Orderly Assembled Macrofiber Based on Bacterial Cellulose Nanofibers:Preparation and Application

Author:Yao Jing Jing

Supervisor:wang hua ping

Database:Doctor

Degree Year:2019

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Pages:120

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Sustainable and high-performance alternatives from renewable resources are intensely needed due to exhausted petroleum feedstock and environmental pollution issues.As abundant organic materials on earth,bacterial cellulose(BC)are secreted by A.xylinum and expected to meet the increasing demand for petroleum-replacing products since its extraordinary mechanical properties,high aspect ratio,low density and abundant hydroxyl group.However,transferring the outstanding performance of nanofibers to a macroscopic counterpart still remains a big challenge.Besides,spatial arrangement of these nanoscale building block to macroscopic dimensions is also significant in applications related to biosensor,electronics,optics,and energy storage.Meanwhile,with the rapid development of nanotechnology,hierarchical and orderly assembly of artificial micro/nano-building blocks have shown the potential for the fabrication of bio-inspired materials with high mechanical and functional performance.Therefore,the exploration of innovative assembly techniques and new combinational styles should be further developed to exploit high mechanical performance,functionality and flexibility from nanoscale to macroscale.Herein,we demonstrated that macrofibers based on aligned BC nanofibers could be continuously prepared through wet spinning.The mechanical properties of the macrofiber were further improved through increasing orientation and interaction force of BC nanofibers.In addition,we controllably prepared CdTe/BC,CNT/BC,PPy@CNT and PPy@BC nanocomposites via active site modification mechanism and orderly assemble these functional building block to obtain luminescent macrofibers and fiber-based electrode by wet spinning.We further explored their application in the field of sensing and energy storage,revealing the effect of designed structure at different scale on the optical and electrochemical performance of macrofibers.(1)The surface of BC nanofibers was modified by TEMPO oxidation system.We studied the effect of oxidation conditions on the surface charge,morphology and dispersion of BC nanofibers.The relationship between crowding factors and dispersion and mechanical behavior of BC nanofibers in water were investigated through rheological test.Then we established a continuous preparation system of macrofiber based on aligned BC nanofibers by the wet spinning strategy.The effect of shear rate and BC nanofiber concentration on the structure and properties of macrofibers were investigated.These results showed that nanofibers with the carboxylate content of 0.65 mmol/g were mostly converted to individual fibrils due to repulsive force.The carboxylate groups formed by TEMPO oxidation were selectively presented on cellulose microfibril surfaces without changing internal cellulose crystallites.The directional flow and draft in the wet spinning process resulted in alignment of BC nanofibers.We found that lower concentration and increase of the spinning rate lead to a higher shear force and less entanglement,subsequently increasing the alignment of the nanofibers in the fiber axis direction.However,porosity or loose structure of the materials caused by low concentrations would significantly decrease the mechanical properties of the macrofibers.Therefore,high elastic modulus(12.0±2.3 GPa)and strength(198.0±53 MPa)could be obtained as the concentration of BC nanofibers was 5.4 wt%,the spinning rate was 18.9m/min.(2)We used post-stretching to improve orientation of the BC nanofibers and achieve dense structure.Besides,multivalent ions constructed cross-linking bridges between nanofibers and thus enhanced mechanical properties of macrofiber in high humidity.The SEM and WAXD results showed that nanofibers in stretched macrofibers were better oriented in the longitudinal axis and less network structure of the layer than the unstressed samples due to the plasticizing effect of water and the improved slippage order degree of the nanofibers.When stretching ratio was up to0.2,the orientation index of macrofibers was 0.72 that leading to their elastic modulus and strength increased by 36%and 25%,respectively.In addition,FTIR indicated that multiple coordination geometries and bonding mechanisms coexist in macrofibers with cross-linking.Trivalent cations had higher tendency than divalent cations to cross-link between two nanofibers because of the greater electrostatic attraction and geometrical needed to interact with three surrounding carboxylate groups.Thus Fe3+showed the high mechanical properties both in dry and wet condition because of stronger intra and interfibrillar interactions than divalent or monovalent cations.(3)We in-situ synthesized CdTe quantum dots(QDs)on the surface of BC nanofibers by aqueous phase synthesis method and explored the influence of synthesis conditions on quantum dot size and optical properties.Color-tunable luminescent macrofibers were obtained using wet spinning by assembling CdTe-loaded BC nanofibers onto one-dimensional structure.The effect of CdTe size on response range and sensitivity of pH and glucose for luminescent macrofibers were studied.The average radius of the CdTe increased in the course of heating and it can be controlled by adjusting the reaction time.The luminescent macrofibers with green,yellow and orange fluorescence could be tuned easily by controlling the size of CdTe QDs.The resulting macrofibers displayed good thermal stability,mechanical properties and knittability.Besides,the luminescent macrofiber showed a sigmoidal dependence with pH,and their pH-dependent behavior indicated different response range with the size of CdTe QDs.Enzyme-modified macrofibers were applied to calculate the glucose concentration in the samples by monitoring the fluorescence change since the fluorescence intensity of the macrofiber was highly sensitive to D-gluconic acid.EMF-CdTe548and EMF-CdTe608 behaved higher sensitivity to glucose concentration.EMF-CdTe548 with smaller size of CdTe presented a sharper response and a higher sensitivity,which showed 0.026 mM detection limit of glucose and could monitor low concentration of glucose in human saliva.(4)The CNT/BC suspension was aggregated into a fibrous structure by wet spinning.The dispersion of BC for CNT and the influence of CNT concentration to fiber structure and properties were investigated.Polypyrrole(PPy)could be easily distributed on the surface of BC nanofibers and CNT by in-site polymerization,resulting in a core-sheath structure.A flexible fiber-based supercapacitor(FSC)with hierarchical core-sheath and porous structure was elaborately designed by self-assembly of multiple nanoscale PPy@CNT/BC electrodes.BC in the FSC not only efficiently prevented the aggregation of CNTs and significantly improved the wettability of supercapacitor,but also acted as electrolyte nano-reservoirs,which could accelerate the diffusion of electrolyte ions.PPy@CNT/BC electrode that assembly of multiple nanoscale supercapacitors had good conductivity and mesoporous structure,facilitating the ion transport and resulting in considerably enhanced electrochemical properties.The assembled all-solid-state FSC exhibited high energy density(6.8?mWh/cm3),power density(391.7?mW/cm3),excellent cycling retention ability and bending ability,possessing great potential as the energy and power in various portable,miniaturized,and wearable electronic devices.