Controllable Construction Of Polymer Nanofiber Aerogels With Oriented Hierarchical Porous Structures And Their Filtration Performances

Oil spills and organic solvent discharge have caused a lot of energy loss and ecological pollution in rivers and oceans,and rise as global issues in recent decades.Thus,the development of highly efficient and economical oil/water separation materials have attracted much attention in recent years.The traditional oil/water separation materials are mainly fibrous filters,especially electrospun membranes with adjustable surface properties and structures.However,in nanofibrous mats,the nanofibers are packed in roughly parallel planes,which have a limited porosity and high fiber packing density,resulting in low flux and high resistance.In addition,the wettability of the oil/water separation materials is very important for selective and efficient separation,which needs be improved urgent.Therefore,this thesis focuses on the design and construction of polymer nanofiber aerogels with aligned micrometer-sized hierarchical porous structures via oriented ice-segregation-induced self-assembly and thermal imidization process.In the first work,polyimide was chosen as an adhesive and cross-linker,OPAN nanofibers were chosen as the main material.The obtained OPAN/PI nanofiber aerogels exhibit high absorption capacities,high oil/water separation efficient and flux.On the basis of this work,the SiO₂/OPAN-PI nanofiber aerogels with aligned micrometer-sized hierarchical porous structures were successfully fabricated.On the one side,SiO₂ nanoparticles are benefit to construct micro-nano structures on the OPAN nanofibers,and enhance the roughness of the OPAN nanofibers.On the other side,the involved SiO₂ nanoparticles can modify the surface properties of the polymer nanofiber aerogels.Thus,the resulted SiO₂/OPAN-PI nanofiber aerogels exhibit excellent wettability,which can meet the selective oil/water separation.The main results obtained in this dissertation are as follows:（1）The polyacrylonitrile（PAN）nanofibers were obtained by electrospinning.Furthermore,the electrospinning factors,such as electrospinning voltage,receiving distance and solution flow rate have been intensely explored.When the spinning voltage is 20 kv,the spinning distance is 25 cm,and the solution flow rate is 0.015 ml min^-1,continuous and uniform PAN nanofibers can be obtained.（2）In order to reduce the filtration resistance of fibrous filters,the OPAN/PI nanofiber aerogels were constructed via oriented ice-segregation-induced self-assembly method and thermal imidization process.In this work,the porous structures can be adjusted by the physical and chemical interaction between PAA and OPAN,and the oriented freeze process.The obtained OPAN/PI nanofiber aerogels have excellent thermal stability,the weight loss is less than 5 wt%at 420℃.Besides,the optimized aerogels have high hydrophobicity（water contact angle 136.0°）,thus they can collect various oily solvents with high absorption capacities up to135 times of their own weight.Moreover,surfactant-stabilized water-in-oil emulsions and oil/water mixture can be fully separated under gravity driving and the filtration efficiency is close to 100%,while the flux is up to 8.7×10⁵ L m^-2 h^-1.Thus,this work can address the tradeoff between filtration efficiency and resistance.（3）To solve the wettability of the oil/water separation materials,the SiO₂/OPAN-PI nanofiber aerogels with aligned micrometer-sized hierarchical porous structures were fabricated via the same procedure of the OPAN/PI nanofiber aerogels.In this work,SiO₂nanoparticles can not only help construct micro-nano structures and enhance the roughness of the OPAN nanofibers,but also modify the surface properties of the aerogels.Thus,the resulted SiO₂/OPAN-PI nanofiber aerogels exhibit excellent double superphobicity and wettability.The optimized SiO₂/OPAN-PI nanofiber aerogels exhibit high filtration efficiency（>99%）,higher flux(oil flux 8.39×10⁴ L m^-2 h^-1;Water flux of 2.87×10⁴ L m^-2 h^-1)and excellent oil/water switching stability.Superhydrophobicity under oil and superhydrophobicity under water can be switched over 50 times continuously and maintain a separation efficiency of more than 98.5%.