Elsevier

Polymer

Volume 55, Issue 1, 14 January 2014, Pages 354-365
Polymer

Novel ultrafiltration membranes with adjustable charge density based on sulfonated poly(arylene ether sulfone) block copolymers and their tunable protein separation performance

https://doi.org/10.1016/j.polymer.2013.09.003Get rights and content

Abstract

A novel poly(arylene ether sulfone) (PAES) block copolymer was prepared from previously synthesized fluoride terminated oligomer (A16) and hydroxyl terminated oligomer (B12) by aromatic nucleophilic substitution polycondensation reaction. PAES was subsequently sulfonated under controlled conditions to yield a copolymer (S-PAES) with sulfonic acid groups selectively in the B12 segments and without chain degradation. Non-solvent induced phase separation method was used to prepare a series of ultrafiltration membranes from blends of S-PAES and PAES with varied ratios and, hence, sulfonic acid group densities. Porous membrane morphologies, structure and surface properties were characterized comprehensively using scanning electron microscopy, Fourier transform infrared spectroscopy in the attenuated total reflection mode, as well as contact angle and zeta potential measurements. Studies of membrane performance revealed systematically increasing water permeabilities and reduced protein fouling tendencies with increasing fraction of S-PAES in the membrane. The protein transmission as function of pH value (and hence protein charge) was studied for two model proteins (bovine serum albumin and lysozyme) and found to be controlled by combined size exclusion and charge effects. The selectivity for the separation of the binary protein mixture could be systematically increased with increasing membrane charge density (by increasing S-PAES fraction). Consequently, the trade-off relationship between permeability and selectivity for conventional ultrafiltration membranes where separation is based on size exclusion solely could be overcome. Due to their high stability and tunable functionality, the PAES block copolymers have also large potential as membrane material for other applications.

Introduction

Purification and concentration of proteins is an important process in biological, medical and food industries [1]. The research and development on protein concentration and purification is stimulated by the increasing demand for high-purity drugs [2]. Several methods have been developed for separation of proteins from their mixtures including liquid chromatography, electrophoretic and membrane based techniques [3], [4], [5]. Among the aforesaid separation techniques, ultrafiltration (UF) has received tremendous attention in concentration and purification as well as separation of protein mixtures in model solutions, fermentation broths and cell culture samples. This process is more efficient and easy to handle than the alternatives and can be scale up at low cost [1], [3], [6], [7]. Various commercial polymers such as polyamides, polyimides, polyacrylonitrile, polyvinylidene fluoride, polysulfone and polyethersulfone (PES) are widely used in the fabrication of UF membranes [8]. However, membrane fouling is a severe problem in UF of protein solutions. Fouling causes significant loss in performance with respect to flux, and often selectivity due to the non-specific protein deposition and adsorption on the external surfaces, at the pore openings, or within the membrane pores [5], [7], [8], [10]. Two types of membrane fouling can occur: (i) reversible protein adsorption or deposition on the membrane surface causes reversible fouling, which could be removed by simple hydraulic cleaning, and (ii) irreversible fouling results from the strong adsorption of protein molecules on the surface or entrapment of protein molecules in the pores [5], [9]. The fabrication of low fouling UF membranes with good mechanical and chemical stability is hence important for practical applications. A useful strategy to suppress the membrane fouling is to enhance the membrane hydrophilicity by two main approaches: (i) surface modification during the membrane formation step; i.e. addition of a small fraction of hydrophilic surface-modifying macromolecules to the casting solution [10], [11] or blending of membrane polymers with larger fractions of a hydrophilic functionalized polymer [12]; (ii) post-manufacturing modification by surface-initiated grafting; i.e. covalent incorporation of functional hydrophilic monomers to the hydrophobic membrane surface by polymerization using plasma, ultra-violet light or γ-radiation excitation or by atom transfer radical polymerization [13], [14], [15], [16], [17].

To further improve separation performance, researchers have made considerable efforts to fabricate low fouling UF membranes with higher selectivity, by anchoring anionic or cationic groups in the barrier layer of the membranes. This has been achieved via chemical modification reactions, or by blending a hydrophilic functionalized polymer with hydrophobic membrane forming polymer [7], [8], [18], [19], [20], [21]. For instance, Rohani et al. have fabricated a series of positively charged UF membranes with different spacer lengths by activation of base cellulose membranes with epichlorohydrin, followed by reactions with different diamines. The protein rejection was increased with increasing alkyl chain length of diamines in the modified membranes [19]. Liu et al. have prepared hydrophilic negatively charged UF membranes using sulfonated polyphenylsulfone random copolymer; the hydrophilicity and antifouling ability of the membranes were enhanced with fraction of sulfonated copolymer [22]. Qiu et al. have fabricated asymmetric positively charged UF membranes by combining the self-assembly of the amphiphilic polystyrene-b-poly-4-vinylpyridine block copolymer with nonsolvent-induced phase separation method, followed by heterogeneous quaternization reaction using 2-chloroacetamide to anchor the cationic groups in the barrier layer of the membrane. The selectivity of this positively charged membrane in separation of bovine serum albumin (BSA) from hemoglobin (Hb) in mixture solution was enhanced 10 times compared to conventional UF membranes [6]. Li and Chung have fabricated negatively charged hollow fiber UF membranes by non-solvent induced phase separation, using the blend of highly sulfonated PES and unmodified PES. The selectivity of the membranes towards BSA and Hb was highly dependent on the charge density. The best separation of Hb from BSA in mixture solution was achieved when the hollow fiber membrane with high charge density was used [23]. Li et al. have also developed negatively and positively charged hollow fiber UF membranes by dual-layer hollow fiber technology, using sulfonated PES and quaternized PES. These membranes were used in the separation of a BSA and Hb mixture model solution. The separation performance of the membranes was dependent on the both size-exclusion and electrostatic interaction [24]. In addition, polysulfone derived amphiphilic graft copolymers have been synthesized by click chemistry approach and used in the fabrication of hydrophilic membranes. The adsorbed amount of proteins was reduced with increasing fraction of amphiphilic graft copolymers in the membranes [25], [26], [27].

Recently, poly(arylene ether sulfone) (PAES) block copolymers have gained much attention in academic and industrial research because of their better thermal stability, tolerance to higher pH and chemical resistance compared to polysulfone [28], [29]. However, the membranes fabricated from PAES block copolymers suffer from low hydrophilicity and water permeability. To overcome these limitations, hydrophilic negatively or positively charged PAES copolymers have been synthesized and used in fabrication of functionalized porous membranes [7], [28]. Kumar and Ulbricht developed positively charged UF membranes by non-solvent induced phase separation method, using blends of PAES and PAES-CH2Br block copolymers, followed by heterogeneous quaternization with trimethyl amine to anchor the quaternary ammonium groups in the barrier layer of the membranes. The hydrophilicity and water permeability of the membranes were improved with increasing the fraction of quaternary ammonium groups in the membranes; membranes had tunable selectivity for transmission of lysozyme versus bovine serum albumin [7]. The combination of size- and charge-based selectivity enhances the separation performance of UF membranes. The transport of proteins across the charged UF membranes is also dependent on the type and strength of electrostatic interactions between the membrane and protein molecules at a specific solution pH [7], [14], [19], [30]. Thus, selective separation of proteins using charged UF membranes is, in principle, possible at a controlled pH and applied transmembrane pressure. To the best of our knowledge, negatively charged UF membranes with tunable protein separation performance based on blends containing sulfonated PAES block copolymers have not been reported till now. The novelty of this work is based on the structure of the poly(arylene ether sulfone) block copolymer (Scheme 1). The B12 blocks have more active sites for sulfonation than the frequently used membrane polymer PES, while the A16 blocks remain unsubstituted under sulfonation conditions. The S-PAES block copolymer, even at a high degree of sulfonation, is not soluble in water, and therefore, the swelling is expected to be low at relatively high charge densities. In addition, other side groups could be anchored onto the backbone of the PAES block copolymer for the development of other charged UF membranes. In this study, efforts have been made to fabricate sulfonated membranes with varied charge density from blends of PAES and S-PAES block copolymers by solution casting and non-solvent induced phase separation (“phase inversion”) method. The fabricated membranes have been evaluated in the selective separation of lysozyme and bovine serum albumin from their mixture solutions.

Section snippets

Materials

The 4,4′-fluorophenyl sulfone (FPS), 4,4′-isopropylidenebis(2,6-dimethylphenol) (IBDP) and 9,9′-bis(4-hydroxyphenyl) fluorene (BHF) monomers were purchased from Sigma–Aldrich. Chlorosulfonic acid (ClSO3H), dichloromethane and disodium hydrogenphosphate were obtained from Acros Organics. Anhydrous potassium carbonate (K2CO3), calcium carbonate (CaCO3) and toluene were purchased from Sigma–Aldrich. N,N′-Dimethyl acetamide (DMAc), sodium dihydrogenphosphate monohydrate and N-methyl pyrolidone

Synthesis and characterization of block copolymers

The 1H NMR spectrum of PAES block copolymer is depicted in Supporting Information (Fig. S1 (C)). It reveals that the protons of hydroxyl substituted phenylene groups had disappeared and the other peaks were consistent with those of parent A16 and B12 oligomers (Supporting Information (Fig. S1 (AB))). Due to the high degree of polymerization, end group analysis by NMR was not possible. Overall, the results support the synthesis of PAES block copolymer (polyA16B12) by SNAr polycondensation

Conclusions

Poly(arylene ether sulfone) and sulfonated poly(arylene ether sulfone) block copolymers were successfully synthesized by SNAr polycondensation and subsequent homogeneous sulfonation reaction. Polymer main chain degradation was not observed after sulfonation reaction using chlorosulfonic acid under controlled conditions. The synthesized block copolymers had multiblock structure and an excellent film forming ability. Novel negatively charged ultrafiltration membranes were successfully fabricated

Acknowledgment

Mahendra Kumar is grateful to Alexander von Humboldt Foundation (Germany) for awarding Alexander von Humboldt Postdoctoral Fellowship.

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