Construction Of Optical Transparent Micro-nanofibrous Membranes And Their Application Performance

With the unique optical structure,optical transparent materials can allow light in the visible range to pass through them efficiently,thus achieving visual transparency,and they have developed into indispensable optical system materials.Traditional transparent materials,such as transparent glass,transparent plastic,transparent metal mesh and transparent ceramics,have high optical transparency and strong mechanical properties,and are widely used in daily life,industry,architecture,military,medical and other fields.Although the development time of traditional transparent materials is long and the preparation technology is relatively perfect,they also have some shortcomings,such as poor air permeability,large rigidity and low flexibility,complex preparation process,and single function.With the progress of the times and the rapid development of science and technology,emerging fields such as personal protection,electronic skin,intelligent wearable,and tissue engineering have put forward new requirements for transparent materials,which require high light transmittance,good breathability and flexibility,stable mechanical properties,and diverse functions.For example,transparent masks as a new type of personal protection tool,they should have excellent light transmission performance and particle interception performance at the same time;transparent electronic skin directly contacts with human skin,which requires not only high transparency and flexibility of the material,but also good air permeability;transparent medical dressing acts on the wound,good air permeability and drug release performance are conducive to wound healing,and excellent light transmission performance is convenient to observe the wound healing,and can avoid secondary wound injury.It is obviously difficult for traditional transparent materials to meet new requirements at the same time.Therefore,it is urgent to develop new transparent materials.Electrospinning fibrous membrane materials are the micro-nanofibrous aggregates prepared by electrospinning technology and stacked by continuous fiber filaments.Compared with traditional fibers,the electrospun fibrous membrane has a fine fiber diameter and can reach nanometer level.It has the advantages of large specific surface area,strong structural adjustability,high porosity,good flexibility,wide source of raw materials,and shows good breathability,absorption and functionality.Through the structural regulation of the fibrous membrane,the transparency of the electrospun micro-nanofibrous membrane material can be realized,which can meet the requirements of the new transparent materials in the above emerging fields at the same time.Hence,it is of great practical significance to develop electrospun micro-nanofibrous membranes with high porosity and transparency.However,due to the serious surface light reflection and internal light scattering of the electrospun fibrous membrane material,it is difficult to make it transparent.The existing preparation methods of transparent fibrous membranes include direct spinning method,impregnation and filling method,chemical deposition method and surface modification method,etc.Although the prepared fibrous membrane materials have high light transmittance,there are still many shortcomings,such as the blockage of the pores between the fibers,resulting in the impermeability of the fibrous membranes,complex post-treatment process and poor independent use performance.In addition,the research on the transparency of electrospun micro-nanofibrous membrane materials is also in the initial exploration stage.The existing research has not made a thorough exploration of the mechanism of the transparency of the fibrous membrane materials,and the mechanism of the visible light propagation in the pores between the fibers is not clear.At the same time,there is also a lack of theoretical guidance for the transparency of micro-nanofibrous membrane materials.This paper summarizes the types of optical transparent materials,the main factors affecting the transparency of electrospun fibrous membrane materials,and the research status of optical transparent electrospun fibrous membrane materials at home and abroad.Aiming at the problems of existing materials,this paper achieved a significant increase in the light transmittance of the electrospun micro-nanofibrous membrane materials by reducing the difference in the refractive index between the fiber and the air and optimizing the structural design to build efficient optical paths,which enriched the preparation methods and control means of transparent electrospun micro-nanofibrous membranes.The relationship between the bulk structure of polymer raw materials and the degree of transparency was deeply studied,and the influence of the bulk characteristic parameters of the fibrous membrane and the transmittance was investigated,the controllable preparation of transparent porous electrospun fibrous membranes was realized.The self-assembly model of electrospun nanofibrous topological structure was constructed,the forming mechanism of nanofibrous topological structure was studied,the mechanism of visible light propagation in the pores between fibers was revealed,and transparent micro-nanofibrous membrane materials with high transparency and large porosity were prepared.The main research results are as follows:（1）Taking several commonly used macromolecular polymers as the research objects,using electrospinning technology and mechanical pressing technology,the porous,flexible and transparent micro-nanofibrous membrane materials were directly prepared without any other additives.The relationship between the structural characteristics of polymer molecular chain and the transparency of polymer fibrous membranes after pressing was summarized for the first time,and the influence mechanism of the flexibility of molecular chain on the transparency of fibrous membranes was proposed.Taking polystyrene（PS）electrospun fibrous membranes as the research objects,the prepared transparent PS fibrous membranes have excellent optical and mechanical properties.The transparent fibrous membranes can achieve high transmittance（≈89%,visible wavelength at 550 nm）,large porosity（10-30%）and strong mechanical tensile strength（≈148 MPa）,which is about 78 times of the original electrospun fibrous membranes.In addition,transparent conductive fibrous membrane materials were also prepared by using vacuum filtration technology to load silver nanowires（Ag NWs）onto the surface of the fibrous membranes and then realizing transparency by mechanical pressing.Compared with indium tin oxide（ITO）conductive films,the prepared transparent conductive fibrous membranes show good conductivity（9Ω/sq,at 78%transmittance）and excellent mechanical properties（can withstand a lot of bending stress）.（2）Taking polyacrylonitrile（PAN）precursor solution as the research object,inspired by the structure of dragonfly wings,the nanofibrous topological structure with high light flux was designed,and the highly transparent and high porosity micro-nanofibrous membrane materials with dragonfly-wing-like structure was prepared by using electrospinning technology and differential electric field distribution.The obtained membranes show excellent optical properties（up to 90%transmittance）and high porosity（>80%）.The electric field simulation conducted by Comsol Multiphysics software and the establishment of the equivalent microscopic model of bionic nanofibrous topological structure reveal the forming mechanism of transparent electrospun fibrous membrane materials with dragonfly-wing-like structure.By comparing the light transmission properties of fibrous membranes with disordered and ordered biomimetic structures,the visible light enhancement mechanism of dragonfly-wing-like structure was proposed.At the same time,the transparent fibrous masks made of transparent fibrous membrane materials have high transparency（>80%）,low pressure resistance（<100Pa）and high PM_0.3 removal efficiency（>90%）.Especially when people wear the transparent masks,they can successfully pass the facial recognition system.（3）Taking the dragonfly-wing-like transparent PAN electrospun fibrous membranes as the research objects,the transparent carbon nanofibrous membranes with"frame-channel"structure holding high-efficiency optical paths were constructed by high-temperature carbonization process.Compared with carbon nanofibrous membranes with disordered fibrous arrangement structure,the light transmittance of the prepared transparent carbon nanofibrous membranes is increased by an order of magnitude,about 18 times of that of disordered structure.At the same time,the transparent carbon nanofibrous membranes also show high porosity（>90%）,excellent flexibility and good mechanical properties.This research work proposes the optical loss mechanism of carbon materials and disordered carbon fibrous membrane materials,and expounds the mechanism of"frame channel"structure for enhancing the transmittance and reducing the absorption of carbon nanofibrous membrane materials.Through in-situ observation of the tensile fracture process of the unit structure in transparent carbon nanofibrous membrane,the failure mechanism of the tensile fracture of the transparent carbon nanofibrous membrane was revealed.In addition,the prepared transparent carbon nanofiber membrane also has high PM_0.3 removal efficiency（>90%）and low air-drop（<100Pa）,and shows good electrical conductivity（resistivity<0.37Ω·cm）.