Highly permeable cellulose acetate nanofibrous composite membranes by freeze-extraction
Graphical abstract
Introduction
Ultrafiltration is important and widely used in water treatment, food industry and life science, and has become more crucial with increasing concerns in the living environment. However, most commercial membranes have a low porous separation layer and a large flow resistance leading to a small permeation flux [1], [2], [3]. Very recently, one-dimensional nanomaterials (e.g. nanofibers, nanowires and nanotubes) have attracted increasing interest and contributed significantly to fabrication of nanofibrous composite membranes [4], [5], [6], [7]. Due to their peculiar properties, they impart unique structural characteristics to the membranes, such as high permeability, high porosity, high surface area and interconnected pores [4]. Currently many one-dimensional inorganic materials are used to produce high efficient nanofibrous composite membranes, including carbon nanotubes [7], [8], [9], inorganic nanowires [10], carbonaceous nanofibers [6], metal oxide nanofibers [11], and metal hydroxide nanofibers [7], [12]. The resulting membranes show a great potential in ultrafiltration. However, most ultrafiltration membranes are prepared from polymers because of their low cost and high processability. Thus, it is highly imperative to develop a facile approach for the fabrication of high efficient ultrafiltration membranes from polymer nanofibers for various separation processes.
Various techniques have been applied to prepare polymer nanofibrous membranes. The electrospinning technique is the most versatile one and has been used to produce nanofibrous membranes with pore sizes usually on the sub-micrometer scale [4], [13], [14], [15]. The diameters of electrospun nanofibers are generally in the range of micrometer or sub-micrometer. However, ultrafiltration membranes fabricated from ultrafine electrospun nanofibers are now a great challenge [16]. The drawback of electrospun nanofibers lies in their poor diameter control and the difficulty in fabricating ultrathin membrane with a controlled thickness [5]. Alternatives have been explored to produce polymer nanofibers, such as molecular self-assembly, lithography, template synthesis, and interfacial polymerization [17], [18], [19], [20], [21], [22], [23]. However, it is difficult to fabricate ultrafiltration membranes by these methods due to their intricate operations and specific outcome of nanofiber. Therefore, the development of facile and versatile pathways of synthesis of polymer nanofibers to fabricate nanofibrous composite membranes for ultrafiltration is an important and interesting task.
In this work, we demonstrate a facile and green approach to fabricate CA nanofibrous composite membranes for ultrafiltration. Briefly, CA nanofiber dispersions were first prepared via the freeze-extraction method and then filtrated on a CA microfiltration membrane to form a free-standing nanofibrous layer. Fig. 1a depicts the as-prepared nanofibrous composite membranes with an ultrathin CA nanoporous layer (separation layer) covered on a CA macroporous membrane (support, Fig. 1b) with a cut-off of 0.20 μm. CA, the acetate ester of cellulose, is one of the earliest materials used and still finds utility in membrane separation processes due to its outstanding formability, reasonable resistance to degradation by chlorine and low fouling. Membranes made from CA are nowadays used in seawater desalination, water purification, wastewater treatment, concentration of fruit juices, and life science [24]. The as-fabricated nanoporous membranes with a tiny membrane resistance show ultrahigh water flux and good ferritin rejection during ultrafiltration.
Section snippets
Materials
CA, with average molecular weight of 30,000 g mol−1 and approximately 54.5–56.0% acetic acid bonded, was purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). CA membranes, with a cut-off of 0.2 μm and 25 mm in diameter, were purchased from Toyo Roshi Kaisha Ltd. (Japan). Dimethylsulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), N, N-dimethylacetamide (DMAc), and glacial acetic acid (GAA), of analytical grade, were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). 5
Preparation and characterization of CA nanofiber dispersions
Freeze-extraction used to prepare CA nanofiber dispersions is a feasible technique that has also been used to prepare microporous membranes and highly porous polymer scaffolds. In this method, a high concentration (usually above 5 wt%) polymer solution is frozen, and then immersed in a non-solvent bath at a temperature lower than the freezing point of the polymer solvent, resulting in the formation of porous structures with porosity of up to 80% [25], [26], [27]. Although this technique has been
Conclusions
In summary, a series of CA nanofiber dispersions was produced from very dilute CA solutions by the freeze-extraction method, and further used to fabricate nanofibrous composite membranes. The diameter of the CA nanofibers is about 10 nm from TEM observation. The as-prepared CA nanofibrous composite membranes have an ultrathin free-standing layer of CA nanofibers covered on a CA microfiltration membrane. This layer has an ultrahigh porosity of up to 71% that is at least 7 times greater than the
Acknowledgments
The research was supported by National Nature Science Foundation of China Grant nos. 21076170, 21107089 and 21376194, the research fund for the Doctoral Program of Higher Education (20120121120013) and the Fundamental Research Funds for the Central Universities (No. 2012121029).
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