| Recently, more applications of natural nanofibers have been found due to the advantages of the high crystalline, excellent biodegrability, and abundant source, and lots of attention has been paid on the nanocellulose composite materials, for the nano-composite materials could exhibit better properties than those of the conventional composites because of the nanometer-sized effect. Cellulose and chitin, as the renewable polymers with fine multilevel structure, have played an indispensable role in the field of materials research. Therefore, the research of biomass materials has been extended to the micro/nano scale in recent years with the development of nanotechnology, and the extraction of nanofibers from natural biological organism has received significant attention due to the problem of resource shortage. In this work, the nanofibers were prepared from the natural cellulose and chitin with mechanical methods, and the micro-morphology, the whole structure, the properties of nanofibers were analyzed. Then, as the reinforcing phases,the obtained nanofibers were composited with PVA to prepare the composite materials, and the structure and properties were also analyzed to show the effects of the interfacial bonding. The main results were summarized as follows:1. Lignin and hemicellulose in wood powders could be removed during a three-step chemical treatment(without the extraction of toluene/ethanol), and the purified cellulose could be prepared. As the removal of lignin and hemicellulose in the three-step chemical treatments, the microfibril or fiber bundles are staggered and arranged parallel or spiral as a net-work structure according to the microstructure analysis, also, the structure of the cell walls of wood powder could not be destroyed in the chemical treatment. By one-step grinder apparatus or high pressure homogeneous methods, cellulose nanofibers(diameter of 15nm) with high aspect ratio could be obtained. However, the preparing efficiency and the nanofibers yield in high pressure homogeneous process were lower than those in grinding process. The preparation efficiency of the nanofibers from purified cellulose was higher in grinding process, and the obtained nanofibers could be distributed more uniformly. The crystal structure or the high crystallinity properties of obtained cellulose nanofibers were similar as those of original cellulose. The crystallinity, the Tmax of the thermal degradation, and the coef?cient of thermal expansion(CTE) of the nanofibers were 75.96%, 342.38°C, 10.2 ppm/K, respectively.2. As the PVA aqueous solutions was mixed with one-time grinding cellulose nanofibers slurry, the transparent PVA nano-composite materials reinforced with cellulose nanofibers could be prepared with solution casting method. The composite film exhibited similar transparency(higher than 90%) to that of pure PVA film, however, the linear light transmittance of the composite film was decreased obviously. Compared with pure PVA film, when cellulose nanofibers(0.5%, 1%, 3% and 1%, respectively, mass ratio) was added in the PVA aqueous solutions to form composite materials, the young’s modulus of the composite film could be increased by 42.5%, 68.3%, 87.4% and 68.3%, respectively, and the tensile strength of the composite film increased by 27.1%, 31.5%, 53.9% and 31.5%, respectively. However, the thermal expansion coefficient of PVA was decreased significantly, for the movement of the PVA molecular chain could be hindered by nanofibers effectively, and it could also be concluded that the cellulose nanofibers could be dispersed uniformly in PVA matrix, and the application of CNF/PVA composite film could be further expanded due to the advantages of higher transparency or mechanical performance, and lower thermal expansion performance.3. The purified chitin was treated by one-time grinding and ultrasonic methods under neutral condition, and the chitin nanofibers(CNFs) aqueous solution could be prepared. Then the chitin nanofibers solution was centrifugated and the microstructure of chitin nanofibers was analyzed. The results showed that the diameter of the chitin nanofibers could reach the nanoscale after onestep grinding process. After further ultrasonic treatment, the diameter range of the chitin nanofibers would be reduced and the diameter distribution was in the range of 15-35 nm. The nanofibers with more uniform diameter distribution range could be retained during the centrifugal treatments, therefore, the light transmittance of the chitin nanofibers film would be increased, however, there was no significant effect on the mechanical properties or thermal stability. The chitin nanofibers film prepared in a vacuum filtration process could exhibit higher mechanical strength or degradation temperature, and lower thermal expansion properties.4、The chitin powders pretreated by chemical methods were treated with grinding and ultrasonication processed, and the CNFs could be obtained. Then the CNFs film(prepared in the vacuum filtration) was soaked in the PVA solution, and finally the CNFs-PVA could be obtained. The diameter distribution of the obtained CNFs was in the range of 30-50 nm and the crystalinity structure was similar to that of α- chitin. The light transmittance of the CNFs-PVA composite ?lm was 86%(6% lower than that of PVA ?lm). The CTE of the CNFs-PVA composite ?lm was 25.09 ×10-6/℃,(80% lower than that of PVA ?lm). The tensile strength and Young’s modulus of the CNFs-PVA composite ?lm were 127 MPa(100% higher than that of PVA film) and 5.70 GPa(140% higher than that of PVA film), respectively. The elongation the composite ?lm was about 4.40%, which was about 80% lower than that of PVA ?lm. The mechanical strength of the PVA film could be increased significantly by CNFs, and the CNFs-PVA composite materials could exhibit better light transimittance, dimensional stability, and lower CTE. |
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