Preparation And Electromagnetic Shielding Performance Of Moso Bamboo Cellulose Nanocrystals-based Composite Conductive Films

As a renewable,biodegradable,biocompatible biomass materials,cellulose nanocrystals(CNC)with the abundance of surface hydroxyl groups,high specific surface area,high mechanical strength and other characteristics,were composited with conductive active material to form conductive composites which is gradually attracted researchers interest.In this paper,separated cellulose nanocrystals of fiber(F-CNC)and parenchyma(P-CNC)were prepared,respectively,considering the different physical and chemical characteristics of bamboo fiber and parenchyma cells.Then,the F-CNC and P-CNC with different morphology and aspect ratio were combined with graphene oxide(GO)respectively to explore the effect of the CNC content,types,crystal structure,size,and preparation processes on the self-assembly behavior,mechanical properties,thermal stability,electrical conductivity,and electromagnetic shielding performance of the composite flexible conductive films.The main conclusions of the above research are summarized as follows:(1)The moso bamboo fiber and parenchyma cells in situ and pretreatment process has different physical and chemical characteristics.In situ,the fiber cells had higher cellulose and lignin content and slightly lower xylan content than the parenchyma cells.In the process of pretreatment,the parenchyma cells showed lower lignin residue,lower crystallinity,higher cellulose fibril aggregates diameter,and higher crystal size.Furthermore,the cellulose conversion efficiency of parenchyma cells was 15.94?4.45%higher than that of fiber.(2)The F-CNC and P-CNC were prepared by separating the bamboo fibers and parenchyma cells and realized the controllable preparation of CNC.Both the F-CNC and P-CNC were rod-like particles,however,the size of F-CNC was slimmer than the P-CNC.The average length,diameters and the aspect ratio of the F-CNC were 399±19 nm,5±2 nm,and 79,respectively,and the corresponding value of P-CNC were 241±21 nm,7±4 nm,and 34.Moreover,both the F-CNC and P-CNC films by vacuum filtration showed strong tensile strength(～250 MPa),while the elongation at break strain of F-CNC film was 0.9%higher than P-CNC film due to F-CNC has larger aspect ratio.(3)One-dimensional CNC,as dispersant and reinforcement at the same time,were mixed with two-dimensional flake GO can form a three-dimensional orderly"brick-and-mortar"lamellar structure through layer-by-layer self-assembly.The CNC inserted into the GO layers could significantly improve the dispersion of GO and the mechanical properties and reduction time of GO films.When 10%CNC was added,the tensile strength of FCNC/RGO and PCNC/RGO composite films increased by 126.84%and 114.17%,respectively,compared with the pure RGO films.For the as-made CNC/RGO composite film,its mechanical strength,conductivity and internal morphology varied with the CNC content and type,and the preparation process.In general,the mechanical strength first increased and then decreased with the increase of CNC content,while the electrical conductivity decreased with the increase of CNC content.Additionally,the CNC/RGO films exhibited more ordered and compact layered structure after the mechanical compression,consequently resulting in 3.28-41.26%higher mechanical strength and 4.22-31.72%higher electrical conductivity.Specially,the FCNC/RGO composite films displayed thinner thickness and higher conductivity than PCNC/RGO films in the case of the same CNC content due to the F-CNC has smaller diameter and larger aspect ratio,and the content of F-CNC(30%)required to achieve the optimal mechanical strength was less than that of P-CNC(50%).(4)The obtained ultrathin,hydrophobic,ultrarobust,flexible and conductive CNC/RGO films can be used in the field of electromagnetic shielding with an absorption-dominant electromagnetic interference(EMI)shielding mechanism.With the same thickness,although the pure RGO film had higher conductivity,the EMI shielding effectiveness(SE)of FCNC/RGO and PCNC/RGO composite films increased by 84.45%and 82.51%respectively compared with the pure RGO film after 10%CNC was added.The as-prepared CNC/RGO composite film with a thickness of 12-18μm has excellent electromagnetic shielding performance with a maximum39.03 dB SE and 11367 dB·cm~2/g specific shielding effectiveness(SSE),as well as outstanding mechanical strength with a maximum tensile strength of 227 MPa.(5)The crystal structure,size and dispersion of the matrix were closely related to the internal morphology,mechanical strength,electrical conductivity and electromagnetic shielding performance of the composite films.Furthermore,the FCNC-II and PCNC-II with smaller aspect ratio and CNF with larger aspect ratio were used as the matrix to composite with 50 wt%GO,respectively,and found only medium aspect ratio of FCNC-II with GO can form a similar layered structure like CNC/RGO composite films.The performance of mechanical strength,electrical conductivity(5555.6 S/m),thickness(12μm),SE(30.38 dB)of 50FCNC-II composite films are similar to those of FCNC/RGO under same CNC content.However,the matrix with too small(PCNC-II)or too much(CNF)aspect ratio were not easily dispersed evenly in the composite films.Therefore,although the 50PCNC-II ans 50CNF composite films had higher conductivity and thickness,their SE value did not significantly improve.Additionally,the nanocellulose/RGO composite film with cellulose II crystal structure has higher thermal stability than that of cellulose I.(6)As for the application of nanocellulose/RGO composite films to electromagnetic interference shielding field,the factors affecting the performance of electromagnetic shielding are not only the thickness and conductivity,but also the aspect ratio and dispersion of matrix.The matrix tends to agglomerate or tangle together with too small or too large aspect ratio,which is not conducive to improving the SE value of the composite film.In addition,nanocellulose/RGO composite films with layered structure have excellent electromagnetic shielding performance,and their mechanical strength is far superior to that of reported carbon-based shielding composites,therefore has great application potential in the rapidly growing fields of flexible electronics.