Preparation And Properties Of Aerogel Fibers Based On Nano-aramid

As a new type of solid material with high-porosity,aerogel is mostly composed of air and has a unique three-dimensional network porous structure with physical characteristics of ultra-low density(0.003-0.3 g/cm³),high specific surface area(100-1600 m²/g)and high porosity(the porosity can be as high as 99.8%or more).It has obvious advantages in the fields of heat insulation protection,electromagnetic shielding,sound absorption,shock absorption and catalyst carrier.The early developed inorganic aerogels based on silica(Si O₂)and other metal oxide aerogels usually have the disadvantage of poor toughness,which limit their applications in many fields.Therefore,so more and more researchers focused on nano-aramid aerogels.Aramid fibers in nanoscale(or named as nano-aramid or aramid nanofibers)are expected to be the constitutional unit or filler materials in nano-size due to their dual advantages of high-performance aramid fibers and nanofibers.As nano building blocks,the assemble of macro aerogel materials based on nano-aramid could further realize the functional and diversified applications of high-performance fibers.At present,most of the studies on nano-aramid aerogels mainly focus on materials such as aerogel blocks,membranes and microspheres.As an important one-dimensional material,fiber materials play an extremely important role both in nature and in life.Its aspect ratio of more than one thousand times makes it show good flexibility.If the nano-aramid could be assembled into aerogel fibers,it is expected to be weaved and owe flexible and wearable characteristics,thus giving it a broader application prospect compared to aerogel materials such as blocks and films.In this paper,the proton-donor-assisted deprotonation method was used to realize the efficient,rapid and high-concentration preparation of aramid nanofibers(ANFs),and then wet spinning was used to prepare ANFs aerogel fibers.At the same time,in the spinning process,the influence of the solid content of the original liquid,the temperature of the coagulation bath,and the content of tert-butanol in the solvent exchange bath on the morphologies and properties of aerogel fibers were studied.Through functional modification of the prepared ANFs aerogel fibers,carbon nanotubes/nano-aramid were fabricated and showed good thermal insulation and electromagnetic shielding performance.The main research contents and results are as follows:(1)Nano-aramid were prepared by deprotonation method and proton donor-assisted deprotonation method,respectively.Their morphologies,structures and properties of the products obtained were studied.The average diameter of aramid nanofibers obtained by deprotonation method is about 20-30 nm,and the length can reach a few microns.The nanofiber presents a slender linear structure with uniform distribution.While the aramid nanofibers obtained by the proton donor-assisted deprotonation method are flat ribbon-like with a diameter of 10-20 nm.The fibers are entangled with each other.Relatively speaking,the aramid nanofibers prepared by the latter method have more the branches with stable characteristics,and the distribution is relatively uniform without obvious agglomeration.The aramid nanofibers prepared by the two methods are still consistent with aramid in chemical structure with roughly the same molecular skeleton and a certain degree of crystallinity.(2)The high-concentration ANFs/dimethyl sulfoxide mixture was used as the spinning stock solution and the assembly of nano-aramid gel fibers was realized by the wet spinning process with the H₂O/Ca²⁺as the coagulation bath.In the spinning dope,the divalent metal ions Ca²⁺in the coagulation bath of wet spinning can form metal coordination crosslinks with the rich oxygen-containing functional groups on the surface of the aramid nanofibers,such as-COOH,-CO-NH-,and promote the assemble process of aramid gel fibers with enhanced mechanical properties.When the temperature of the coagulation bath is 90 ^oC and the solid content of the spinning dope is 5%in the spinning process,the obtained aerogel fiber displays the best mechanical properties and the highest specific surface area.In the study,the microstructures of the obtained aerogel fiber were adjusted by adjusting the composition and ratio of the solvent replacement liquid.The results show that when the volume ratio of tert-butanol to water in the solvent replacement bath is 1:1,ANFs aerogel fibers have good formability.SEM test results show that the nano-aramid fibers inside are loosely arranged and present a fluffy network structure.At the same time,its specific surface area can reach 165.4 m²/g,and its tensile rupture strength can reach 4.2 MPa.With the increase of water content in the solvent replacement bath,the specific surface area of aerogel fibers gradually decreases and the mechanical strength gradually increases;When H₂O is a solvent replacement fluid,the nano-aramid fibers are neatly aligned within the formed structure,and its breaking strength can reach 328.7 MPa,but with greatly reduced specific surface area and porosity.(3)In order to further expand the application of nano-aramid aerogel fibers,ANFs aerogel fibers were hybridized with carboxylated carbon nanotubes(CNT-COOH),and a series of composite aerogel fibers with different CNT-COOH loads were prepared.It was found that CNT-COOH and ANFs are connected by?-?stacking and hydrogen bonds.When the addition amount of CNT-COOH is too high,it is not conducive to its dispersion,resulting in reduced mechanical properties.The highly porosity of ANFs/CNTs aerogel fabrics endow their good thermal insulation performance,and the multi-layer composite aerogel fiber fabric shows better thermal insulation performance at high temperature.When the of loading the CNT-COOH is 60%,the maximum value of electromagnetic shielding for the ANFs/CNTs-COOH composite aerogel fiber can reach 36.6 d B,which is higher than the requirements of commercial standards for practical applications.