Thermally induced phase separation followed by in situ sol–gel process: A novel method for PVDF/SiO2 hybrid membranes
Introduction
Organic–inorganic hybrid materials have been extensively investigated as a promising choice for separation membranes [1], [2], [3]. The hybrid membranes usually show attractive advantages including expected separation performances, excellent mechanical properties, and high thermal and chemical stabilities [4], [5], [6]. These advantages are believed to be originated from the synergistic effects of the organic phase and the inorganic component. Therefore, the hybrid membranes have received intense attention in the fields of separation science [7], [8], [9], gas transportation [10], [11], heterogeneous catalysis [12], [13], [14] and fuel cells [15], [16], [17]. For example, Pereira et al. [18] found that significant improvement can be obtained in ionic exchange capacity and proton conductivity by hybridizing Nafion membrane with mesoporous silica (SiO2) containing sulfonic acid groups. Liang et al. [19] blended poly(vinylidene fluoride) (PVDF) matrix with nano-ZnO to optimize the membrane pores, and the hybrid membranes then exhibited excellent anti-irreversible fouling property. Saxena et al. [20] prepared poly(vinyl alcohol)/SiO2 organic–inorganic hybrid membranes with charges to efficiently separate protein mixtures under coupled driving forces.
Inorganic particles are the most useful ones to be incorporated into polymeric matrixes to form the hybrid membranes. Nanoparticles are preferred inorganics such as CaCO3 [21], [22], TiO2 [23], [24], [25], Al2O3 [26], [27], ZnO [28], [29], and SiO2 [30], [31], [32]. Generally, they are directly blended with polymers in solvents, and the resulting suspensions are transferred into a nonsolvent to induce phase separation. This route is known to be the simplest and well-used method to prepare the hybrid membranes. However, the direct blending/phase separation method is always difficult to avoid agglomeration of the nanoparticles, especially in those cases with high particle content. Particle agglomeration causes large defects and limits the improvement of membrane performance. Recently, the sol–gel process, as a classic way for the nanoparticle formation of SiO2 [33], [34], has been introduced to combine with phase separation to prepare organic–inorganic hybrid membranes [35], [36]. In the multiple processes, hydrolysis and polycondensation reactions take place to form nanoparticles in the presence of polymer networks during membrane formation. The confined growth of nanoparticles in polymer matrix can effectively prevent the aggregation of nanoparticles and control the final particle size. Xu et al. [37] prepared PVDF/SiO2 hollow fiber membranes by combining the sol–gel process of tetraethoxysilane (TEOS) with the wet-spinning method. The in situ formed SiO2 particles were homogenously dispersed in PVDF matrix, and apparently improved the mechanical property, thermal stability, permeation and antifouling performance of the hybrid membranes. However, by far, the sol–gel process is always combined with the nonsolvent induced phase separation (NIPS) method, which limits the development of hybrid membranes undoubtedly. Thermally induced phase separation (TIPS) is known to be another useful method for the preparation of porous membranes. Compared to NIPS, TIPS has several advantages such as ease of control, low tendency to defect formation and ease to fabricate diverse microstructures, which are desirable for various applications of membrane. Accordingly, TIPS has been applied to many polymers including PVDF [38], [39], [40], polyethylene [41], [42], polypropylene [43], [44], [45], and polyacrylonitrile [46], [47], [48], [49]. To our best knowledge, no literatures have reported the combination of TIPS and the sol–gel process of TEOS to prepare organic–inorganic hybrid membranes based on polymers and SiO2 particles. One remaining challenge is that the sol–gel process is still difficult to take place simultaneously during the cooling step of TIPS. It is well known the sol–gel process of TEOS is always performed with a basic aqueous medium. Therefore, a two-steps method is required to overcome the challenge as mentioned above, that is, TIPS followed by the sol–gel process of TEOS. Nevertheless, the normally used diluents in TIPS and TEOS are usually liquid at room temperature. TEOS prefers to elute out when immersing the nascent membrane into the aqueous reaction solution, which makes the sol–gel process unfulfilled in the organic matrix.
In this work, organic–inorganic PVDF/SiO2 hybrid membranes were prepared via TIPS followed by the sol–gel process of TEOS. Dimethyl sulfone (DMSO2) was used as a crystallizable diluent. It crystallizes into solid upon cooling, which can hold TEOS in the diluent phase, ensure the sol–gel process happens inside the membrane matrix, and reduce the loss of formed SiO2 particles. We report the effects of TEOS content in the binary diluent system on the pore size, surface porosity, overall porosity, surface hydrophilicity, water flux as well as mechanical properties of the prepared hybrid membranes. The stability of water flux under high pressure was used to characterize anti-compression property of the hybrid membranes. Furthermore, initial protein separation studies were conducted with the as-prepared membranes. Bovine serum albumin (BSA, pI=4.7, Mw=66 KDa) and bovine hemoglobin (BHb, pI=7.0, Mw=64.5 KDa), which have nearly identified molecule weight, were selected as model proteins.
Section snippets
Materials
PVDF (Mn=110,000 g/mol, Solef 6010) is a commercial product of Solvay Solexis, Belgium. It was dried to constant weight before use. DMSO2 (99%) was purchased from Dakang Chemicals Co., China. Tetraethoxysilane (TEOS), ethanol (C2H5OH), ammonium hydroxide (28 wt%, NH3·H2O), N,N-dimethylformamide (DMF), disodium hydrogen phosphate (Na2HPO4·12H2O), potassium dihydrogen phosphate (KH2PO4), sodium chloride (NaCl), bovine serum albumin (BSA, pI=4.7, Mw=66 KDa) were supplied by Sinopharm Chemical Reagent
Role of TEOS in membrane formation
PVDF/DMSO2 binary system was studied to prepare PVDF membranes via TIPS in our previous work [52]. Solid−solid (S−S) phase separation was confirmed for this system according to the simultaneous crystallization of PVDF and DMSO2. Herein, we introduce a third component, TEOS, to form a PVDF/DMSO2/TEOS ternary mixture as the casting solution for the preparation of PVDF/SiO2 hybrid membranes. The cooling process was tracked by real-time polarized optical microscopy (POM) for the ternary mixture (
Conclusion
A new method is proposed to prepare PVDF/SiO2 hybrid membranes by TIPS followed by the sol–gel process. TEOS can not only reduce the phase separation rate of PVDF/DMSO2/TEOS, but also act as a pore-forming agent for the hybrid membranes. Two types of pores can be formed in the membranes: large tubular pores due to the diluent crystals and small round pores shaped by TEOS droplets. SiO2 particles are in situ generated inside the small round pores after the hydrolysis and polycondensation of
Acknowledgments
The research is financially supported by the National Natural Science Foundation of China (Grant no. 21174124) and the Ph.D. Programs Foundation of the Ministry of Education of People׳s Republic of China (Grant no. 20120101110123).
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