Preparation Of Micro-nano PVDF/CNC Membrane By Electrospinning And Its Purification Performance For Oily Wastewater

Frequent oil spills and the growing industrial oil waste emissions have caused serious environmental pollution problems.The original waste oil adsorption material has many defects,such as complex operation,small adsorption capacity and unsustainable use.It forces researchers to focus on the design of micro-nano multi-level structure of materials and develop micro-nano structure adsorption materials with high oil absorption efficiency,excellent mechanical strength and reliability.Nanofibers produced by electrospinning technology have the characteristics of high porosity and easy control of micro-nano structure,which is an effective forming technology for micro-nano multistage structure of composite materials.Cellulose nano crystals（CNC）are widely used as polymer reinforcement materials and hydrophilic amendments due to their high Young’s modulus（～130 GPa）,excellent tensile properties（～7 GPa）,abundant hydroxyl group and hydrophilic properties.Therefore,PVDF/CNC composites were prepared using CNC as reinforcement material and polyvinylidene fluoride（PVDF）as matrix by electrospinning technology.Using the humidity sensitivity of CNC,the phase separation between polymer and solvent was promoted,and the porous fiber membrane with the structure of nanospheres was prepared.In addition,inspired by the hierarchical structure of trees,organic branching salt was added to the spinning solution to improve the conductivity of the solution,so as to prepare the dendritic nanofiber membrane,which provides a theoretical basis for its application in the field of oily sewage.The main research contents and results are as follows:（1）PVDF/CNC nanocomposites were prepared by electrospinning with CNC as reinforcement and PVDF as polymer matrix.The effects of CNC addition on the micromorphology,mechanical properties and thermal properties of the composites were investigated.The results showed that with the increase of CNC content,the overall structure of the fiber changed from a single fiber to a cross-linked state between fibers,and the average diameter of the fiber increased to 1.946 nm.The introduction of CNC can effectively improve the mechanical properties of PVDF fiber membranes.When the content of CNC was 8wt%,the fiber membrane had the best tensile strength,Young’s modulus and elongation at break,which were 6.4±1.25 MPa,72.62±6.71 MPa and 43.96±3.69%,respectively.In addition,the glass transition temperature and maximum degradation temperature of PVDF/CNC nanocomposites were also studied to understand their thermal stability.In general,this study lays a theoretical foundation for the subsequent preparation of micro-nano structured fibers.（2）Based on the research of PVDF/CNC nanocomposites,the porous fiber membrane with nanosphere structure was prepared by using CNC as the water molecular inducer to promote the phase separation process by regulating the ambient humidity in the electrospinning process.The surface morphology,pore size analysis,mechanical strength and adsorption properties of the porous fiber membrane were systematically analyzed.By adjusting the relative humidity,a large number of nano-micro spheres were generated on the surface of PVDF/CNC fibers,and porous structure formed inside the fibers were interconnected.Benefiting from this unique structure,the specific surface area of the fibrous membrane was increased up to 12.22 m² g^-1 and the adsorption capacity for engine oil reaches 73.04 g/g.The adsorption capability of the membrane to toluene under high humidity retained over 95%after ten cycles.The one-step electrospinning manufacture of mechanically strong and highly porous PVDF/CNC fibrous membranes provides a new approach to develop high-performance membrane materials for adsorption and separation of oil-bearing wastewater.（3）On this basis,in order to solve the problem of too large aperture caused by irregular fiber stacking and large diameter in the previous study.By adding organic branched salt（TBAC）into the spinning solution to improve the conductivity of the solution,the dendritic nanofiber membrane was prepared.The surface morphology,pore size,mechanical strength and oil-water separation performance of dendritic fiber membrane were analyzed systematically.The results showed that when the content of TBAC was 0.02 mol/L,the fibers showed a tree-like network structure.The diameter of the main fiber was 1.168±0.25μm,and the diameter of the branching fiber was 53.3±14.5 nm.This structure gived the composite the best porosity（85.02%）,average pore size（6.06 nm）and pore volume(0.002 cm³ g^-1).In addition,when the TBAC content was0.02 mol/L,the membrane showed rapid selective adsorption of various oils and organic solvents,especially for engine oil,the adsorption capacity was 64.11 g/g.Meanwhile,as an oil-water separation material,the flux of the surfactant stabilized toluene water emulsion was as high as4529.3 L/（m²h）.1-18μm droplets were not observed on the filtered optical microscope image.This composite fiber membrane has great potential for wastewater purification.