Fabrication of dual-responsive cellulose-based membrane via simplified surface-initiated ATRP

Abstract

An independently temperature- and pH-responsive membrane was developed by simultaneously grafting poly(N-isopropylacryamide) (PNIPAAm) and poly[(2-(diethylamino)ethyl methacrylate] (PDEAEMA) from different sides of a crosslinked cellulose membrane. The synthesis was simplified by using surface-initiated activators regenerated by electron transfer for atom-transfer radical polymerization in a diffusion device. The grafted membrane was heterostructured. The grafted polymer layer thickness was linearly related to reaction time. The wettabilities of the double-membrane sides responded individually and reversibly to temperature and pH. The surface grafted with PNIPAAm shifted from hydrophilicity to hydrophobicity above the lower critical solution temperature. The PDEAEMA side was hydrophilic in acidic aqueous solution and hydrophobic under basic conditions. This dual-response cellulose membrane has potential applications in water treatment, separations, and other membrane applications.

Introduction

Stimuli-responsive polymers have gained considerable interest because of their fast and reversible structural and morphological transformations in response to surrounding stimulus changes. These polymers are used in the fabrication of stimuli-responsive membranes, which exhibit abrupt property changes such as pore size or surface wettability (Lin et al., 2012, Lue et al., 2008, Pan et al., 2012, Tokarev and Minko, 2009, Zhao et al., 2011) as a result of their sensitive behaviors in response to environmental changes such as temperature, pH, and ionic strength.

Membranes with responsive properties are promising for a range of applications, including drug delivery (Chen et al., 2011, Lue et al., 2008, Lue et al., 2011), separation (Pan et al., 2010a, Pan et al., 2010b, Xiong et al., 2010), biosensor (Tokarev & Minko, 2009), and water treatment (Cai et al., 2011, Wandera et al., 2011), and multi-responsive membranes are preferred in many situations because their property changes in response to multiple stimuli. Drug delivery systems with multi-responsive properties, including temperature/pH, pH/ionic strength, and temperature/magnetic field responses, are suitable for insulin delivery and site-specific delivery of anticancer drugs to tumor sites (Gordijo et al., 2011, Kaiden et al., 2011). In the water separation and treatment industries, temperature, pH, and ionic strength are among the key factors that can influence membrane performance. Multi-responsive membranes have therefore been built to improve separation capacity (Wang, Zhang, Ji, Qin, & Liu, 2010), antifouling, and cleaning performance (Wandera et al., 2011). Among them, cellulose based membranes have been extensively studied with responsive properties have been widely exploited (Lindqvist et al., 2008, Wang et al., 2011a, Wang et al., 2011b, Xiong et al., 2010) in the applications of anion exchange, separation, controlled release, water treatment, etc. (Liu et al., 2010, Lqbal et al., 2007, Pan et al., 2010a, Pan et al., 2010b, Schmitt et al., 2011, Wu et al., 2010). Being a natural polymer, cellulose is a renewable resources and possessing excellent mechanical strength and suitable modifications can result in improved performance (Qiu, Tao, Ren, & Hu, 2012). However, double-side modifications have never been reported before, and no exceptional for cellulose based membrane.

Presently, stimuli-responsive membranes have been extensively studied, there is still scope for further exploitation of multi-responsive membranes (Kaiden et al., 2011, Shaikh et al., 2010, Wang et al., 2010). There are several methods of fabricating multi-responsive membranes from stimuli-responsive polymers. Alginate-gel membranes with sub-micrometer pores were prepared by salt-induced phase-separation of sodium alginate and gelatin (Gopishetty, Roiter, Tokarev, & Minko, 2008). The membrane had a range of functions mimicking natural skin, and most of the functions were regulated by pH and ionic strength. Zhang, Song, Ji, Wang, and Liu (2008) developed a polyelectrolyte multi-layer membrane on a hydrolyzed polyacrylonitrile hollow fiber membrane using a dynamic negative-pressure layer-by-layer technique. The morphology and pervaporation selectivity for ethanol/water were highly dependent on salt concentration, pH, and oxidant. A layer-by-layer technique was also used by Allen et al. to fabricate glass-supported assembly membranes that responded to solute and temperature (Allen, Tan, Fu, Batteas, & Bergbreiter, 2012). A hybrid polymer membrane was prepared by casting a mixture of poly(ether sulfone) and an amphiphilic block polymer of Pluronic F127-b- poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA) (Yi et al., 2010). The added Pluronic F127-b-PDEAEMA endowed the poly(ether sulfone) membrane with smart properties which responded to temperature and pH stimuli. Grafting techniques are intriguing methods for fabricating stimuli-responsive membrane brushes, since the stimuli-responsive polymers are able to graft from/onto the membrane surface efficiently, and the mechanical strength of the membrane is maintained (Lue et al., 2011). This advantage is of vital importance for membrane industries, especially for water treatment and separation (Cai et al., 2011, Lue et al., 2011), because robust membranes are required in these industries.

One of the most promising methods for grafting membrane brushes is surface-initiated atom-transfer radical polymerization (ATRP). ATRP initiators are first immobilized on the membrane, and responsive polymers are then grafted consecutively in a multi-step process (Lindqvist et al., 2008, Pan et al., 2012, Zhang et al., 2009). The advantage of the ATRP technique is that it has the potential to tailor membrane properties, e.g., by enabling adjustment of the grafted polymer thickness, and can be carried out in aqueous solutions under mild conditions. The fabrication of multi-responsive membranes by surface-initiated ATRP typically requires multiple steps after immobilization of ATRP initiators on the membranes. The fabrication of each responsive grafting layer comprises polymerization and purification steps (Pan et al., 2012, Zhang et al., 2009). Adding one more step therefore leads to an inevitable loss of a small, but not negligible, amount of reactive polymer chains, accompanied by formation of polymeric impurities (Schmid et al., 2012, Weiss and Laschewsky, 2012). Also, multi-polymerization is labor-intensive. It is worth noting that ATRP is a redox-initiated polymerization reaction; the transition metal is susceptible to reaction with oxygen or other oxidizers, and the activity of the metal is quenched after exposure to oxidizers. ATRP polymerization therefore requires an oxygen-free environment throughout the entire process, and specific equipment and critical operations are necessary.

To simplify the preparation of dual-responsive membranes and obtain a novel heterostructured dual-responsive membrane, a diffusion device was used to fabricate dual-responsive cellulose membranes by surface-initiated activators regenerated by electron transfer (ARGET) ATRP in the present work. ARGET ATRP was developed to enable reactions to be conducted under a limited oxygen atmosphere by adding reducing agents, and it overcomes the limitations mentioned above (Matyjaszewski et al., 1998, Matyjaszewski et al., 2007a, Matyjaszewski et al., 2007b). ARGET ATRP has been used elsewhere to prepare membrane adsorbers (Bhut, Conrad, & Husson, 2012). In this work, an ATRP initiator was first immobilized on a crosslinked cellulose membrane (CCM) prepared according to our previous work (Qiu et al., 2012), and then PNIPAAm and PDEAEMA were simultaneously grafted from each surface of the membrane in one step. The prepared membrane was heterostructured, one surface grafted with PNIPAAm and the other surface grafted with PDEAEMA. The chemical structures of the membrane brushes were characterized by attenuated total reflectance infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy (XPS) to verify the fabrication of dual-responsive cellulose membranes. The morphologies and thicknesses of the grafting layers were observed by scanning electron microscopy (SEM). The dual-responsive behaviors were characterized by contact angle measurements. This work aims to offer a simple method of preparing heterostructured dual-responsive membranes for water treatment, separation industry, and other membrane industries.