| Cellulose is the most abundant renewable polymer on earth,and the research and application of nanocellulose has far-reaching significance for green and sustainable development.Nanocellulose can be prepared by ordinary acid treatment,enzyme treatment and mechanical treatment.It is mainly divided into two categories:cellulose nanocrystals(CNC)and cellulose nanofibrils(CNF),but their size is limited to above fibrils.fibrils or aggregates of fibrils.Due to the compactness of the crystalline structure of cellulose,the further dissociation of fibrils is full of challenges,so its dissociation mechanism,properties and applications are rarely studied.We found that the use of glacial acetic acid,an acidic polar organic solvent,and the oxidation of hydrogen peroxide can weaken the binding force between cellulose molecular chains.Finally,combined with ultrasonic dispersion,the effective disassembly of the cellulose sub-fibril structure can be achieved.untie.And successfully prepared cellulose nanoribbon(Cellulose nano Ribbon,CR)with the thickness of molecular layer.In this dissertation,we systematically studied the impregnation of fiber raw materials with glacial acetic acid and hydrogen peroxide system,and the change of cellulose crystal structure by ultrasonication,thereby explaining the dissociation mechanism of CR.Then,CR was characterized in detail,and further applied to Pickering emulsion and 3D In printing,the main research conclusions are as follows:1.CR preparation,influencing factors and peeling mechanism.The method of preparing CR by ultrasonic treatment after immersion in glacial acetic acid and hydrogen peroxide is applicable to several different types of fiber materials such as eucalyptus,wheat straw,bamboo,metasequoia and poplar.During the dipping process,the two solvents acted on the crystalline surface of the fiber through polar infiltration and oxidation,prolonging the dipping time,the interplanar spacing of the crystalline plane(110)increased significantly,while weakening the crystalline plane(110)internal hydrogen bonding,and trace amounts of CRs with a length of 1-6 μm,a width of 2-18 nm,and a height of 0.4-1.2 nm appeared in the supernatant.During the ultrasonic process of the fibers immersed in glacial acetic acid/hydrogen peroxide,the interplanar spacing of the crystal plane(110)was further increased,so that more fibers slipped along this plane to obtain CR;when the ultrasonic time at 120 min,the length of CR was 1.47±0.48 μm and the height was 0.8±0.4 nm,corresponding to 1-3 cellulose molecular layers.2.Performance characterization of CR.The carboxyl group content of the fiber raw material impregnated with glacial acetic acid and hydrogen peroxide was 0.5 mmol/g,more charged groups were exposed after ultrasonication,and the carboxyl group content of CR increased to 2.86 mmol/g,showing good dispersion stability in water.Compared with traditional CNF and CNC,the thermodynamic stability of CR is slightly decreased,but the viscosity of CR is more than 2 orders of magnitude larger than that of traditional CNF and CNC,and the storage modulus is high,and it reaches the state of gel at a concentration of 0.3%;The low affinity for the cellulase adsorption domain showed super-hydrophilic properties,which also verified that the CR was stripped from the hydrophilic surface.However,the micromechanical properties of CR with only the molecular layer thickness did not decrease significantly.On the contrary,the Young’s modulus of CR at the kink was as high as 36 GPa,showing flexible properties.3.CR has excellent Pickering emulsion stability.The CR-stabilized Pickering emulsion(CRPe)is an oil-in-water emulsion;CR can prepare a stable Pickering emulsion with two oil phases with different polarities,soybean oil and cyclohexane at a low concentration of 0.05%.After 60 d,it still has a stable emulsion phase volume;the emulsion prepared with 0.3%CR-stabilized cyclohexane gels;the Pickering emulsion prepared with soybean oil as the oil phase has a larger volume fraction of the emulsion phase;the volume of the oil phase is increased,the volume fraction of the emulsion phase can be increased.In the centrifugal stability test,CR-Pe with a concentration of 0.05%～0.5%did not break the demulsification under the centrifugal action of 2000～8000 rpm,and 0.5%CR-Pe could resist the centrifugal action of 8000 rpm without breaking the demulsification,no oil leakage,no precipitation.As the control CNC-Pe,the oil and water are separated after centrifugation,and the CNF-Pe will appear demulsification and oil leakage.In the rheological test,both CR-Pe have shear thinning ability,and the storage modulus(G’)is greater than the corresponding loss modulus(G").The Pickering emulsion prepared with cyclohexane as the oil phase is viscous,the elasticity is better,and the apparent viscosity of 0.5%CR-Pe is as high as 2.24×107 mPa s,which is twice higher than that of 0.5%CNF-Pe.4.The application of CR in 3D printing.The Pickering emulsion of CR-stabilized cyclohexane was printed by the 3D printing technology of direct ink writing.Without adding any cross-linking agent,0.3%CR-Pe has formability and structural stability,and 0.5%CR-Pe has formability and structural stability.The printed cube has a high resolution,and it shrinks slightly in the longitudinal direction after being placed at room temperature for 1 d,but the texture is relatively hard and can support a weight of 1000 g;the freeze-dried printed model is only a few milligrams,and the density is 0.018 g/cm3,its internal structure is a uniform pore and bridge structure,under the compression test,when the strain is 64%,the stress is 216 kPa,showing better elasticity. |
PDF Full Text Request