Superamphiphobic nanofibrous membranes for effective filtration of fine particles

https://doi.org/10.1016/j.jcis.2014.04.026Get rights and content

Highlights

  • Novel fabrication of superamphiphobic nanofibrous filtration membranes.

  • Bonding/non-bonding structured composite membrane were constructed.

  • Mechanical properties were significantly improved.

  • Functionalized nanofibrous media showed enhanced air filtration performance.

Abstract

The worldwide demands are rising for an energy-efficient and cost-effective approach that can provide advanced nanofibrous membranes with high filtration performance and superior antifouling properties. Here we report a novel synthesized fluorinated polyurethane (FPU) modified nanofibrous membrane optimized to achieve oil and non-oil aerosol particle filtration. By employing the FPU incorporation, the polyacrylonitrile/polyurethane (PAN/PU) composite membranes were endowed with superhydrophobicity with a water contact angle of 154° and superoleophobicity with an oil contact angle of 151°. Morphology, surface wettability, porous structure, and filtration performance could be manipulated by tuning the solution composition as well as the hierarchical structure. Furthermore, the as-prepared membranes can capture, for the first time, a range of different oil aerosol particles in a single-unit operation, with >99.9% filtration efficiency, by using the combined contribution of fiber diameter and surface roughness acting on the objective particles. Exemplified here by the construction of superamphiphobic nanofibrous membrane, numerous applications of this medium includes high efficiency particulate air filters, ultra-low penetration air filters, and respiratory protection equipment.

Introduction

Clean air is a vital resource for human life. However, population growth and enhanced human activities, together with the expansion of industrial and agricultural production, are creating unprecedented demands on clean air supplies all over the world [1], [2]. The World Health Organization has reported that more than two million premature deaths each year can be attributed to the effects of urban outdoor air pollution and indoor air pollution, particularly in developing countries [3]. Fine particles, especially particulate matter with an aerodynamic diameter smaller than 2.5 μm (PM2.5), proved to be a major cause of adverse health effects ranging from the human respiratory tract to extrapulmonary organs [4], [5]. Conventional filter materials based on nonwoven fibers (e.g. melt-blown fibers, glass fibers, and spun-bonded fibers) are incapable of capturing fine particles due to the micro-sized fiber diameter [6], [7]. Nanofiber based filters are attractive for air filtration because of their enhanced filtration performance and improved service life in actual operating environments [8], [9].

Nanofiber fabrication techniques include template synthesis, phase separation, sea-island spinning, etc. [10], [11], [12]. Although these methods can be used to construct nanofibrous membranes according to applications, they are usually difficult-to-control, time- or energy-intensive. Electrospinning, as an alternative to these methods, is increasingly becoming the subject of filter material investigation [13], [14], [15]. Owing to the tunable fiber diameter, high porosity, remarkable specific surface area, and interconnected porous structure, electrospun fibrous membrane is considered an effective medium for air filtration [16], [17], [18], [19]. A series of electrospun filter materials have been fabricated by designing the filter structure or exceptional properties in combination with filtration modeling and mechanism [9], [20], [21], [22]. However, the antifouling and mechanical properties of these membranes remain insufficient to allow them to compete with commercial polypropylene (PP) nonwoven membranes in practical working circumstance.

On the other hand, combining the advantages of electrospun nanofibers with surface modification to yield hybrid functional membranes has provided a facile way to construct filter media with controllable wettability [23], [24], [25]. In contrast to superhydrophobicity, the super-repellence to oil fluids seems more challenging, but shows great potential in antifouling filter media from hazardous chemical and biological contaminants [26], [27]. Fluorinated polyurethane (FPU) containing perfluoroalkane segments (Fig. 1a), a class of low surface free energy materials with a wide range of interesting characters including low surface free energy, resistant to abrasion, and excellent hydrolytic stability, which make it a promising material for functional membranes with distinctive wettability [28], [29]. To date, although it has been known for some time that nanofibers could be prepared from FPU [28], nearly no effort has been devoted to the development of FPU modified superamphiphobic air filter media.

In this contribution, we present the fabrication of superamphiphobic polyacrylonitrile (PAN)/PU nanofibrous filter media with robust fine particle filtration performance by the introduction of a novel synthesized FPU, as shown in Fig. 1a. With this aim in mind, PAN/PU composite membranes with interpenetrating bonding/non-bonding structures were constructed via regulated jet ratios of PAN and PU solutions. Key to our development design is that the use of FPU endowed the PAN/PU nanofibers with hierarchically roughened surface, which showed great influence on the wettability, porous structure, and filtration properties of resultant membranes. Furthermore, the FPU modified PAN/PU fibrous membranes were proven to be serviceable filter media with substantially high filtration performance to oil and non-oil aerosol particles, which is capable of providing robust operational stability, and effectively extending the service life.

Section snippets

Materials

PAN (Mw = 90,000) was purchased from Kaneka Co., Ltd., Japan, PU (Elastollan1 2280A10) and 4,4-methylenebisphenylisocyanate (MDI) were purchased from BASF Co., Ltd., Perfluoro-1-decanol (CF3(CF2)7CH2CH2OH) (TEOH-8) was purchased from Hengtong Fluorine Co., Ltd., China. Polytetrahydrofuran (PTMEG, Mn = 1000), triethylene glycol (TEG), N,N-dimethylformamide (DMF), dimethylacetamide (DMAc) and methanol were supplied by Shanghai Aladdin Chemical Co., China. All chemicals were of analytical grade and

Results and discussion

Representative FE-SEM images of PAN nanofibers resulted from the varied concentration of PAN solutions are shown in Fig. 2, revealing the randomly oriented three-dimensional (3D) nonwoven structures. As shown in Fig. 2a, the PAN-7 fibrous membranes in which beads (indicated by dotted circle) of both small and large sizes coexist in the individual nanofibers (average diameter of 175 nm) were formed from 7 wt% PAN solutions. A possible explanation for beads formation is that the molecular chains in

Conclusions

In summary, we have described the fabrication of a novel superamphiphobic nanofibrous membrane exhibiting robust air filtration performance by the combination of electrospun PAN/PU nanofibers and a new synthesized FPU containing terminal perfluoroalkane segment. The interpenetrating bonding/non-bonding structured PAN/PU presented relatively high tensile strength (12.28 MPa), excellent air permeability (706.84 mm/s), and comparable abrasion resistance by varying the concentration of PAN solutions

Acknowledgments

This work is supported by the Key Technologies R&D Program of China (No. 2013BAC01B02), the National Natural Science Foundation of China (Nos. 51173022 and 51322304), the Fok Ying-Tong Education Foundation for Young Teachers in the Higher Education Institutions of China (131070), the Program for New Century Excellent Talents in University, the Fundamental Research Funds for the Central Universities in China, and the “DHU Distinguished Young Professor Program” and Deanship of Scientific Research

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    These authors have contributed equally to this work.

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