Robust superhydrophobic and superoleophilic filter paper via atom transfer radical polymerization for oil/water separation
Introduction
Materials with extreme wettability, including superhydrophobic, superoleophilic, superhydrophilic, and superoleophobic, have attracted considerable academic and industrial interest in the past decades due to their promising potential applications in self-cleaning, anti-icing, antifouling, smart membrane, microfluidic devices, and oil/water separation (Chen et al., 1999; Wang, Liu, Yao, & Jiang, 2015). Some creatures in nature, such as lotus leaves, cicada wings, and mosquito eyes, present fascinating superhydrophobicity. A droplet of water on its surface remains almost spherical and easily rolls off, removing dirty substances in their path. Fish scales of Crucian Carp exhibit superoleophilicity in air and superoleophobicity underwater. A thin layer of mucus and multiscale structures could trap water and form a composite interface to resist oil. Especially, functional materials integrated with both superhydrophobicity and superoleophilicity, were effective in the oil/water separation, which might play an important role in oil spill accidents and the increasing industrial oily wastewater. Bioinspired combination of low surface-energy substances and hierarchical micro/nanostructured surface, a variety of artificial superhydrophobic/superoleophilic materials have been developed, e.g., carbon nanotube sponge (Gui et al., 2010), metal mesh (Wang, Song, & Jiang, 2007), and polymers (Zhang et al., 2016, Zhang and Seeger, 2011).
With the increasing environmental interest in the use of renewable resources and biodegradable materials, cellulose has attracted intense attention for the fabrication of functional superhydrophobic and superoleophilic materials. Cellulose is one of the most common organic polymers in nature and is considered as an almost inexhaustible source of raw material for the increasing demand in environmentally friendly and biocompatible products in coating, laminates, optical film, sorption media, pharmaceuticals, foodstuffs, and cosmetics (Joubert, Musa, Hodgson, & Cameron, 2014; Tian & He, 2016). However, due to the large number of hydroxyl groups on surface of filter paper, cellulose is hydrophilic and easily wetted by water. To meet the practical application, physical or chemical modifications like coating (Du, Wang, Chen, & Chen, 2014; Gao et al., 2015; Li, Yang, Li, Lan, & Peng, 2017; Li, Zhang, & Wang, 2008; Lin et al., 2016; Wang, Li, & Lu, 2010; Wen, Guo, Yang, & Guo, 2017; Zhang, Lu, Qian, & Xiao, 2014; Zhao, Xu, Wang, & Lin, 2012), acetylation (Frisoni et al., 2001; Zhou et al., 2016), silylation (Andresen, Johansson, Tanem, & Stenius, 2006; Goussé, Chanzy, Excoffier, Soubeyrand, & Fleury, 2002; Sai et al., 2015), and graft polymerization (Carlmark & Malmstrom, 2002; Deng et al., 2010; Gao et al., 2016; Guo, Wang, Shen, Shu, & Sun, 2013; Jain, Xiao, & Ni, 2007; Larsson, Pendergraph, Kaldeus, Malmstrom, & Carlmark, 2015; Li et al., 2015; Lindqvist et al., 2008; Meng et al., 2009; Nystrom, Lindqvist, Ostmark, Hult, & Malmstrom, 2006; Xiao, Li, Chanklin, Zheng, & Xiao, 2011; Xue, Guo, Ma, & Jia, 2015; Zampano, Bertoldo, & Bronco, 2009), are generally applied to adjust the surface property.
Among those various preparation methods, the graft polymerization technique is an versatile method to bond hydrophobic polymers covalently onto cellulose and has been the subject of intensive research (Carlmark and Malmstrom, 2002, Deng et al., 2010, Gao et al., 2016, Lindqvist et al., 2008, Meng et al., 2009, Nystrom et al., 2006, Xiao et al., 2011, Zampano et al., 2009). The covalent bonds formed between cellulose and polymers enhance the stability of the superhydrophobicity and prolong the lifespan of the superhydrophobic material. Under radiation-induced graft polymerization, 1H,1H,2H,2H-nonafluorohexyl-1-acrylates were grafted onto the cotton fabric (Deng et al., 2010). The modified cotton fabric exhibited superhydrophobicity and the reported water contact angle was approximately 160°. However, the water droplets pinned on the surface and were unable to slip off irrespective of the direction in which the surface is tilted.
Surface-initiated atom transfer radical polymerization (SI-ATRP) is a versatile approach for the polymers to grow from initiating sites or immobilized initiators on the surface of cellulose, providing graft chains with controllable molecular weight and narrow molecular weight distribution (Joubert et al., 2014, Matyjaszewski and Xia, 2001). Carlmark et al. reported that poly(methyl acrylate) was grafted from filter paper by SI-ATRP. The grafted paper became more hydrophobic with an increase in grafted chain lengths and the contact angle reached 133° (Carlmark & Malmstrom, 2002). Wood pulp cellulose fibers were grafted with poly(ethyl acrylate), which is of great potential as value-added paper products (Zampano et al., 2009). Xiao et al. demonstrated poly(butyl acrylate)-grafted cellulose microfibrils (CMF) with controllable hydrophobic chains by varying the reaction temperature, the type of solvents, and the use of catalyst (Xiao et al., 2011). This hydrophobic-modified CMF is a promising reinforcement for biocomposites.
Cellulose-based filter paper is a porous microtextured material with the capability for rapid liquid–liquid and solid-liquid separations. Although many hydrophobic monomers (Carlmark and Malmstrom, 2002, Deng et al., 2010, Joubert et al., 2014, Lindqvist et al., 2008, Meng et al., 2009, Nystrom et al., 2006, Xiao et al., 2011, Zampano et al., 2009) have been used to graft onto cellulose, the fabrication of functional cellulose-based membranes with both superhydrophobicity and superoleophilicity has not been explored extensively. It is desirable to fabricate filter papers with superwettability by graft polymerization for the oil/water separation in the complicated practical applications.
In this study, a robust superhydrophobic and superoleophilic filter paper membrane was prepared by grafting of poly(perfluorooctylethyl methacrylate) (PFOEMA) on filter paper using ATRP method. The covalent chemical bond linking between low-surface-energy PFOEMA and filter paper surface provides high chemical resistance to withstand various harsh conditions such as acidic or alkaline solutions. The PFOEMA-modified filter paper has high efficiency and reusability for the separation of oil/water mixture by a simple and convenient filter approach.
Section snippets
Materials
Filter papers were purchased from Hangzhou Special Paper Co., Ltd, China. 2-(perfluorooctyl)ethyl methacrylate (FOEMA, 97%) were obtained from Shanghai Aladdin Reagent Co., Ltd, China. N,N,N',N′′,N′′-pentamethyldiethylenetriamine (PMDETA, 99%) and 2-bromisobutyryl bromide (BiBB, 98%) were obtained from Tokyo Chemical Industry. Copper (I) bromide (CuBr, >99%) were purchased from Adamas. N,N-dimethylformamide (DMF), dichloromethane (CH2Cl2), toluene, pyridine, acetone, and ethanol were supplied by
Modification of filter paper
The surface of filter paper contains large amounts of hydroxyl groups, which are active sites for various modifications to form controlled architecture and functionalized materials by ATRP. BiBB reacts with the accessible hydroxyl groups on the filter paper surface by esterification reaction (Scheme 1), forming a monolayer of cellulose-Br covalently binding on the surface of pristine paper. The cellulose-Br containing active terminal bromine was used as macroinitiator for the grafting of PFOEMA
Conclusion
Superhydrophobic and superoleophilic PFOEMA-grafted filter paper for oil/water separation is demonstrated in this study. The filter papers with different DG were prepared by controlling the amounts of added monomer. When the DG exceeded 11.2%, the cellulose-g-PFOEMA reached a steady superhydrophobicity and the water contact angle was about 157°. The covalent bond of polymer to filter paper, renders cellulose-g-PFOEMA high chemical resistance to withstand different pH solution conditions. The
Acknowledgments
This work was supported by the National Natural Science Foundation of China (31470598, 21774021), the Award Program for Minjiang Scholar Professorship, and Technology Cooperation and Exchange Project of Fujian Agriculture and Forestry University (KXb16002A).
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