Structural Design And Functionalization Of Nanocellulose-Based Composite Materials

Recently,cellulose nanofiber(CNF)as an emerging biomaterial has gained widely attentions both from academical and industrial fileds due to its superior mechanical property,excellent thermal stability,tunable physical-chemical properties,and sustainability.Great progresses have been reported about the functionalization of the CNF materials in diverse fields such as papermaking,food,vehicle,aerospace,energy storage,environmental,biomedical and so on.However,most of these works are based on the directily utilization of the novel physical-chemical properties arised from the nano-sized effect of the CNF,or just in forms of composites with simply mixed structure.The overlook of the relationship between the functions and the substructure of the material has lead to a limited property and inefficient mateiral utilization.In this case,we have paid more attentions on the structural engineerng of the functional materials.Several methods have been demonstrated for the structure design and control of a variety of forms of CNF-based functional materials,including the electrostatic induced self-assembly,solvent evaporation induced self-ssembly,directional freeze-drying template method,and surface patterning method.Based on these methods we systematically studied the influences of substructure on the targeted function of the CNF-based functional materials,and further improved the performances of these functional materials by rational structural design and modification.The main work of this dissertation includes:Firstly,we designed and fabricated a novel one dimensional(1D)conductive nanofiber based on the conformal assembly of carbon black(CB)on the CNF back bone,which lead to a more efficient space utilization and significantly decreased the interface resistance between CB paticles.Charge transfer kinetic studies illustrate that the high loading nanopaper electrode(up to 60 mg/cm2 lithium iron phosphate loading)based on the 1D conductive nanofiber demonstrated a much faster ion and electron conductivities than commercial PVDF(Polyvinylidene Fluoride)based electrode.The ion conductivity of the nanopaper electrode is an order of magnitude higher than the PVDF electrode,while the electron conductivity of the nanopaper electrode is 4 times higher than the PVDF elelctrode.Given the low cost raw materials together with the scale up processability,the conductive nanofber design provides a promising strategy for the develop of thick electrode.Secondly,inspired by the "coffee ring" effect,we systematically studied and revealed the solvent evaporation induced self-assembly behavior of the CNF materials,which allows us to design and tailor the microstructure of the CNF deposition.Based on this method,we constructed a high-performance bilayer soft actuator with micro-patterned CNF as the active layer and PI as the passive layer,which demonstrates a fast(response time less than Is),powerful(?1000 times lifting weight ratio),and controllable response to external environmental stimuli.Proof of concept demonstrations of the micro-patterned soft actuators as mechanical arms and soft walking robots indicate their great potential for soft robots and biomimetic systems.Thirdly,we report a cellulose nanofiber/carbon nanotube hybrid aerogel with low tortuosity pore structure based on the directional freeze-drying method,and further demonstrate a Janus solar evaporator by changing one side of the CNF/CNT aerogel into hydrophobic,which can perform a stable steam generation without salt deposition even in a highly concentrated briny water(up to 12 wt%).Salt diffusion analysis based on the Fick's first law illustrates that the low tortuosity pore structure plays an important role in facilitating the salt rejection between the solar evaporation interfaces and the bulk solution.This low-tortuosity architecture design with enhanced salt rejection property provides a promising solution for long-term solar-assisted water generation from seawater and other industrial waterFourthly,we reported a novel strategy for the construction of programmable water responsive shape memory polymers(SMPs)based on the surface wettability control and patterned design by using CNF percolation network/PVA composite as the model.Based on this strategy,for the first time we demonstrate a water responsive SMP with multiple shape memory effects.It is believed that the strategy proposed here will open new door for the design of water responsive SMPs and significantly extend the potential applications of water responsive SMPs.