Polyacrylonitrile Membrane Porous Structure Tailoring And Its Application In The Composite Membrane Preparation

With the development of society,issues like water pollution and resource shortages have become more common.Membrane technology can allow fluid to transport across membranes and remove harmful substances through mechanisms such as size screening and dissolution diffusion.Separation membranes are the key to membrane technology.The development and practical application of membrane technology are hindered by a variety of problems,including the limited improvement of membrane performance,the aggravation of membrane fouling,the trade-off phenomenon between membrane permeability and selectivity,and the challenge of the precise tailoring of membrane pore structure.In this thesis,a method for the preparation of polyacrylonitrile（PAN）ultrafiltration（UF）membraneswithidealpore structure—atomization-assisted non-solvent induced phase separation（AA-NIPS）—was developed.Moreover,the phase behavior during solidification and the mechanism tailoring membrane pore structure were theoretically studied by drawing thermodynamic phase diagrams of ternary systems.To fulfill the demands of greater separation accuracy,the AA-NIPS technique was additionally introduced for the preparation of thin film composite（TFC）membranes and interlayered thin film nanocomposite（TFNi）membranes.The comprehensive performance of composite membranes was effectively improved through the modification and modulation of the support and separation layers.The following are the results and research contents:（1）In this thesis,a series of high-performance PAN UF membranes were continuously prepared by the AA-NIPS method in response to the challenging problems of the difficulty in accurately tailoring the pore structure and membrane fouling.The UF membrane,composed of a 3D network porous sublayer and surface layer with narrow pore size distribution,was prepared by designing the nonwoven fabric atomization direction mode under the protection device on the nascent film surface coated with casting solution during the atomization pretreatment stage.This method achieved clean and effective preparation without the use of chemical pore-forming agents.It has been established through the methodical analysis of the UF membrane’s micromorphology and pore structure that these characteristics can be precisely tailored by adjusting the atomization process.The pure water flux（PWF）of UF membranes after atomization pretreatment increased more compared to the conventional NIPS-prepared UF membrane,while the irreversible fouling ratio decreased.The advantages of AA-NIPS technology include controllable structure and simple functionalization,which are predicted to be used in fields like biotechnology,medicine,and food processing.（2）In view of the complex mechanism and lack of characterization methods for the wet preparation of microporous membranes’solidification phase separation process,this thesis,based on the Flory-Huggins theory,extended the establishment of the thermodynamic model of the PAN-DMF-H₂O ternary system.The theoretical thermodynamic phase diagram of the PAN-DMF-H₂O system is drawn through the theoretical equations of binodal and spinodal lines,which are continuously and iteratively calculated using a self-designed Python language program based on the least squares method after the experimental determination of fundamental physical quantities such as binary interaction parameters.The track of the solidification process was acquired by measuring the system composition of AA-NIPS and NIPS under different atomization and solidification times during the wet film preparation.The behavior of the solidification phase and the mechanism of pore structure tailoring were then fully investigated.The cloud point method was also used to explain how the ternary system components influenced the pore structure and phase behavior of membranes.The research methodology presented in this thesis offers a fresh approach and concept for the investigation of the mechanism underlying the solidification phase separation process.It can therefore better guide the wet preparation of membranes and facilitate the precise tailoring of membrane pore structure.（3）In a bid to take care of the performance limitations of the TFC membrane and the self-limiting nature of interfacial polymerization,the AA-NIPS method was used in the preparation of the TFC membrane support layer of this thesis.The influence of different support layers on TFC membranes was investigated through systematic characterization and testing of the structure and performance of the composite membranes.The support layers suited for thin film composite nanofiltration（TFC-NF）and thin film composite forward osmosis（TFC-FO）membranes were chosen.Based on the modification of the support layer,the PA layer of the TFC membrane has been successfully tailored by exploring the optimal interfacial polymerization reaction conditions and using methods like combined cosolvent-assisted interfacial polymerization.These approaches work in conjunction with the support layer to enhance the overall membrane performance and surpass the self-limit of interfacial polymerization.A new approach to the preparation and modification of the optimal structural support layer is offered by the AA-NIPS process.（4）In view of the long-term trade-off between the permeability and selectivity of NF membranes,this thesis first tried to construct a PVA/ATP nanocomposite interlayer by connecting the natural nanomaterial attapulgite（ATP）into the cross-linked network of polyvinyl alcohol（PVA）and glutaraldehyde（GA）in the form of a covalent bond on the PAN microporous membrane with a symmetrical network porous structure prepared by the AA-NIPS method,replacing the traditional vacuum filtration deposition method,which is difficult to prepare continuously.The water transport channel is greatly improved by the porous structure of the support layer,and a high-quality PA layer with a large surface area may be formed on the support layer owing to the design of the PVA/ATP interlayer.Under the manipulation of the adsorption-diffusion behavior and spatial-temporal distribution of the aqueous monomers at the interface of the interlayer,an ultra-thin PA layer with crumpled textures was formed,which further enhanced the permeability of the TFNi-NF membrane.The trade-off between NF membrane permeability and selectivity is no longer obvious,which has shown great potential for aqueous electrolyte purification and seawater desalination.