Plasma-modified PVDF Membrane And Its Membrane Flux Under Electric Field Study

The shortage of freshwater resources has become an important factor restricting sustainable economic and social development.As a new desalination technology,membrane distillation can effectively solve the problem of water shortage.Traditional PVDF（Polyvinylidene Fluoride,PVDF）membrane is prepared by adding inorganic nanofillers and using electrospinning technology,and this paper proposes an electric field-driven synergistic plasma modified PVDF membrane saline-alkali water desalination method for traditional membrane distillation such as low membrane flux and poor stability.In view of the poor permeability of traditional PVDF polymer membranes,firstly,by electrospinning process,the experimental parameters are effectively controlled,and silica is introduced as a modified material to prepare PVDF electrospun fiber membranes with good hydrophobicity and long-term operation stability.After testing,the porosity and surface static contact angle of the composite polymer hydrophobic film with a concentration of 15wt%,a spinning voltage of 16kV,a spinning time of 120min and a silica content of lwt%increased by 9.8%and 6.2%,respectively,and the average membrane flux could reach 0.223kg/（m²·h）,which had good modification potential.Secondly,in order to further improve the membrane flux,the plasma modification method was used to surface treat the PVDF membrane.The mechanism of fluorination modified PVDF was comprehensively explored by micro-mesoscopic and macroscopic multi-scale tests,and the influence of plasma modification on the hydrophobicity of PVDF membranes was further explored through FTIR,XPS,SEM,contact angle test and flux test experiments.It is found that fluorine element with-CF₃ group as the main side group is effectively grafted at the original PVDF side chain group,and plasma treatment effectively reduces the filament diameter size and promotes the effective permeation of water molecules and OH^-and other active groups.Through plasma modification of 13.5kPa and 7kV/10kHZ,the contact angle of the obtained plasmamodified PVDF membrane increased to 139.9°,and the membrane flux increased to 0.234kg/（m²·h）,which was increased by 5%compared with the unmodified PVDF film.At the same time,using first-principles calculations,the effects of fluorination modification on the charge probability distribution,electron density analysis,electrostatic potential analysis,and electron orbit occupation of PVDF molecular segments were comprehensively explored from the microscopic perspective.It is found that compared with the original fluorine atom in the PVDF segment,the charge density of the plasma grafted-CF₃ group is weakened and the electronegativity is reduced,which effectively inhibits the affinity of water molecules and OHand other active groups,which is conducive to the improvement of membrane flux.Finally,the membrane flux of PVDF hydrophobic membranes under different electrodes before and after plasma modification was experimentally studied.It is found that the improvement effect of negative polarity electric field is better than that of positive polarity electric field,and electric field distortion has a promoting effect on water evaporation.Under the single-needle negative electrode and multi-needle negative electrode,the plasma modified PVDF membrane flux increased by 44%and 35%,respectively.In this paper,the electrostatic field and plasmonic particle modification on the distillation process of electrospun fiber membrane are comprehensively explored by nano-doping method,and the influence of electric field parameters and plasma modification strategy on the water flux of composite nanofibers is explored through multi-scale research methods.The results of this paper can provide technical reference and new ideas for improving membrane flux in the process of membrane distillation.