Construction Of Flexible And Elastic Zirconic Fibrous Materials And Their Applications In Eliminating Chemical Warfare Agents

In recent years,due to the multi-polarization of the world,international armed conflicts,terrorist acts and other factors,military wars and public security incidents caused by the use of chemical warfare agents(CWAs)have occurred from time to time,posing serious threats to human life,ecological environment and social development.In order to better cope with the damage to the human body caused by chemical warfare,terrorist attacks,CWA leakage and other emergencies,countries around the world have successively developed a variety of chemical protective clothing,gas masks and other personal protective equipment after nearly a century of development.The core materials can be divided into physical adsorption modified activated carbon and physical barrier butyl rubber according to the mode of action.However,neither of the above two materials can destroy the molecular structure of chemical poisons,and it is easy to cause secondary pollution and induce secondary toxicity.Therefore,the development of high-performance self-detoxifying chemical protective materials is of great significance from the fundamental and strategic aspects.At present,a series of nanomaterials with unique structures(such as metal(hydro)oxides,metal-organic frameworks,polyoxometalates,etc.)have been reported for the catalytic decomposition of CWAs and their analogs.Among them,Zr-based nanomaterials rich in coordinatively unsaturated metal ions and surface hydroxyl groups showed better catalytic activity and shorter half-life.However,Zr-based materials still face many practical bottlenecks before they are widely used.First,these nanomaterials mainly exist in the form of powder or particles,which are not ideal for manufacturing chemical protective products,such as gas filters,clothing and boot covers.At the same time,the easy agglomeration of particles will reduce the accessible catalytic sites,which in turn will bring down the degradation activity of the materials.Secondly,these fine particles are difficult to recycle,and a large amount of dust will be generated during their processing or transportation,causing potential health risks and environmental pollution.In order to solve the above problems,researchers have loaded or fixed Zr-based nanoparticles on fiber substrates by dipping,coating,blending,atomic layer deposition and other methods to satisfy the practical requirement.However,these fiber-based composites usually have relatively weak interfacial bonding forces so that the particles tend to fall off easily,resulting in the poor durability.In addition,nano-scale Zr-based particles will inevitably aggregate or be embedded in the fibers during the preparation process,making some active sites not available for effective contacts toward toxins.Therefore,the catalytic degradation efficiency of the material can be seriously affected.On the other hand,personal protective equipment also has certain necessary requirements for the mechanical properties of materials such as bending resistance and compression resistance.Therefore,how to construct Zr-based fibrous materials with both excellent mechanical properties and high catalytic degradation performance to promote their practical applications in the fields of public security and military chemistry is an urgent problem to be solved.In this dissertation,a series of researches have been carried out on the controllable preparation of flexible and elastic Zr-based nanofibrous materials and their catalytic degradation performance toward CWA simulants.Two-dimensional flexible Zr(OH)₄ nanosheet-assembled nanofibrous membranes and three-dimensional elastic Zr(OH)₄-based fibrous aerogels were designed and constructed based on the robust surface-confined strategy.Our methodology allows for the vertically aligned amorphous Zr(OH)₄ nanosheets to be directly grown in situ on surface of nanofibers,thus significantly improving the specific surface area and catalytic activity of the materials.The hydrolysis reaction mechanism between Zr(OH)₄ and dimethyl methylphosphonate(DMMP)molecules was fully explored.In addition,flexible Zr-doped TiO₂ nanofibers were prepared based on the hetero-ion reinforcement strategy,and three-dimensional elastic Zr-doped TiO₂ nanofibrous composite aerogels were obtained by self-assembly with graphene oxide nanosheets via unidirectional freeze-drying method.The effects of the introduction of carbonaceous phase on the photothermal performance for the binary aerogels were investigated,and the plausible photo-initiated decomposition mechanism of the composite aerogels toward DMMP was analyzed.The main research results are summarized as follows:(1)Highly flexible and freestanding Zr(OH)₄ nanosheet-assembled nanofibrous membranes with core-shell structure were prepared by electrospinning technology and in situ co-precipitation method,realizing ultrathin and highly tortuous Zr(OH)₄ nanosheets vertically directed growth on nanofiber surfaces.The in situ constructed Zr(OH)₄ nanosheets(about 5 nm in thickness)were uniformly distributed on the surface of polyvinyl butyral(PVB)nanofibers,increasing the specific surface area by an order of magnitude(from 0.7 m~2 g^-1 to 99.7 m~2 g^-1).The half-life of the fibrous membranes for decomposing the nerve agent simulant DMMP was 4 min.DFT simulation and calculation results showed that the addition-elimination reaction between Zr(OH)₄ and DMMP was more likely to occur kinetically and thermodynamically.Benefiting from the interconnected pore structure and high porosity of the electrospun nanofibrous membranes,the as-prepared Zr(OH)₄nanosheet-assembled nanofibrous membranes exhibited high water vapor transmission rate(10.05kg m^-2 day^-1)and good air permeability(50.19 mm s^-1).At the same time,the composite fibrous membranes also showed excellent mechanical flexibility and robust fatigue resistance,displaying invisible plastic deformation after one million cycle tests at a buckling strain of 50%.In addition,the intriguing membrane-type catalysts demonstrated high-efficiency catalytic activity and robust durability against CWAs in vapor or liquid forms.(2)Choosing electrospun flexible zirconium n-butoxide/PVB nanofibers and flexible SiO₂nanofibers as the basic building blocks of aerogels,and selecting fully hydrolyzed methyltrimethoxysilane sol as a bonding and crosslinking agent,and the freeze-forming method was used to construct the fibrous aerogels.The elastic Zr(OH)₄-based fibrous aerogels with parallel-arrayed conduits were prepared by the post-treatment process of ammonia fumigation.This three-dimensional space-confined synthetic method realized the in situ construction of ultrathin and wrinkled Zr(OH)₄ nanosheets on the surface of aerogel fiber skeleton,which could significantly improve the specific surface area and catalytic activity.The fundamental formation mechanism of generating the inerratic honeycomb-networks inside the aerogel was revealed,and it was found that the ordered fibrous networks with strong interfacial bonding could endow the aerogels with prominent shape-memory property and robust fatigue resistance,even under the compressive strain of 90%.The aerogels were able to sustain a large stress of ten thousand times higher than their own weight and showed only 1%plastic deformation after 10~6 cyclic loading-unloading tests at 40%compressive strain.The Zr(OH)₄-based fibrous aerogels had only a half-life of 3 min for DMMP hydrolysis.After five cycles of use,the aerogels could still maintain the original porous cell structure,showing excellent reusability and service durability.In addition,the porosity of the aerogels could reach up to 99.987%,and the three-dimensional interconnected firous networks endowed the materials with excellent air permeability(>54.6 L m^-2 s^-1),which held enormous potential to be used in personal protective gears and military equipment,such as chemical protective suits,gloves,boots,and military tents.(3)The flexible Zr-doped TiO₂ nanofibers were prepared based on the hetero-ion enhancement strategy,together with graphene oxide nanosheets were used as the building blocks for contructing nanofibrous aerogels.The elastic Zr-doped TiO₂ composite aerogels were synthesized by the unidirectional freeze-shaping method.The binary aerogels exhibited the hierarchically entangled fibrous frameworks,which were enhanced by the van der Waals interaction between graphene flakes and Zr-doped TiO₂ nanofibers,as well as the strongly coupled?-?conjugation among reduced graphene oxide(RGO)nanosheets.The highly porous aerogels featured integrated properties of ultralow density,superior elasticity,and outstanding fatigue resistance.The maximum stress under80%compressive deformation could reach to 14.07 k Pa,and only 4%plastic deformation could be observed after 10~6 compression cycles.Subsequently,the microstructural variations and local stress distributions of the aerogels under compression were comprehensively analyzed by virtue of in situ FIB-SEM observations combined with the finite element simulation method,revealing the buckling behaviors of nanofibers and cell walls.In addition,the introduction of RGO nanosheets could not only increase the specific surface area of the aerogels(from 57 m~2 g^-1 to 134 m~2 g^-1),but also improve the photon absorption rate of Zr-doped TiO₂ and enhance the absorbancy in visible light and near infrared regions.Compared with the room light(cool white LED)irradiation,the aerogels showed a faster degradation rate(0.115 min^-1)and a shorter half-life(6 min)toward DMMP under the irradiation of simulated solar light.The DMMP catalytic degradation efficiency of Zr-doped TiO₂ composite aerogels could retain 92.2%after 5 on/off cycles(under simulated solar light),demonstrating their excellent multi-cycling reusability and photostability.