| Aramid nanofibers(ANFs)are kinds of building blocks with the advantages of aramid fibers as well as nano scale effects.ANFs have excellent strength,modulus,temperature resistance and rich active groups,which have great application potential in many materials fields,such as structural weight reducing materials,electrical insulation materials,battery separators,flexible electrode materials and so on.However,ANFs have not yet achieved industrial application.At present,the most widely used method for the preparation of ANFs is KOH/DMSO alkali dissolution method,which is simple and has excellent product properties.However,the preparation of ANFs by this method and building materials still faced the following major challenges:firstly,the conversion efficiency of aramid fiber into ANFs is low,which can not meet the needs of industrial production and large-scale application.In addition,ANFs often need to be precipitated from the homogeneous system through structural transformation,and then used as building elements.Moreover,there are few corresponding material construction strategies and control approaches,which are difficult to meet the needs of rich and diverse applications for complex structures.The root of the above problems lies in the unclear understanding of the micro interaction between ANFs and DMSO alkaline system,so it is difficult to effectively control the preparation of ANFs and the macro material construction process.The results showed that the intra-intermolecular hydrogen bonds and the polymer solvent hydrogen bonds play an important role in the formation and precipitation of ANFs.Therefore,clarifying the hydrogen bonding of ANFs in DMSO alkaline system and further realizing effective regulation have important theoretical significance for improving the preparation efficiency of nanofibers and enriching the means of material construction.In view of the above problems in the preparation and material construction of ANFs,this study focused on the material interaction and hydrogen bonding in the DMSO alkaline system of ANFs,and specifically carries out the following research work:(1)Study on the influence of key factors in the formation process of ANFs on the preparation efficiency of ANFs.Through in-situ tracking study on the conversion of aramid fiber to ANFs,a new theoretical hypothesis was put forward for the formation process and existing form of ANFs:the electronegativity of free OH" in DMSO system was greater than that of C=O in polymer,which promoted the conversion of some intra-intermolecular hydrogen bonds of aramid fiber into N-H and OH-intermolecular hydrogen bonds.Finally,the above process transformed macro fibers into ANFs.The formed ANFs were stably dispersed by hydrogen bond interaction with DMSO,rather than exist in the deprotonated state.Based on the above theory,the kinetic process was optimized to efficiently obtain free OH-,and the formation time of nanofibers can be as short as 10 min.This strategy is the most efficient ANFs preparation method at present,and it is universal for aramid products from different sources.The mechanical properties and breakdown properties of ANFs assembled membrane materials prepared by this method are better than those of other schemes.The temperature resistance and light transmission properties are not damaged.The tensile strength of ANFs membrane is 184 MPa,and the breakdown strength reached 101 kV·mm-1.The membrane still has 40%mass retention at 800℃.Based on the new theoretical hypothesis,it is beneficial to expand the understanding of micro interaction in ANFs/DMSO alkaline system.The efficient preparation method based on this theory is helpful to promote the industrial application of ANFs.(2)Study on self-organization behavior and its ordered structure of ANFs/DMSO alkaline system.The reaction-diffusion system was constructed by using ANFs/DMSO alkaline system and water.Under the action of H2O,the hydrogen bond between ANFs and solvent was transformed into the hydrogen bond between polymer and water and the intra-intermolecular hydrogen bond of polymer,and hydrogels were formed.Through the dynamic study of the advancing law of the gel liquid interface,it is proved that the system can form an ordered structure through self-organization,and the process conformed to the common law of the self-organization behavior of rigid polymer reaction-diffusion system.Through the characterization of the ordered gel structure,the regularity of its complex ordered structure was clarified:from the surface to the inside,the transition between layered structure,oriented structure and radial structure was presented.The anisotropy of mechanical properties of hydrogels prepared in 0.5%acid coagulation bath was 22.3,which was 14.6 times that of 0.5%alkaline coagulation bath.The study of control variables showed that the diffusion rate and the internal stress in the liquid phase are the key factors affecting the formation of ordered structures.The above research on the self-organization behavior of ANFs/DMSO alkaline system is helpful to understand the self-organization behavior of polymers.The self-organization behavior of ANFs/DMSO alkaline system can realize the construction of ANFs materials with ordered structure,which is helpful to expand the construction and control means of ANFs materials with ordered structure.(3)Study on the effect of the interaction between ANFs/DMSO basic system and metal cations on the ordered structure.Under the coordination between metal cations and ANFs/DMSO alkaline system,metal ions enhanced the interaction between ANFs in the process of hydrogen bond reconstruction,thus changing the ordered structure of ANFs gel.Through the characterization of the ordered gel structure formed,the regularity of the regulation of metal ions on the structure was clarified:the influence degree of gel structure was related to the strength of coordination effect.Cu2+ with strong coordination effect could directly affect the gel process of ANFs,and the mechanical properties of the induced hydrogels were greatly improved.The hydrogels prepared in 0.25 M Cu2+ coagulation bath were 2.6 times that of the blank group,which had a significant impact on the structure of ANFs gel.The structure of the hydrogel changed from oriented structure to layered structure.By changing the concentration of Cu2+,the thickness and spacing of the layered structure can be adjusted.Under the influence of Cu2+,the ordered gel is formed,and the inorganic substance containing the metal ion is also formed.The above results showed that the interaction between metal cations and ANFs/DMSO alkaline system can be used to regulate the microstructure of ANFs gel.The influence of the interaction between metal cations and ANFs on the structure of ANFs gel was revealed.This method also provided a new way to form ANFs based organic/inorganic composite gel with ordered structure.(4)Study on the formation of ANFs aerogel with ordered structure by DMSO crystallization-sublimation in ANFs/DMSO alkaline system.With the crystallization-sublimation of DMSO in the system,the hydrogen bond between ANFs and solvent was transformed into intra-intermolecular hydrogen bond of polymer,and aerogel was formed under the template of continuous phase crystallization.The comparative study showed that compared with the current ANFs homogeneous precipitation aqueous phase freeze-drying method,ANFs aerogel materials prepared by using the continuous phase crystallization template of ANFs/DMSO system have significant structural differences.By characterizing the ordered structure of the formed aerogel gel,the regularity of its structure construction is clarified:the ANFs aerogel obtained by DMSO crystallization sublimation has a typical anisotropic ordered structure,and the vertical direction of DMSO crystallization growth will form a multi-layer structure with pore structure between layers.Depending on the porous nature of the gel,the compression recovery performance of the aerogel gel can reach 90%.At the same time,it can also adsorb 27 times its own weight of organic solvents,which has excellent adsorption performance.The above studies showed that ANFs can directly construct anisotropic ordered aerogel gel materials from homogeneous system through hydrogen bond reconstruction and crystallization template.The structural regulation law enriched the design strategies and construction approaches of ANFs aerogel gel materials. |
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