Preparation and characterization of flexible composite membranes with MOFs grown inside hollow nanofiber
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摘要: 基于金属有机框架 (Metal organic frameworks, MOFs) 的柔性复合膜材料在气体分离、污染物吸附、药物可控释放等领域具有重要的作用。利用同轴静电纺丝技术，在聚偏氟乙烯 (Polyvinylidene fluoride, PVDF) 的高分子溶液中添加有机配体2-甲基咪唑 (2-Methylimidazole, 2-mI) 作为外壳纺丝液，在甘油中添加锌离子作为内芯纺丝液，制备得到的核壳结构纳米纤维进行水热反应。在内芯甘油溶解形成中空结构的同时，锌离子向外扩散接触到外壳高分子层内表面的有机配体，在中空纳米纤维 (Hollow nanofibers, HNFs) 内部原位生成了MOF晶体ZIF-8，得到了ZIF-8@HNFs柔性复合膜材料。同时研究了金属盐与有机配体的不同比例、水热生长时间和温度对中空纳米纤维内部生长ZIF-8的影响。最佳金属盐与有机配体的物质的量比为1∶40，65℃水热生长4 h。采用XRD、SEM、FT-IR和氮气吸附-脱附实验等对ZIF-8@HNFs柔性复合膜的结构和性能进行了表征。结果表明，ZIF-8原位生长在中空纳米纤维内部，负载量为3.351%，ZIF-8@HNFs复合材料的比表面积为38.189 m2/g、孔体积为0.204 cm3/g、孔径分布在4.678 nm 和7.573 nm，并且可以耐受200℃高温、多次弯折和纯水、碱液的浸泡4 h依旧保持结构的稳定性。而ZIF-8本身在酸性条件下会发生解离的特性，使得ZIF-8@HNFs柔性复合膜材料在污染物吸附-解吸、药物可控释放等领域具有潜在的应用价值。Abstract: The flexible composite membranes materials based on metal organic frameworks (MOFs) play a significant role in many fields such as gas separation, pollutant removal and controlled drug release system. Utilizing coaxial electrospinning technology, the composite nanofibers of core-shell structure was prepared by mixing organic ligand 2-methylimidazole (2-mI) in polyvinylidene fluoride (PVDF) polymer solution as shell spinning solution, and adding zinc ion to glycerol as core spinning solution. With hydrothermal synthesis, the glycerol in the core layer dissolves to form a hollow structure, while the zinc ion diffuses outward and contacts the organic ligands on the inner surface of the polymer shell layer at the same time. The in-situ growth of ZIF-8 MOF crystals inside PVDF hollow nanofibers (HNFs) were obtained as the ZIF-8@HNFs flexible composite membrane material. The effect of different metal salt and ligand proportions, different hydrothermal growth time and temperature on the formation of ZIF-8 in HNFs were studied. The molar ratio of zinc ion and 2-mI ligand was 1:40, and the hydrothermal growth condition was at 65℃ for 4 h with water as solvent, which proved to be the optimal preparation circumstance. The structure and performance of ZIF-8@HNFs was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry analysis (TGA) and the nitrogen adsorption-desorption test, etc. The results indicated that the ZIF-8 crystals were in-situ synthesized and loaded inside the PVDF HNFs with 3.351% content. The Brunner-Emmet-Teller measurements (BET) surface area was 38.189 m2/g, the pore volume was 0.204 cm3/g and the pore size distribution were at 4.678 nm and 7.573 nm. Furthermore, the ZIF-8@HNFs still maintains its structural stability after being treated with 200℃ high temperature, multiple bending and soaking with water or alkali solution for 4 h. The characteristic of ZIF-8 that itself can dissociate under acidic conditions, which makes the ZIF-8@HNFs flexible composite membrane material potentially valuable in areas such as contaminant adsorption-desorption and controlled drug release system.
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Key words:
- coaxial electrospinning /
- hollow nanofibers /
- metal organic frameworks /
- ZIF-8 /
- in-situ growth
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图 1 ZIF-8@中空纳米纤维(HNFs)复合膜的制备流程图
Figure 1. Flow chart of preparation of ZIF-8@Hollow nanofibers(HNFs) composite membranes
图 2 ZIF-8粉末、HNFs和ZIF-8@HNFs复合膜的XRD图谱
Figure 2. XRD patterns of ZIF-8 powder, HNFs and ZIF-8@HNFs composite membranes
图 3 锌离子与2-甲基咪唑的摩尔比分别为1∶10、1∶20和1∶40时制备的ZIF-8@HNFs复合膜的XRD图谱
Figure 3. XRD patterns of ZIF-8@HNFs composite membranes synthesized by adding different molar ratios of zinc ions and 2-methylimidazole: 1∶10, 1∶20 and 1∶40
图 4 水热反应温度分别为45℃、65℃和85℃时生长4 h制备的ZIF-8@HNFs复合膜的XRD图谱
Figure 4. XRD patterns of ZIF-8@HNFs composite membranes synthesized by different hydrothermal growth temperature at 45℃, 65℃ and85℃ for 4 h
图 5 水热反应时间分别为2 h、4 h、8 h和12 h时在65℃制备的ZIF-8@HNFs复合膜的XRD图谱
Figure 5. XRD patterns of ZIF-8@HNFs composite membranes synthesized by different hydrothermal growth time for 2 h, 4 h, 8 h and 12 h at 65℃
图 6 不同制备条件下ZIF-8@HNFs复合膜的SEM图
Figure 6. SEM images of ZIF-8@HNFs composite nanofiber membranes under different preparation conditions
图 7 ZIF-8@HNFs复合膜的EDS能谱图 (氟、碳、锌和氮)：(a) 大范围的纳米纤维膜；(b) 放大的单根纳米纤维
Figure 7. EDS maps of ZIF-8@HNFs composite membranes (F, C, Zn and N): (a) wide scale elemental mapping of nanofiber membranes; (b) enlarged elemental mapping of single nanofiber
图 8 锌离子与2-甲基咪唑的摩尔比为1∶10、1∶20和1∶40时制备的ZIF-8@HNFs复合膜的FT-IR图
Figure 8. FT-IR spectra of ZIF-8@HNFs composite membranes synthesized by adding different molar ratios of zinc ions and 2-methylimidazole: 1∶10, 1∶20 and 1∶40
图 9 ZIF-8粉末(a)和ZIF-8@HNFs复合膜(b)的水接触角照片
Figure 9. Water contact angle photos of ZIF-8 powder (a) and ZIF-8@HNFs composite membranes (b)
图 10 ZIF-8粉末、HNFs和ZIF-8@HNFs复合膜的TGA 曲线
Figure 10. Thermogravimetry curves of ZIF-8 powder, HNFs and ZIF-8@HNFs composite membrane
图 11 不同温度处理后ZIF-8@HNFs复合膜的XRD图谱
Figure 11. XRD patterns of ZIF-8@HNFs composite membranes treated by different temperature
图 12 ZIF-8粉末、HNFs和ZIF-8@HNFs复合膜的N2吸附-脱附曲线(a)和孔径分布(b)
Figure 12. N2 adsorption-desorption curves (a) and pore size distribution (b) of ZIF-8 powder, HNFs and ZIF-8@HNFs composite membranes
图 13 ZIF-8@HNFs复合膜：(a)尺寸和(b)质量；柔性测试：(c)对折、(d)弯曲和(e)缠绕
Figure 13. ZIF-8@HNFs composite membranes: (a) Size and (b) Mass; Flexibility testing: (c) Folding, (d) Bending and (e) Wrapping
图 14 ZIF-8@HNFs复合膜经过水、酸性环境和碱性环境洗涤4 h后的XRD图谱
Figure 14. XRD patterns of ZIF-8@HNFs composite membranes after washing 4 h in water, acidic and alkaline conditions
表 1 ZIF-8粉末、HNFs和ZIF-8@HNFs复合膜的吸附性能数据
Table 1. Adsorption performance of ZIF-8 powder, HNFs and ZIF-8@HNFs composite membranes
Sample SBET/(m2·g−1) Pore Volume/(cm3·g−1) Pore size/nm ZIF-8@HNFs 38.189 0.204 4.678, 7.573 HNFs 32.708 0.169 4.678, 7.838 ZIF-8 484.327 0.273 0.926 Notes: SBET is the specific surface area calculated by the Brunner-Emmet-Teller (BET) method. 
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