RSC Advances PAPER View Article Online View Journal | View Issue A durable thin-film nanofibrous composite nanofiltration membrane prepared by interfacial Cite this: RSC Adv.,2017,7, 18001 polymerization on a double-layer nanofibrous scaffold Yin Yang,a Xiong Li,a Lingdi Shen,b Xuefen Wang *a and Benjamin S. Hsiao*c A novel kind of thin-film nanofibrous composite (TFNC) nanofiltration membrane consisting of a polypiperazine amide (PPA) barrier layer, an ultrathin electrospun poly(acrylonitrile-co-acrylic acid) (PAN–AA) transitional mid-layer and an electrospun polyacrylonitrile (PAN) nanofibrous supporting layer, was successfully fabricated by interfacial polymerization with piperazine (PIP) and trimesoyl chloride (TMC) onto the PAN–AA/PAN double-layer substrate. The PAN–AA nanofibrous mid-layer played two important roles between the PPA barrier layer and the PAN nanofibrous supporting layer. It could be swollen in the alkaline aqueous monomer (PIP) solution to form an intermediate hydrogel film, which Creative Commons Attribution 3.0 Unported Licence. acted as the transitional mid-layer to cover the majority of the large surface pores of the electrospun PAN nanofibrous substrate. On the other hand, the hydrophilic PAN–AA hydrogel film could capture and reserve abundant PIP monomer to facilitate interfacial polymerization with TMC to form an endurable ultrathin PPA barrier layer, resulting in an integrated composite membrane confirmed by the mechanical Received 15th January 2017 properties. The resultant TFNC membranes demonstrated a high rejection rate (98.2%) and high Accepted 17th March 2017 À2 À1 À1 permeate flux (64.4 L m h ) for MgSO4 aqueous solution (2.0 g L ), and also exhibited excellent DOI: 10.1039/c7ra00621g structural stability due to the strong interactions between the barrier layer and the nanofibrous support rsc.li/rsc-advances that were enhanced by the transitional PAN–AA mid-layer. This article is licensed under a 1. Introduction received much attention as the support layer to fabricate TFC membrane due to its high permeability that derived from its Open Access Article. Published on 24 March 2017. Downloaded 6/4/2019 10:37:28 PM. In recent decades, thin lm composite (TFC) membranes have high porosity (up to over 80%), interconnectivity, micro scale been heavily researched for developing nanoltration (NF) interstitial space, and a large surface-to-volume ratio.9,10 This membranes, which have an asymmetric structure composed of kind of TFC membranes was named as thin lm nanobrous a thin functional barrier layer on a porous substrate.1,2 It is composite (TFNC) membranes, which have been proven to be considered as a key advantage that each layer in a TFC an effective media for microltration (MF),11 ultraltration – – membrane can be optimized. The thin dense barrier layer is (UF)12 14 and NF,8,15 17 as the permeability of the composite usually obtained through dip-coating,3 layer-by-layer self- membrane was enhanced while maintaining good solute assembly4,5 or interfacial polymerization,6,7 by which the ltra- rejection capability. tion performance of TFNC membranes is mainly determined. However, high permeability of the nanobrous substrates Meanwhile, the conventional reinforced porous matrix is from its high porous structure will bring new challenges for the a dense asymmetric membrane made by phase inversion preparation of the top barrier layer onto the nanobrous methods which could provide enough mechanical strength to substrate. For example, it is difficult to avoid the permeation of support a thin barrier layer.8 Recently, an alternative highly coating solution into the inside of the nanobrous scaffold porous substrate made of electrospun nanobrous mat has resulting in a much thicker barrier layer during the conven- tional surface coating process. Especially for the barrier layer prepared by interfacial polymerization method, the low viscous a State Key Laboratory for Modi cation of Chemical Fibers and Polymer Materials, aqueous phase monomer solution held in nanobrous College of Materials Science and Engineering, Donghua University, Shanghai, 201620, P. R. China. E-mail: [email protected]; Fax: +86-21-67792855; Tel: +86- substrate could be easily drained away from the surface during 21-67792860 the interfacial polymerization process, readily leading to bSchool of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou, a defective and ultrathin barrier layer. In addition, the diame- 221116, China ters of most polymer bers generated by electrospinning are in cDepartment of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA. the range of 100–1000 nm, the mean pore sizes of electrospun E-mail: [email protected] This journal is © The Royal Society of Chemistry 2017 RSC Adv.,2017,7,18001–18013 | 18001 View Article Online RSC Advances Paper nanobrous membranes (ENMs) are relatively large (more than Acrylonitrile (AN), acrylic acid (AA), azobisisobutyronitrile 300 nm) according to the relationship between membrane pore (AIBN), methanol, ethanol, N,N0-dimethylformamide (DMF) and size and ber diameter in an electrospun non-woven structure, dimethylsulfoxide (DMSO) was kindly supplied by Shanghai i.e., the mean pore size of nanobrous membranes is about 3 Æ Chemical Reagent Plant. 1 times the mean ber diameter.18,19 Therefore, the ultrathin interfacial polymerized barrier layer supported by ENMs with 2.2 Synthesis of poly(acrylonitrile-co-acrylic acid) the relatively large pore size may cause new problems about the Polyacrylonitrile-co-acrylic acid (PAN–AA) with a viscosity- stability and durability of the TFNC membrane. To overcome 5 À1 averaged molecular weight (Mh) of 2.4 Â 10 g mol was these shortcomings, various approaches have been attempted synthesized by radical polymerization according to the method such as soaking in a coagulant bath,20,21 treating the nano- reported by Takaomi Kobayashi et al.26 AN was dried over brous substrates by hot-pressing,22 introducing a dopamine molecular sieves and distilled under atmospheric pressure at intermediate layer7,15 or coating a regenerated cellulose nano- 78 C to remove the inhibitor. AA was distilled under reduced whiskers layer on the nanobrous substrate.13,17,23,24 The intro- pressure (16 mmHg, 50 C). AIBN was recrystallized in super- duction of intermediate transitional layer not only can reduce saturated ethanol solution. All other reagent grade chemicals the surface pore size of nanobrous substrates, but also can were used without further purication. Radical polymerization introduce functional groups such as hydroxyl groups, carboxyl was carried out in DMSO solution as follows. In a reaction vessel groups,23 and amino groups15 leading to a more hydrophilic with 500 mL capacity, 30.4 g (566 mmol) of puried AN, 7.51 g surface for further modication. Herein, the intermediate (104 mmol) of AA, 110.5 g of DMSO and 0.22 g of AIBN were transitional layer can ll the gaps between functional barrier introduced. Polymerization was carried out at 60 C for 6 h in layer and nanobrous substrate and build up the interconnec- nitrogen ow. The reaction was terminated and the mixture was tion on both sides, and the integrity of TFNC membranes will be poured into a large quantity of water to precipitate the crude signi cantly enhanced. copolymer. A er washed with hot deionized water and meth- Creative Commons Attribution 3.0 Unported Licence. In the present study, a double-layer nano brous substrate anol alternatively several times, the copolymer was then dried containing an ultrathin poly(acrylonitrile-co-acrylic acid) (PAN– under vacuum at 40 C for at least 24 h. The copolymerization AA) nano brous transitional layer and a thick PAN nano brous 1 was conrmed by FTIR and H-NMR (d6-DMSO, 400 MHz) support layer was used to prepare the TFNC membranes. The À measurements, as the absorbance peaks of FTIR n /cm 1: more hydrophilic PAN–AA nanobers could be swollen in max 3470 (–OH), 1732 (C]O), 1627 (–COOH) and chemical shisof aqueous amine solution and merged into an ultrathin hydrogel 1H-NMR d /ppm: 12.87 (–COOH of AA), 2.05 (2H, –CH – of AN), lm which could cover most of the large pores (0.3–1.0 mm) on H 2 1.85 (2H, –CH – of AA). The result were in accordance with the the surface of PAN nanobrous substrate and enhanced the 2 reports in literature.27 The degree value of the acrylic acid (D ) membrane surface hydrophilicity.25 The swollen PAN–AA tran- AA This article is licensed under a was about 9.95%, which was calculated by following equation: sitional layer not only can form interconnection with PAN 2A nanobrous substrate by hydrogen bond between the nitrile D ð%Þ¼ COOH Â 100% AA A þ A (1) group of PAN and the carboxylic acid group of PAN–AA, but also CH2ðAAÞ CH2ðANÞ Open Access Article. Published on 24 March 2017. Downloaded 6/4/2019 10:37:28 PM. can capture the aqueous amine monomer for the following where A is the peak area of protons from carboxyl group interfacial polymerization process, i.e. the ow away of aqueous COOH (–COOH) attached to AA, A is the peak area of protons monomer in the process of conventional interfacial polymeri- CH2(AA) from methylene (–CH –) attached to AA, and A is the peak zation could be avoided effectively, and further improved the 2 CH2(AN) area of protons from methylene (–CH –) attached to AN.27 The interfacial bonding between the polyamide barrier layer and the 2 peak area of protons from carboxyl group (A ) was 1.0, and PAN nanobrous supporting layer. The effect of the transitional COOH the peak area of protons from methylene (–CH2–) was 20.11 in PAN–AA mid-layer on the fabrication of the interfacial poly- total. From the polymer structure, peak area of protons from amide barrier layer onto the nanobrous support was investi- methylene attached AA (ACH (AA)) was relative 2 times of the gated in details.