Controllable Fabrication Of Ultrafine Twodimensional Nanonets And Their Application In High-efficiency Air Filtration

Haze pollution in our country has become more and more serious with the development of urbanization and industrialization in recent years.Only <1% of 500 largest cities of China could meet the air quality standards suggested by the World Health Organization(WHO).In terms of particulate matter(PM)pollution,PM2.5(aerodynamic diameter ?2.5 ?m)is particularly harmful because it can travel long distances,penetrate small airways and carry abundant bacteria/viruses.In 2010,the PM pollution has become the world's seventh largest cause of death,leading to the death of more than 3 million people,and the WHO for the first time classified it as a primary carcinogen in 2013.Therefore,creating high efficient filter materials that can reduce the harmness of pollutants is a major issue related to people's livelihood.Conventional filter materials such as glass fibers and melt-blown fibers are incapable of capturing fine particles due to their micro-sized fibers and large pore sizes.Nanofiber based air filters have gained intensive attention due to their dramatically decreased diameters.However there are still some drawbacks,such as inadequate removal efficiency,especially for PM0.3 removal,due to their relatively thick fibers(diameter of >100 nm).The nanonets,as two-dimensional(2D)network nanomaterials,exhibit a more remarkable slip-effect for air flow and physical intercept capability for PM particles originating from the nanoscale diameter(~20 nm)and small pore size.Therefore,the novel 2D nanonets have the potential to be an ideal filter materisals.In this thesis,we pointed out that the formation mechanism and structure-performance relationship of 2D nanonets were two critical factors for air filtration application.Herein,we reported novel numerical models for clarifying the origin,evolution,and regulation of the nanonets via combining the ejection modes of Taylor cone apex and the phase separation of charged droplets.Researching the structure-function relationship of the 2D nanonets matreials to fabracate filters with high PM removal efficiencies and low pressure drops.The detailed contents are summarized below:(1)We developed numerical models that are capable of linking the solution properties and operating parameters to the forms and dynamics of the fluids issued from a Taylor cone.Our models propose two critical condition formulas for the generation of jets and droplets(jets threshhold:(?)droplets threshhold:(?),and a lower critical solution temperature phase diagram combined with concentration sweep paths for the nanonets or non-porous films.The resulting model predications exhibit excellent agreement with the experimental results obtained via varying the solution properties and process parameters.(2)We first established a strategy to design and fabricate tailoring mechanically robust poly(mphenylene isophthalamide)(PMIA)nanofiber/nets(PMIA NF/N)for ultrathin high-efficiency air filter via electrospinning/netting.PMIA membranes with high nanonet coverage rate could be prepared by varying solutions concentration,relative humidity and dodecyl trimethyl ammonium bromide concentration.By regulating the scaffold structures,the coverage rate of nanonets and the basis weight of the membrance,PMIA NF/N filter with the integrated properties of superlight weight(0.365 g m-2),ultrathin thickness(~0.5 ?m),and high tensile strength(72.8 MPa)could be prepared.It was demonstrated that our PMIA NF/N filter could achieve the grade of high efficiency particulate air filter with high filtration efficiency(>99.97%)and low pressure drop of 92 Pa for 300~500 nm particles by sieving mechanism.(3)We demonstrated a facile and scalable strategy to fabricate the novel air filters consisting of nanofiber/nets and three-dimensional cavity structure via embedding PMIA staple fibers or PET filament.Morphology,porous structure,bulk density and filtration performance could be regulated via adjusting PA-6 concentrations,PMIA staple fiber concentrations,diameters of polyethylene terephthalate(PET)filaments and framework gaps of PET filaments.We found that the capacity of particle capture and air penetration could be mainly improved by the structural characters of the membrane filters.For example our PA-6/PMIA nanofiber/net filter could filtrate the ultrafine particles with high removal efficiency of 99.995% and low air resistance of 101 Pa.(4)A novel highly integrated polysulfone/polyacrylonitrile/polyamide?6(PSU/PAN/PA-6)air filter for multilevel physical sieving airborne particles was successfully fabricated for the first time via sequential electrospinning.The PSU microfiber layer,PAN nanofiber layer,and PA-6 nanofiber/nets layer were successively constructed by employing different polymer systems and optimizing their respective spinning durations.The resultant PSU/PAN/PA-6 integrated filter exhibits gradually varied pore structures with size ranging from ?2.2 and ?0.6 to ?0.27 ?m and can cut off penetration of particles with certain sizes that exceed the designed threshold level.Benefiting from its elaborate gradient structure,our highly integrated filter even can filtrate the 300 nm Na Cl particles with a high removal efficiency of 99.992%,a low pressure drop of 118 Pa.(5)We introduced the equilibrium factor(?r)for the first time to perfect the ejection modes and obtained its required critical charge state(jets threshold:(?),droplets threshold:(?).And then we introduced a simple strategy to capture the dynamics of the phase separation of PMIA/PU/DMAc solutions during solvent evaporation by monitoring their light transmittance variation.Next,we designed and processed an efficient and scalable methodology for creating high-efficiency and transparent air filters consisting of 2D self-assembled nanonets filters,which exhibit the integrated properties of a superlight base weight(0.12 g m-2),an extremely low thickness(~350 nm)and free-standing strength,and they can achieve 99.984% removal efficiency and <68 Pa air resistance for PM0.3 filtration at ~85.6% transmittance.We further performed in situ SEM study to study the PM dynamic capture process of the filters.It was demonstrated the filtration mechanisms of their multilevel hierarchy ranging from integrated filters and nanonets to nanowires.To provide insight into the slip-effect of airflow within the nanonets,we introduced for the first(?)time,a correction factor ? to establish a novel filtration model for the filters Furthermore,we also carried out field tests on days with PM2.5 concentrations of 123–150 ?g m-3 in Shanghai,China.After 120 h,the ULTRA NET air filter still maintained a real PM2.5 purification efficiency of >99.7%,and its pressure drop increased slightly from 25 to 28 Pa,indicating their robust removal efficacy and long lifetime for haze governance.