Construction And Separation Applications Of High-performance Polyamide Thin-film Composite Membranes Supported By Electrospun Nanofibers

Electrospun nanofiber membranes possess very broad application prospects in separation fields,attributed to their unique advantages such as high porosity,low tortuosity and interconnected pore structure.Especially,polyamide thin film composite（TFC）membranes supported by electrospun nanofiberous substrates exhibited much superior separation performance than those supported by conventional polymeric substrates by non-solvent phase separation method.However,electrospun nanofiber substrates generally suffer problems of poor mechanical performance,large surface pores size and poor solvent resistance,et.al.To this end,various modification strategies of nanofiber substrates are conducted in this thesis,including the hydrazide thermal cyclization and synthesized monomer optimization of the polyimide（PI）nanofiber substrates,the cyclization modification of polyacrylonitrile（PAN）nanofiber substrate,and the nanofiber alignment,for constructing high-performance polyamide TFC membranes.Effects of different modification strategies on physicochemical properties,microstructure and separation performance of nanofiber-based TFC membranes were also studied.Firstly,polyimide nanofiber substrate was modified by a one-step simple thermal cyclization modification with terephthalic hydrazide（TPDH）,and employed as substrate for TFC membrane in the application of strong polar aprotic solvents recovery for the first time.The thermal cyclization modification of the TPDH/PI nanofiber substrate can simultaneously improve the solvent resistance,increase the mechanical properties and reduce the surface pore size of the nanofiber substrate.The modification mechanism involved,effects of TPDH loading and thermal cyclization temperatures on solvent resistance and pore structure,were studied.The solvent permeance of TFC membrane supported by the thermal cyclized TPDH/PI nanofiber substrate reached up to 9.4L·m^-2·h^-1·bar^-1（LMH/bar）,coupled with a 99.8%rejection in Rose Bengal（RB）/DMF system and a good long-term stability.Secondly,intrinsic insoluble PI nanofiber substrate was further developed by a two-step method including electrospinning of poly（amic acid）synthesized by polycondensation reaction between various monomers and subsequent thermal imidization,and employed as substrate for TFC membrane in strong polar aprotic solvents recovery.The lower thermal imidization temperature of PI nanofiber substrate was achieved by monomer optimization to alleviate the flux decline caused by the excessively high thermal treatment temperature.The effects of different dianhydride and diamine monomers and different thermal imidization temperatures on the solvent resistance and microscopic morphology of nanofiber substrates were also studied.The solvent permeance of corresponding TFC membrane supported with the insoluble PI nanofiber substrate reached up to 11.2 LMH/bar,with 99.7%rejection in RB/DMF system,and good long-term stability.Thirdly,to reduce the high cost of raw materials,a novel facile cyclization modification of low-cost PAN nanofiber substrate was also proposed by a simply thermal treatment and subsequent alkaline hydrolysis,and employed for the fabrication of TFC membrane for the first time.The reaction mechanism involved,strong polar aprotic solvents resistance and pore structure of PAN nanofiber substrate brought by the modification were investigated.In addition,the“trade-off”relationship between the permeance and solvent resistance of chemically crosslinked polymeric-based TFC membranes was effectively broken.The TFC membrane supported by the modified PAN nanofiber substrate exhibited ultrahigh solvent permeance up to 13.3 LMH/bar coupled with a 99.7%rejection in RB/DMF system and a good long-term stability.To overcome the membrane permeance decline caused by the high modification temperature of nanofiber substrates involved in above works,the well-aligned nanofiber substrates with slit-shape pores were also designed to modulate the pore structure and to construct high-performance TFC membrane for the first time.The physicochemical properties of nanofiber substrates,the micro-structure and separation performance of resultant TFC membranes were systematically investigated.The slit-shape pores with the better pore interconnectivity and larger inner pore size of the well-aligned nanofiber substrate facilitates the fast transportation of solvent and solutes and increased permeance of TFC membrane.The forward osmosis tests were used to evaluate the effects of aligned nanofiber substrate on permeance and selectivity of TFC membrane and membrane structural parameter.In addition,the solvent permeance in strong polar aprotic solvent systems of TFC membrane supported by the modified aligned PAN nanofiber substrate was40%higher than that of TFC membrane supported by the modified PAN nanofiber substrate in previous work,and exhibited a good long-term stability.