Study On The Construction And The Property Of Nanocellulose-Based High-Dielectric Composites

The rapid development of fine electronics has dramatically increased the use of dielectric capacitors,but currently commercially available dielectric capacitor materials are based on non-renewable and non-degradable petroleum-based polymers,which undoubtedly cause a heavy burden on both resources and the environment.As a natural polymer from a wide range of sources,cellulose has great potential for the preparation of high dielectric composites due to its high polar functional group content.However,compared with petroleum-based polymers,research on the preparation of cellulose-based high dielectric materials is still lacking and is limited to physical doping modifications.Therefore,this project focuses on cellulose nanofibres(TOCNF)produced by the oxidation of TEMPO(2,2,6,6-tetramethylpiperidine oxide),and analyses the effect of carboxyl group content on the dielectric properties of TOCNF.On this basis,the dielectric constant of the composite film is further improved by introducing nanoparticles with high dielectric properties and increasing the flexibility of the polar functional groups of the TOCNF itself.The main studies are as follows:Firstly,TOCNF was prepared by the TEMPO oxidation method,and the carboxyl content of TOCNF was regulated by adjusting the concentration of the NaClO and the oxidation time respectively.And a rapid detection method based on derivative spectroscopy was developed to accurately quantify the carboxyl contents of TOCNF.A series of TOCNF films with different carboxyl group contents were prepared by vacuum filtration,and their dielectric properties were tested.The results show that the dielectric constant and dielectric loss increase with increasing carboxyl group contents.At 103 Hz,the dielectric constant and dielectric loss of TOCNF films with a carboxyl group content of 1429 μmol/g were 21.4 and 1.52,which were 62.9%and 94.9%higher than those of the nanocellulose films without TEMPO oxidation,respectively.Then,the dielectric properties of composite films are improved by introducing barium titanate(BTO)which has high dielectric properties.In order to maximise the dispersion of the nanoparticles at high addition levels and the bonding strength between the nanoparticles and the substrate,and they were modified by cationisation.Firstly,polydopamine coating modification is used to introduce activity Group-NH2.After that,ring-opening reaction with-NH2 via the cationic etherifying agent(GTMAC)to introduce a quaternary ammonium group,and finally the positively charged BTO nanofiller in an aqueous dispersion is obtained.The high dielectric composite films were investigated to find out the effects of different PDA/BTO modification conditions and the optimum ratio on the dielectric properties of the materials.The results show that the cationized BTO nanoparticles can combine with the negatively charged TOCNF by ion exchange,thereby improving the dispersion,so that when the filler content is as high as 40 wt.%,the dielectric constant reaches 116.2,and the dielectric loss is 0.92 at 103 Hz,and the dielectric strength is 57.7 kV/mm,which is 695.9%,19.2%and 51.4%higher than the pure TOCNF film,respectively.Finally,the TOCNF suspension was treated with saturated NaOH-Cu2+solution to prepare TOCNF coordinated by Cu2+(Cu2+-TOCNF),and the hydrogen bond between molecules was broken,thereby increasing the activity of the original polar molecular chain of TOCNF to achieve the improvement of dielectric constant.The results show that the dielectric constant of Cu2+-TOCNF film is 88.3 at 103 Hz,which is 461.4%higher than that of pure TOCNF film,but the mechanical properties of Cu2+-TOCNF decrease by 89.9%due to high concentration NaOH,which is difficult to meet usage requirements.Therefore,using Cu2+-TOCNF as a filler to enhance dielectric properties and TOCNF as the substrate to ensure the final mechanical properties,and a TOCNF/Cu2+-TOCNF composite film was prepared,and it was finally found that when the addition amount of Cu2+-TOCNF was 60 wt.%,the dielectric constant at 103 Hz was 32.3 and the mechanical properties were 32.6 MPa,compared with the TOCNF film,the dielectric constant increased by 105.3%,and compared with the 100%Cu2+TOCNF film,the mechanical properties are increased by 301%,which improves the dielectric constant to some extent while meeting the mechanical requirements.