Micro-and Nano-fibrillation Of Pineapple Leaf Fibers And Functional Regulation Of Their Composite Films

Cellulose nanofibers（CNF）,as a one-dimensional nanomaterials derived from natural plants,has some advantages,such as high aspect ratio,adjustable surface chemical properties,and good biocompatibility,thus it has great application potential in the field of composite material reinforcement.Two-dimensional nanomaterials,graphene and transition metal carbide/nitride（MXene）with high electrical and thermal conductivity,have outstanding advantages in the construction of electromagnetic shielding film and thermal conductivity film.However,macroscopic films assembled from graphene or MXene usually have poor mechanical properties and flexibility due to the weak interlayer interactions.CNF can effectively improve the interlamellar bonding force,but their insulative characteristic will inevitably reduce the conductivity of films.In addition,agricultural waste pineapple leaves,as a source of natural fibers,has been extensively studied in textile,pulp and paper,fiber-reinforced composite materials and other fields in recent years.However,there is a lack of exploration about the high-value utilization of pineapple leaf fibers.In view of this,pineapple leaves was used as the raw material in our paper,and the basic properties of pineapple leaf fibers,and the structural morphology/characteristics of fibers after micro-and nano-fibrillation,as well as the properties of nanocellulose film were systematiclly studied.In addition,the extracted and separated pineapple leaf nanofibers（PALNF）was used as the enhancement phase of MXene and graphene to construct the bio-based functional composite films,and their comprehensive properties were also improved through the composition and structure regulation.Thus,our work provides new ideas and theoretical guidance for realizing high value transformation of pineapple leaf fibers.It mainly includes the following parts:Firstly,the basic properties（including chemical composition,microstructure and mechanical properties,etc.）of pineapple leaf fibers were studied and compared with other three species of leaf fibers（such as,banana,sisal and sugarcane）.The results showed that the mechanical properties of different fibrils bundles were positively correlated with the content of cellulose,aspect ratio and crystallinity of the fibers,and negatively correlated with microfibril angle and lumen area.Among the four species of leaf fibers,pineapple leaf fibers had the highest cellulose content,crystallinity and aspect ratio,and the lowest microfibrillar angle and lumen area.The corresponding pineapple leaf fibrils bundles showed the highest tensile strength（1033.6±402.5 MPa）and Young’s modulus（81.7±21.6 GPa）.Apparently,pineapple leaf fibrils bundles is a high-quality raw material for preparation of high-strength bio-based materials.Secondly,the structural morphology,physical and chemical properties of pineapple leaf fibers after micro-and nano-fibrillation were studied,and compared with other five species of fibers（such as,Nordic pine,poplar,cotton,flax and bamboo）.The results showed that the sizes of six species of nanocellulose were obviously different under the same fibrillation conditions due to the differences of fiber’s crystallinity and cell wall structure.In addition,the nanocellulose films were prepared.It was found that the mechanical properties of the nanocellulose films were positively correlated with their crystallinity and aspect ratio.Among the six species of nanocellulose,PALNF has the highest aspect ratio（～1260）and high crystallinity index（74.5%）.The corresponding PALNF-based nanocellulose film shows the highest tensile strength（229.0±9.8 MPa）and toughness（33.9±2.9 MJ/m³）.This work suggested that the highly spindly and crystalline PALNF is suitable as an enhancement for the functional films.Thirdly,the MXene/PALNF composite films with mussel brick-mortar structure were prepared by blending and filtration using PALNF as the enhancement phase of MXene.The composite film has excellent mechanical properties,low thickness and high specific electromagnetic shielding efficiency（EMI SE）.The MXene/PALNF composite film with only 50 wt%PALNF at a low thickness of 28μm,their tensile strength is up to 159.56 MPa and the specific EMI SE is up to 6541.1 d B cm² g^-1.Therefore,PALNF showed the great potential to simultaneously improve the mechanical properties and electromagnetic shielding efficiency of the composite films.Fourthly,the MP/PALNF/SiO₂ hydrophobic film with laminated structure was constructed through the stratified filtration and dip coating of hydrophobic silane.The laminated structure gives the composite film excellent mechanical properties and electromagnetic shielding properties.When the thickness of MP layer is about 9.4μm,the tensile strength of composite film is up to 256.23±6.38 MPa,toughness is up to 33.68±1.84MJ/m³.Meanwhile,the specific EMI SE of composite film can reach to 22695.8 d B cm² g^-1,which is more than commercial Cu foil(～7800 d B cm² g^-1)and Al foil(～18000 d B cm² g^-1).Thus,the stratified filtration strategy solves the problems of low tensile strength and poor shielding performance of the composite film that obtained by blending filtration.Furthermore,the hydrophobic layer improves the surface wettability,and the surface water repellent property,as well as the conductive stability of the composite film.In addition,the excellent joule thermal performance of the composite film makes it have a potential application prospect in the field of electromagnetic shielding and thermal management.Fifthly,a high strength and high specific EMI SE MrGO/PALNF-DA/PBAn composite film with lightweight characteristic was constructed,based on the intercalation effect of small size MXene on large size reduced graphene oxide（r GO）,the hydrogen bond interaction of dopamine（DA）grafted PALNF andπ-πinteraction of PBAn molecule with pyrene ring structure at four ends.The maximum tensile strength and toughness of the composite film with a grammage of 22 g/m² and thickness of 7.9μm can reach to 469.2±16.9 MPa and37.7±2.2 MJ/m³,respectively.The conductivity of composite film is simultaneously up to10241.3 S/m,and the corresponding specific EMI SE is up to 35459 d B/cm.At the same time,the high-efficiency of electrothermal conversion ability and heat dissipation ability make the surface temperature of composite film can reach the balance of rising and cooling within 5 s under the low voltage of 1 to 5 V.In addition,a flexible strain sensor based on the MrGO/PALNF-DA/PBAn composite film was designed,which can be used to accurately monitor physiological signals in different parts of human body.This lightweight and multifunctional integrated composite film has potential applications in high-end consumer electronics,automotive electronics and other cutting-edge industries.