Preparation And Properties Of Heterocyclic Aramid Aerogel Fibers Cross-linked By Metal Coordination Bonds

Aerogel materials have received extensive attention in thermal insulation due to their high specific surface area,high porosity and low density,and are considered to be one of the most excellent thermal insulation materials.However,traditional 3D aerogel materials have limited applications due to their block shapes.One-dimensional fibrous aerogel materials have better morphological designability,but the preparation process involves the sol-gel transition of the spinning solution.If the transition is too fast,the spinnability will be affected,on the contrary,if the transition is too slow,during the exchange process,phase separation will occur,so that a uniform gel cannot be formed,and the production efficiency is low,making it difficult to achieve continuous fiber production.In addition,the traditional sol-gel transition is an irreversible process,once a gel is formed,it is difficult to revert to a sol state,which makes the recycling and reuse of aerogel materials very difficult.Therefore,it is of great significance to develop a class of systems with reversible sol-gel transition properties and to continuously prepare uniformly structured aerogel fibers on this basis.In this paper,a new type of reversible crosslinking system was constructed using heterocyclic aramid fiber resin as raw material through metal coordination bond crosslinking.Metal coordination crosslinking aerogels of heterocyclic aramid fiber were prepared by continuous wet spinning,and the reversible sol-gel transformation of the crosslinking system was realized.First,by adding metal ions into the heterocyclic aramid resin solution,the sol-gel transition is achieved by complexing the metal ions with the imidazole groups on the main chain of the heterocyclic aramid to form a physical cross-linked structure.Under the action of external conditions such as temperature and shear force,the gel can return to the sol state.The formation of physical cross-linked structures was demonstrated by infrared spectroscopy and transient fluorescence spectroscopy.The storage modulus,loss modulus and viscosity of the heterocyclic aramid resin solution before and after crosslinking under different solvent components,metal ion species,and the molar ratio of metal ions to imidazole groups were characterized by advanced rotational rheometer.The results show that when the volume ratio of NMP to DMAc in the solvent is 1:2,the molar ratio of Cu²⁺to imidazole groups is 1:2,and the mass fraction of resin is 0.5 wt%,the gel has a good cross-linking structure and exhibits good comprehensive performance.On this condition,the storage modulus increased from 1.2 Pa to 15.6 Pa,and the loss modulus increased from 0.36 Pa to 2.6 Pa.What's more,the viscosity increased from 1.6 Pa·s to 20.9 Pa·s at low shear rates,and the viscosity increased from 0.02 Pa·s to 0.3 Pa·s at high shear rates.Keeping other conditions unchanged,changing the resin mass fraction can change the sol-gel transition temperature of the cross-linked system.When the resin mass fraction in the Cu²⁺-heterocyclic aramid resin system is 0.5 wt%,0.75 wt%and 1 wt%,the sol-gel transition temperatures of the system is 45?,63?and 75?,respectively.Secondly,using the above metal ion-heterocyclic aramid resin solution as a spinning solution,the aerogel fiber is prepared by a continuous wet spinning method.The spinning solution is extruded through the spinneret into the coagulation bath in the sol state.When the temperature of the coagulation bath is higher than the sol-gel transition temperature,the sol-gel transition occurs to obtain gel fibers.After solvent exchange and freeze-drying,the final aerogel fibers with high specific surface area,high porosity and low density are obtained.The effects of process conditions such as negative draw ratio at the spinneret,spinning speed,the concentration of spinning solution and coagulation bath temperature on the structure and properties of aerogel fibers were investigated.The optimum spinning conditions were determined by nitrogen adsorption-desorption test,SEM,tensile test and thermal conductivity test,with negative tensile ratio of-50%and spinning speed of4 m/min at the spinneret.On this basis,the fiber has the best morphology structure and mechanical properties when the resin mass fraction is 1 wt%and the coagulation bath temperature is 80?.Meanwhile,the fiber has the best thermal insulation properties when the resin mass fraction is 0.5wt%and the coagulation bath temperature is 60?.