High-efficiency Preparation Of Aramid Nanofiber And Its Application In Paper-based Insulating Materials

Aramid nanofiber(ANF),serving as one novel nanoscale polymer material,has become one of the promising building blocks due to its extraordinary performances in reinforcing polymer materials.It can be combined with polymer matrix through physical/chemical/self-assembly,playing an important role in strengthening the interface bonding and toughening the material.Therefore,it has drawn intensive attention and interest in high-performance nanocomposites,such as electrical insulation materials,heat-resistant filtration mediums,battery separators,flexible electrodes,and biological tissue.The current application of ANF in the field of advanced materials is based on the deprotonation method proposed by professor Kotov in 2011.Compared with other ANF preparation methods,the ANF prepared by deprotonation method possesses the best performance and therefore has the most extensive application.However,there are numerous challenges related to the preparation of ANF,such as the lengthy preparation cycle(7-10 days),low preparation concentration(0.2 wt %),and high difficulty in quantitatively judging the endpoint of the deprotonation reaction.Based on these issues that limits the development of ANF towards large-scale,functional and diversified applications in the future,the evolution rules of PPTA fiber micromorphology,structure and performance during the deprotonation process,and the critical points of reaction time and fiber scale between macro PPTA fiber and ANF were investigated,clarifying the fiber formation mechanism,structural reconstruction and performance maintenance mechanism.Then the fibrillation/ultrasound/proton donor coupling deprotonation are proposed to realize the controllable and efficient preparation and differential regulation of ANF.Then,ANF was used as the nano reinforcing material of cellulose nanofiber(CNF)based insulating material and PPTA paper based insulating material,which significantly improved the comprehensive performance of traditional paper based insulating material,and explored the strengthening mechanism of ANF in multi-scale interface based composite material.The main research work is as follows:Firstly,the evolution of micromorphology and chemical structure from macroscale PPTA fibers to nanoscale ANF were studied systematically.The deprotonation process does not change linearly with reaction time,but there is a critical point between PPTA fiber and ANF on reaction time and fiber scale.In the KOH/DMSO system,PPTA fiber has experienced "amide bond deprotonization--negative charge accumulation on molecular chain--electrostatic repulsion breaking hydrogen bond network--overall dissociation of fiber layer and core--fiber splitting along axis/radial--ANF" During the deprotonation process,ANF retains the crystal structure and chemical structure of PPTA,thus maintaining the excellent properties of PPTA fibers.The endpoint of deprotonation can be quantitatively determined by Raman characteristic peak,cation demand(CD)and the degree of deprotonation(?).Using water as a proton donor can complete the protonation reduction of ANF,realizing the structural reconstruction of ANF,and maintain the integrity of its chemical structure.The obtained colloidal ANF can achieve high concentration,long time storage,and excellent redispersion.Secondly,based on the structure properties of PPTA fiber,the fibrillation/ultrasonic-assisted deprotonation methods was proposed,breaking the traditional ANF preparation process,and making it happen as follows: "fibrillation/ultrasonic--layer stripping--longitudinal splitting--exposure/swelling core layer--destroy the protonation of the hydrogen bond network--ANF",which makes the preparation cycle shortened to 1 day.Further,the proton donor-assisted deprotonation stretagy was also propoesd,which significantly shortened the traditional preparation cycle from 7 days to 4 h,and is the most efficient method reported in the literature thus far.It has overwhelming advantages in the size,preparation cycle,strength and industrialization prospect over the traditional methods.Based on this efficient preparation strategy,a small trial of ANF large-scale production was carried out,and the problems to be solved in the large-scale production process of ANF were explored,which is expected to promote its industrial production.In addition,functional and differentiated ANF were also developed,which enriches the structure of ANF products,allowing the application of ANF to be expanded from simple 1D nanofiber reinforced phase to functional advanced composite materials with complex 3D structure.Thirdly,based on the similarity of the compatibility and microstructure between ANF and cellulose nanofiber(CNF),and easy to bond through hydrogen bonding,the properties of CNF and ANF were comprehensively investigated.ANF show significant advantages over CNF in dispersibility,water resistance,wet strength,thermal stability and UVblocking ability.Those impressive characteristics of ANF make it an ideal alternative building block to CNF used in harsh conditions.In order to improve the issues of CNF insulating paper,such as poor water and heat resistance,easy to aging,a promising strategy to synergistically improve the water resistance,wet strength and UV-blocking property of CNF nanopaper via layer-by-layer(LBL)assembly of ANF film was proposed.The CNF@ANF nanocomposite insulating paper with a sandwich structure synergeticcally enhanced the overall performance of CNF insulating paper through intermolecular hydrogen bonding and coating effect of ANF film,exhibited excellent mechanical strength(101.3 MPa)and dielectric strength(29.6 k V/mm),outstanding water resistance and ageing resistance(contact angle of 92°,rention of wet strength reaches 86%).Finally,based on the highly homologous of the structure and composition of ANF and PPTA paper,ANF and ANF nanofilm were utilized as the self-reinforcing materials to synergeticcally improve the overall performance of PPTA paper.The multi-scale ANF matrix composite materials interface enhancement mechanism was also explored.The different bonding actions of ANF reveal its bridging,inlaying,filling and self-assembly coating enhancement effects on the mechanical properties and dielectric strength of the PPTA paper.Moreover,the adhesivefree ANF-films laminated PPTA papers(PFP and FPF)with tailored lamination structures were developed,exhibiting appealing mechanical strength(136 MPa),extraordinary dielectric strength(100 k V/mm)and desired flexibility,making them suitable for a variety of high-end insulation application.Besides,an efficient and high value-added approach was proposed for the recovery of ANF and ANF insulating nanopaper from PPTA broken paper.It not only solves the problem of resource waste,but also realizes high value utilization of waste polymer,which is beneficial to sustainable development.This study solves the bottleneck problems that restricting the large-scale production and application of ANF(such as low efficiency and high cost of production).It provides certain theoretical guidance and technical basis for promoting the industrial production and diversified application of ANF,and expands the application of ANF in the field of advanced electrical insulating paper-based materials.The development of this research has important scientific value and practical significance for enriching the existing preparation technology and theory of nanofibers and developing new 1D nanofibers and nanocomposite reinforcement technology.