnitude. A systematic effect of side chain length on permeability was seen when varying temperature, ionic strength, and pressure. Contrary to expectations, the membrane size cutoff (~0.8 nm) for charged rigid molecular probes in deionized water was independent of the comb side chain length. This new finding can be explained by modeling the hydrophilic domains as opposing swollen polymer brushes of uniform density acting as a physical gel. The gel mesh size (distance between chains) is independent of side chain length, and controls the size cutoff in good solvent conditions matching those in which the membrane was equilibrated during fabrication. In poorer solvent conditions, a decrease in the brush height, progressing to complete collapse of the PEO gel, can be expected to create differentiation based on domain size (i.e. side chain length). This is consistent with the finding that retentions of dyes increased with decreasing side chain length in saline solution, as salt is known to reduce PEO-water miscibility. Fluorescently labeled peptides germane to proteomics research were filtered and both chromatographic and size-selective membrane behavior was observed-the first demonstration of size-based nanofiltration of peptides. Based on this finding, two different peptides of molecular weights 1.3kDa and 1.5kDa were fractionated to achieve a six-fold increase in the concentration of the larger peptide relative to the smaller peptide in two filtration steps. The electrostatic selectivity of the PEO comb membranes could also be varied. Terpolymers consisting of PAN-g-PEO with 1-2% charged sulfopropyl acrylate (SPA) or 5% N,Ndimethyl- N-(2-methacryloyloxyethyl-N-(3-sulfopropyl) ammonium betaine (SPE) were synthesized and coated onto PAN base membrane. The divalent salt (Na2SO4) retention of the resulting TFC membranes increased from ~20% for the PAN-g-PEO copolymer to ~45% and 82% for the SPE and SPA terpolymers, respectively. Retention of monovalent NaCl was substantially lower, characteristic of commercial NF membranes. The charged comb membranes did not completely resist fouling by a 1 g/L BSA solution, losing 2% of the initial flux after 24 h exposure. Forming a trilayer TFC, with a layer of PAN-g-PEO coated over a charged terpolymer, reduced membrane fouling compared to the charged layer alone. In summary, the goal of this study was to demonstrate control of membrane selectivity in fouling-resistant PAN-g-PEO NF membranes. An important finding was that the PEO gel created in the hydrophilic domains leads to similar size cutoffs over a wide range of side chain length. To access the desired spectrum of size cutoffs, the quality of solvent for the swollen PEO brush must be reduced. In spite of these limitations, the membrane was shown to have useful fractionating properties as demonstrated with labeled peptides of varying molecular weight. The retention of salts was enhanced by incorporating small amounts of charged monomer into the comb backbone, but at the expense of fouling resistance.