Preparation And Properties Of Crosslinked Regenerated Cellulose-based Dielectric Composites

Traditional dielectric polymers are synthetic polymers,which produce toxic substances during synthesis or decomposition and means inconformity to the development needs of green environmental protection.With the depletion of resources and environmental degradation,there is an increasing need for green polymers to replace such synthetic dielectric polymers.Cellulose,as one of the most abundant materials in nature,is biocompatible,non-toxic,biodegradable and recyclable.Therefore,cellulose has become one of the most promising green polymers and the development of cellulose-based dielectric materials has attracted a lot of attentions from researchers.However,the preparation of cellulose-based dielectric films is complicated due to the difficulty of cellulose solubility in common organic solvents.Cellulose films usually have many defects,which are very unfavorable for suppressing the inherent dielectric loss of the cellulose matrix.To address this issue,it is important to investigate the dependence of dielectric properties of cellulose materials on the intrinsic microstructure from the perspective of elucidating the microstructure-property relationship.In addition,reducing the dielectric loss by constructing the transition layer of filler and matrix is essential for the preparation of cellulose-based composites with excellent dielectric properties.In this context,regenerated cellulose（RC）dielectric films with dense structures were prepared by crosslinking cellulose with Glutaraldehyde（GA）and two-dimensional conductive filler MXene was introduced into the matrix to prepare cellulose-based dielectric composite films.The changes of the dielectric and mechanical properties of the cellulose films during the GA crosslinking process were investigated.We studied the effects of the variation of filler contents and the presence of transition layers on the dielectric properties of the composites.The main conclusions of the study are as follows:（1）RC films were prepared by dissolving cotton cellulose in LiOH/Urea/H₂O solvent,and a series of crosslinked regenerated cellulose（CRC）films were prepared by adjusting the crosslinking methods,GA concentration and crosslinking time.The structure and properties of the prepared samples were analyzed,and it is found that the CRC has stronger interactions,smaller surface roughness and more uniform structure than the RC films.When the concentration of GA is 6 vol%and the crosslinking time is 30 min,the CRC films obtained by post-gel crosslinking have lower dielectric loss（0.03,at 1 k Hz）,higher breakdown strength(336.55 MV m^-1)and better mechanical properties（the tensile strength and tensile modulus of76.8 MPa and 6.08 GPa）.（2）The rich surface functional groups of MXene were introduced into the cellulose matrix to improve the dielectric constant of the materials.To further improve the dielectric properties of the composites,GA was used to reduce the dielectric loss of the materials and increase their breakdown strength.The structure and properties of the prepared cellulose/MXene dielectric composite films were analyzed,and good hydrogen bonding interactions between MXene and cellulose are found.It is found that the dielectric constant of the composite films is 8.24,the dielectric loss is 0.071,and the breakdown strength is 246.78MV m^-1 at a MXene content of 2 wt%and an electric field frequency of 1 k Hz.（3）In order to solve the problem of high dielectric loss of MXene in cellulose matrix,polydopamine（PDA）was used as the transition layer between MXene and matrix to prepare RC/MXene@PDA and CRC/MXene@PDA dielectric composite films.The structure of the filler was further studied,and it is found that the coating of PDA can not only increase the interlamellar spacing of MXene,but also facilitate the uniform dispersion of MXene in the polymer matrix.Besides,by tuning the contents of MXene@PDA,composites with good dielectric properties were obtained.When the mass fraction of MXene@PDA is 5%and the electric field frequency is 1 k Hz,the dielectric constant of the composite film increases to30.28,and the dielectric loss is only 0.06.Meanwhile,the breakdown strength of the film is maintained at a high level(254.08 MV m^-1),and the energy storage density is 8.65 J cm^-3.