Structure Manipulation And Performance Optimization Of Organic Solvent Nanofiltration Membrane Based On Polyethylenimine

Organic solvent nanofiltration(OSN) is a burgeoning technology for membrane separation in organic solvents, which applicatively separates molecules or nanoparticles in the 200-1000 Da range by simply applying a pressure gradient. The development of OSN has triggered much attention owing to its easy operation, green, and high efficience. Therefore, the fabrication of OSN membranes with tunable transport properties is urgently required to accommodate sophisticated solvent systems for practical applications. The nanofiltration properties of OSN membranes are mainly derived from the membrane microstructures, which highly rely on the membrane material and preparation approach. Coupled hybridizing with cross-linking, OSN membranes with tunable transport properties were fabricated in this thesis by embedding the functional nano-particle/-tube into polymer matrix to regulate the microstructures. The composition and microstructure of as-obtained membranes were characterized extensively by TEM, SEM, FTIR, TGA, and contact angle measurements. Solvent uptake, area swelling, peameate flux, and rejection were elaborately measured. To achieve the high-performane OSN membranes, there were two critical issues encountered in this process, including the formation mechanism and control approach of membrane microstructure, as well as the close relationship between membrane material-microstructure-nanofiltration performance. The results were summarized as follows:(1) Inspired by the biomineralization, a series of composite OSN membranes with polymer-nanoparticle hybrid active layer were prepared via interfacial polymerization. Polyethyleneimine(PEI) was employed as polymer matrix, which catalyzed the tetraethoxysilane(TEOS) to synthesize inorganic nanoparticles with tunable structure. The microstructures and nanofiltration performances of the resultant membranes were systematically explored. It was found that the incorporation of well-dispersed inorganic nanoparticles endowed the composite membranes with enhanced thermal and high solvent resistance properties by inhibiting the polymer chain mobility. These factors facilitated the transfer ability for polar solvents, resulting in the increase of permeate flux. Meanwhile, the excellent compatibility of two interfaces dramatically elevated the rejection of membrane and displayed the promising long-term operation stability.(2) Owning to the hydrophilic shell and hydrophobic cavity, cyclodextrins(CDs) were utilized to construct the hydrophilic/hydrophobic pathways within the membrane, benefiting for the transport of polar/apolar solvents. In this study, the CDs with controllable cavity size and functional group were uniformly dispersed into PEI matrix to prepare composite OSN membranes, constructing a dual-pathway nanostructure. The relevant methods and theories on multi-scale synergies were tentatively proposed on the basis of extensive characterizations and measurements, affording some guidance to fabricate OSN membranes with tunable transport properties. It was found that the exquisite cavities of CDs served as hydrophobic pathway within the membrane to promote the transfer of apolar solvents. Increasing cavity size of CD benefited the smaller solvent molecule to migrate. Meanwhile, the free volume cavities of PEI matrix acted as hydrophilic pathways for polar solvents, which were tuned by the PEI/CDs interfacial interactions. For instance, the incorporation of β-CD-NH(1.5 wt% to PEI) elevated the isopropanol and n-heptane fluxes up to 41.6 and 24.2 L m-2 h-1 under 10 bar, and the rejection of PEG 400 was higher than 94.9 %.(3) The directional alignment halloysite nanotubes(HNTs) could construct continuous transfer pathways for organic solvents within the membrane. The surface of HNTs was first modified by β-CDs in the aid of epichlorohydrin. Afterwards, the HNTs-β-CD was incorporated into PEI to prepare the composite OSN membranes with high transfer characteristics. The mechanisms on microstructure formation and control method were rationally proposed to attain the relevant methods and theories for the enhanced transfer properties. It was found that after the modification, the HNTs-β-CD was successfully aligned vertically on the PEI matrix due to the directional formation and hydrophobic interactions. As a result, the transport properties of polar solvents were dramatically enhanced. In addition, the formation of polymeric layer coated on the surface of HNTs denoted the membrane with excellent compatibility, and thus the high rejection ability and potential long-term operation stability were obtained. Particularly, when the loading of HNTs-β-CD increased to 1.5 wt%, the isopropanol flux was up to 67.8 L m-2 h-1 and the rejection of PEG 200 was higher than 94.3 % under 10 bar.