Antifouling and antibacterial behavior of membranes containing quaternary ammonium and zwitterionic polymers

Highlights

• A dual-functional ultrafiltration membrane containing quaternary ammonium and zwitterionic polymers was prepared.

• The antifouling and antibacterial behavior of the membrane was studied.

• The membrane showed the improved organic-/bio fouling resistance.

• The membrane showed high-efficiency broad-spectrum antibacterial performance.

Abstract

To overcome the organic-/bio- fouling of the membrane, a dual-functional ultrafiltration membrane containing quaternary ammonium and zwitterionic polymers via quaternization and surface radical polymerization was designed, and its antifouling and antibacterial behavior was studied. In this work, poly(vinylidene fluoride)/poly(methyl methacrylate-co-dimethylamino-2-ethyl methacrylate) (PVDF/P(MMA-co-DMAEMA)) blend membrane was quaternized by p-chloromethyl styrene (p-CMS), and the double bonds were introduced onto the membrane surface, which further participated in the polymerization of zwitterionic monomers on the membrane surface. The results indicated that the resultant membrane exhibited obviously improved hydrophilicity and weak positive charge (isoelectric point, 7.49). The membrane presented higher flux recovery ratio and lower protein adhesion compared with the pure PVDF membrane. Meanwhile, the membrane showed high-efficiency broad-spectrum antibacterial performance, that is, the bacteria killing efficiency of S. aureus and E. coli reached 98.2% and 97.0%, respectively. Moreover, the membrane effectively inhibited bacterial adhesion, which is important for the long-term antibacterial properties of membrane. This antifouling and antibacterial PVDF membrane may have potential in the long-term filtration process, especially when dealing with microbiologically contaminated water.

Introduction

Ultrafiltration (UF) membrane has been widely used in water treatment, electronic, energy, biotechnological and chemical areas [1] due to its advantages of low energy consumption and mild operating conditions [2]. However, membrane usually suffers from organic and biological fouling during long-term separation process because of their inherent nature of low surface energy and hydrophobicity [3], [4]. Therefore, various strategies have been applied to improve the antifouling or antibacterial properties, including the direct modification of the membrane material [5], [6], blending [7], [8], [9], coating [10], [11], and grafting modification [12], [13].

For antifouling substances, poly(ethylene glycol) (PEG) and zwitterionic polymers are two common types of polymeric segments employed for hydrophilic modification [13]. However, PEG is now considered to be auto-oxidized rather rapidly, especially in the presence of oxygen and transition metal ions found in most of the biochemical-relevant solutions [14], [15]. Therefore, zwitterionic polymers have been specially focused since these polymers possess remarkable ability to attract and bind water molecules much stronger than hydrophilic materials via electrostatic interactions [16], [17], [18]. Li et al. [19] summarized the fabrication, properties and various applications of zwitterionic polymer grafted surfaces, showing that zwitterinonic polymers have significant potentials particularly in biological systems. Chiang et al. [13] and Tang et al. [20] reported that the zwitterionic sulfobetaine methacrylate (SBMA) was grafted on the PVDF membrane surface via ozone activation and surface-initiated atom transfer radical polymerization (ATRP). The polySBMA-grafted membrane effectively resisted the protein adsorption and showed promising antifouling properties.

Among various bactericidal substances, including metal ions [11], [21], [22], [23], [24], nanoparticles [25], [26], quaternary ammonium compounds [27], [28], [29], antimicrobial polymers [30], graphene-based materials [31], [32], and carbon nanotubes [33], the polymers containing quaternary ammonium salt are very promising for practical application because of their excellent antibacterial activity [34], [35]. It is well known that the positive charge of quaternary ammoniums and other cationic polymers could bind to the outside of the bacteria, penetrate the outer membrane and finally disrupt the cytoplasmic membrane to inhibit bacterial growth [29]. This is because the cell surface is generally negatively charged regardless of bacterial species such as Gram-positive and Gram-negative bacteria [36]. Wu et al. [37] prepared quaternized poly(vinyl chloride) (QPVC) UF membrane through soaking PVC in trimethylamine solution and hydrochloric acid solution successively. Antibacterial research testified the PVC membrane was endowed antibacterial properties after quaternization. Liu et al. [38] synthesized an amphiphilic copolymer polyvinylidene fluoride-g-poly(N-(3-dimethyl aminopropyl) methacrylamide) (PVDF-g-PDMAPMA) and prepared the quaternized blend PVDF membrane. This membrane showed good potency against microorganisms.

Nevertheless, the antifouling surfaces could not kill attached bacteria, the small amount of attached bacteria will form colonies and subsequent biofilm, which causes severe biofouling [39]. Similarly, the antibacterial surfaces could not prevent the adhesion of foulants and dead bacteria, the increasing and accumulated contaminants will lead to the decline or loss of antibacterial property. Therefore, an ideal membrane surface should be capable of killing bacteria on the surface to inhibit bacterial reproduction, as well as releasing the deposited dead cells and other foulants to maintain the long-term antibacterial performance [40].

The effect could be expected if the zwitterionic polymers and quaternary ammonium compound can be combined, which has been adopted in some works [41], [42]. However, strict experimental conditions with very high energy consumption are usually required. Otherwise, the non-covalent binding of functional polymer chains to the substrate may not be firmly during the long-term usage.

Herein, based on our previous work about poly(methyl methacrylate-co-dimethylamino-2-ethyl methacrylate) (P(MMA-co-DMAEMA), denoted PMD [43], [44] and the reaction of tertiary amine groups with benzyl chloride [45], [46], we proposed an ultrafiltration membrane containing quaternary ammonium and zwitterionic polymers via quaternization and surface radical polymerization and studied its antifouling and antibacterial behavior. Referring to some work [44], [47], it is easy and cost-effective to blend PMD into PVDF matrix during non-solvent induced phase separation (NIPS) process. The tertiary amine groups of DMAEMA on the blending membrane (PVDF-PMD) were then quaternized by p-chloromethyl styrene (p-CMS), meanwhile, the double bonds were also introduced onto this quaternized membrane (denoted QA membrane) surface. Sulfobetaine methacylate (SBMA) was then grafted onto the membrane surface by surface radical polymerization. This modified membrane (denoted QAZ membrane) with both quaternary ammonium and zwitterionic polymers not only prevents the membrane from adhesion of protein and bacteria, but also possesses broad-spectrum antibacterial ability.