Controlled Splitting Of Aramid Fibers And Mechanical Properties Of Their Nanopaper-Based Materials

Aramid paper is a two-dimensional sheet material made of aramid fiber（PPTA）as raw material with wet forming and hot pressing,which is widely used in aerospace,rail transit,and other fields because of its high strength,high modulus and high temperature resistance.Macro aramid fibers can be gradually split into nanofibers by strong base（KOH）deprotonation treatment in dimethyl sulfoxide（DMSO）solvent.It is reported that paper-based materials formed by the assembly of aramid nanofibers（ANFs）have the higher mechanical strength and insulation properties than those of macro aramid paper.However,due to the characteristics of ANFs with high orientation,high crystallinity and high modulus,the toughness of ANFs paper-based materials is low,and the improvement of toughness often accompanied by the decrease of strength.Starting from the controlled split of aramid fibers,ANFs were prepared by dissolving flexible polymers and inorganic salts in DMSO or deionized water and then splitting macro-aramid fibers,finally assembled into two-dimensional paper-based materials.Then,the effects of polymer types and properties,and inorganic salt types and contents on the microstructure and mechanical properties of ANFs paper-based materials were studied,and the strengthening and toughening mechanism of ANFs was revealed.The main research contents are as follows:Firstly,the ANFs were obtained by splitting the PPTA through a deprotonation process,while the silk fibroin（SF）was obtained by degumming,dissolving and dialyzing the silkworm cocoon,which was then directly introduced into the ANFs suspensions to form composite films.The effect of the SF on the micromorphology,chemical structure,mechanical property,thermal stability,wettability,and insulation property of the SF/ANFs composites were thoroughly investigated.When the amount of SF is 0.6 wt.%,the tensile strength,toughness,and elongation at break of the SF/ANFs film are 257.4 MPa,37.6 MJ·m^-3,and 23.6%,respectively,which are 59%,272%,and 148%higher than those of pure ANFs film.When the SF content is up to 1.0 wt.%,the dielectric breakdown strength is 61.9 kV·mm-1,which is 153%higher than ANFs film.Subsequently,polyacrylic acid（PAA）,polyacrylamide（PAM）,thermoplastic polyurethane（TPU）,and polyvinylpyrrolidone（PVP）were selected as additives to improve the mechanical strength and toughness of the ANFs film.The results show that all these polymers can improve the strength,elongation at break,and toughness of the ANFs film.Compared with pure ANF film,the TPU/ANFs composite film shows the highest strength of 277.9 MPa,an increase of 64%,and the ANFs/PAA exhibits the highest elongation of 29.7%and the highest toughness of 48.3 MJ m^-3,increased by 191%and 335%,respectively.The strengthening and toughening mechanisms can be attributed to the structural densification,hydrogen bonding,and dipole-dipole interactions.The TPU and PAM that contain amide structures can form ordered networks of hydrogen bonds and show strong strengthening capacities,and the PAA and PVP show robust enhancement toughening capacities.Meanwhile,taking polystyrene（PS）as additive,PS/ANFs films were prepared and characterized.We prove that the nonpolar polymer shows weak enhancing ability on the mechanical strength of ANFs film due to the poor interfacial compatibility and severe phase separation.Finally,we used different kinds of metal ions(Ca²⁺,Mg²⁺,and Al³⁺)to insitu assist the splitting of PPTA to prepare their nanofibers,which successfully growth the CaCO₃,Mg（OH）₂,and Al（OH）₃ nanoparticles（NPs）on the surface of ANFs under the alkaline environment.The nanopapers assembled from the inorganic NP modified ANFs show higher mechanical and insulation properties than the pure ANFs film,as well as the thermal stability and water flux.The Mg²⁺/ANFs-10 shows the increase of 66%,150%,334%,and 28.3%on stress,strain at break,toughness,and dielectric breakdown strength,respectively.The enhanced mechanical and insulation property is attributed to the structural densification and improved interfacial interactions,while the increased water flux is caused by the hydrophilicity of the inorganic NPs.After heat treatment at 400℃for 2 h,the dielectric breakdown strength is further improved due to the dense microstructures,but the stress and elongation are decreased.