Study On The Structure Regulation Of Anisotropic Nanocellulose Aerogels And Their Mechanisms Of Performance Influences

As the third-generation biomass aerogels after inorganic aerogels and polymer aerogels,nanocellulose aerogels combine the excellent natures of traditional porous aerogels(low density and high porosity)and nanocellulose itself(easy to modify and good biocompatibility),which have been widely developed in many fields(e.g.,adsorption,energy,and biomedicine).However,the current nanocellulose aerogels are difficult to transmit directionally to electricity,heat and magnetism because of their unitary function,and disorder and uncontrollability in structures.Besides,their complex transmission paths also restrict the application scope of materials.Inspired by natural woods,this paper intended to construct multifunctional nanocellulose aerogels with anisotropic structures by directional freeze-drying.First,through the reasonable control for the directional freezing process,the microstructure of aerogels could be regulated according to requirements.On this foundation,different functional nanomaterials were introduced to fabricate nanocellulose composite aerogels.And the anisotropic distributed percolation behavior and electromagnetic interference(EMI)shielding mechanism of nanofillers inside aerogels were explored to endow composite aerogels with special electrical,thermal,and magnetic diversified functional applications.Further,an insight for preparing an ultra-long and oriented aerogel was proposed based on the growth law of ice crystals.And the efficient adsorption and fast transport ability for organic dyes of composite aerogels were studied by loading adsorbents,and the adsorption mechanism was also revealed.The main contents and research results are shown as follows:(1)A cellulose nanofibril(CNF)aerogel with anisotropic structures was constructed by using adjustable low-temperature ethanol(-30?)and 2,2,6,6-tetramethylpiperidin-1-oxyl(TEMPO)-oxidized CNFs as the directional cold source and raw materials,respectively.It had dense honeycomb-like porous structure in the transverse direction and well-organized oriented pore channels due to the directional induced growth of ice crystals in the longitudinal direction,showing structural anisotropy.This aerogel possessed ultra-low density(?7 mg/cm³),high porosity(>99.5%)and water adsorption capacity(>120 g/g).It also exhibited a bearing capacity of 4000 times greater than its own weight in the longitudinal direction and a fast directional solvent transport capability due to the introduction of directional structures,which were much better than that in the transverse direction or other types of aerogels(conventional refrigerator freezing,directional or non-directional freezing using liquid nitrogen),achieving anisotropic performances.(2)In order to enrich the properties and realize multifunctional applications of CNF aerogels,anisotropic CNF/silver nanowire(Ag NW)composite aerogels were obtained by directional freeze-drying using high conductivity and aspect ratio(>200)of Ag NWs as functional fillers.Since Ag NWs had better contact and connectivity with each other in the longitudinal direction,the composite aerogel obtained low percolation threshold(0.1 vol.%),high electrical conductivity(1.52 S/cm)and thermal diffusivity(1.39 mm²/s),and excellent compressive properties(24.5 k Pa)in this direction,better than that in the transverse direction,achieving performance anisotropy.Based on this feature,it also showed potential applications in many fields.(3)To further improve the EMI shielding performances of the CNF aerogel,magnetic Fe₃O₄nanoparticles were first grown on Ag NWs by a chemical coprecipitation method to fabricate Ag NW@Fe₃O₄ composite nanomaterials successfully.After introducing them into the CNF system,the CNF/Ag NW@Fe₃O₄ composite aerogels with anisotropic porous structures were constructed by directional freeze-drying.The composite aerogel exhibited low density(16.76mg/cm³),good saturation magnetization(4.21 emu/g)and electrical conductivity(0.02 S/cm),as well as anisotropic EMI shielding performances.This was due to the introduction of the directional structure and the composite aerogel dominated by absorption loss,which could effectively absorb and dissipate EM waves through the combined effects of dielectric loss and magnetic loss.And the addition of magnetic Fe₃O₄ was also able to reduce the proportion of reflection loss in the total shielding effectiveness(7.2%),effectively alleviating the secondary EM pollution.(4)Based on the growth law of ice crystals and the existing directional freezing technology,a simple and efficient method for preparing ultra-long uniform and oriented CNF aerogels was proposed to solve the problem of forming an uneven pore structure with a small bottom and a large top when using the existing method to construct long-range oriented aerogels.The long-range uniform oriented growth of ice crystals could be successfully controlled by applying an external speed of 0.04 mm/s opposite to the growth direction of ice crystals to balance the driving force of the ice crystal growth,realizing the transition from short-range to long-range orientation.Besides,this long-range oriented structure also endowed the aerogel with a fast directional long-distance solvent transport capability,and the ethanol could be transported up to40 mm within 50 s.(5)Rectorites with good cation exchange properties were introduced in the long-range ordered CNF aerogels.And the adsorption capacity and removal rates of methylene blue of the obtained composite aerogel reached 120 mg/g and 96.1%at the adsorption equilibrium,respectively,which were superior to those of pure CNF aerogels and rectorite powders.This was owing to the strong electrostatic interaction between CNFs and dyes,and the good cation exchange adsorption capacity of rectorites in the three-dimensional highly porous composite aerogel.The adsorption process of methylene blue by the composite aerogel fitted the pseudo-second-order kinetic model and the Langmuir adsorption isotherm model,which was influenced by both boundary layer diffusion and intraparticle diffusion.And the monolayer adsorption occurred on the uniform adsorbent surface.Through the study of adsorption thermodynamics,it was found that it was a spontaneous exothermic process.Furthermore,the composite aerogel also exhibited the abilities of highly efficient long-range adsorption and selective adsorption of dyes,which could realize the real-time purification of dyes.