Preparation And Investigation Of TEMPO-Oxidized Cellulose Nanofibers And Their Composite Films

TEMPO(2,2,6,6-tetramethylpiperidine-1-oxyl radical)/Na Br/Na Cl O is an effective regioselective oxidation system for cellulose oxidation. The C6 primary hydroxyls in cellulose can be selectively oxidized into carboxyl groups when this free radical system is applied to native celluloses in aqueous media. Nanocellulose could be obtained through a mild homogenization treatment of TEMPO-oxidized cellulose. This kind of nanocellulose preparation method is mild and energy-saving. In addition, the obtained nanocelluloses(i.e. TEMPO-oxidized cellulose nanofibers) have homogeneous size, large aspect ratio and good dispersity. Thus, TEMPO-mediated oxidation is a promising method to prepare nanocellulose. However, the study on smart control of TEMPO-mediated oxidation as well as the improvement in oxidation efficiency, and the application of TEMPO-oxidized cellulose nanofibers(TOCNs) in composite materials remain extraordinary academic value and practical significance.This dissertation mainly focused on improvement of TOCNs manufacture method and its application. It included influence of formic acid pretreatment on TEMPO-mediated oxidation of cellulose; Effect of compositions of carbonate buffer solution on TEMPO-mediated oxidation and the oxidized cellulose; Strengthening performance of TOCNs in electrospun polyvinyl alcohol(PVA)/waterborne polyurethane(WPU) nanofibers and properties of hydroxypropyl guar(HPG)/TEMPO-oxidized cellulose nanofibers(TOCNs) composite films.Formic acid was used to pretreat the bleached softwood kraft pulp, and the pretreated sample was subsequently subjected to TEMPO-mediated oxidation. The obtained results indicated that formic acid pretreatment could effectively resulted in a 10% increase in carboxyl content of TEMPO-oxidized cellulose. The comparison of various volume concentrations such as 2%, 5%, 7%, 9%, 15% and 18% was conducted to study their influence on TEMPO-oxidized cellulose, and the carboxyl contents of the forementioned oxidized cellulose were in the range of 0.981-1.769mmol/g cellulose. Not only the degradation and dissolution of cellulose, but also the destruction of cr ystal region in cellulose affects the carboxyl content of TEMPO-oxidized cellulose. Taking into consideration of the carboxyl content and yield of oxidized cellulose, the proper concentration of formic acid for pretreatment is 5%(V/V). Moreover, the pretreatment time and temperature also had significant influence on the successive TEMPO-oxidized cellulose. The results showed that the optimal pretreatment time is 24 h and the proper temperature is 20°C. The SEM observation certificated that the formic acid pretreatment could effectively destroy the cellulose structure and fibrillate the cellulose fibers, resulting in increased amount of exposed hydroxyl. XRD characterization showed that formic acid pretreatment could break the crystalline region in cellulose, especially when the harsh condition was adopted. Specifically, crystallinity of cellulose dramatically decreased after the formic acid pretreatment. Additionally, TEM observation showed that the nanocellulose obtained in this research had diameters of 3-7 nm and lengths of 300-400 nm.The factors that influenced the TEMPO-mediated oxidation in carbonate buffer solution was investigated. It mainly focused on the effect of composition ratio of carbonate buffer solution on TEMPO-mediated oxidation. The results indicated that the carboxyl content of TEMPO-oxidized cellulose varied with the change of molar ratios of Na2CO3 and Na HCO3. It could attribute to the initial p H value differences and the different ranges of p H fluctuation resulting from the variation of composition ratios of buffer solution. The results showed that the upmost carboxyl content could be attained when using the carbonate buffer solution with the Na2CO3/Na HCO3 molar ratio of 7/3. Otherwise, the oxidation reaction rate was also related to the molar ratio of Na2CO3 and Na HCO3, that is, the bigger molar ratio of Na2CO3 to Na HCO3, the higher the reaction rate was. A p H increase at mild alkaline conditions was accompanied with a substantial increase of oxidation rate. And the initial p H increased with the increment of molar ratio of Na2CO3 to Na HCO3. Furthermore, the carboxyl content of oxidized cellulose improved with the upgrade of reaction temperature while the yield reduced with the temperature increase. Higher temperature would intensify the disruption of cellulose crystalline region, leading to improved accessibility, which would make the C6 primary alcohol hydroxyl easier to be oxidized. But meanwhile, it would aggravate the cellulose degradation. The dosage of main oxidant(Na Cl O) had a big influence on oxidized product, too. Specifically, the carboxyl content increased with the increase of Na Cl O consumption while the yield decreased at the same time. Moreover, the reaction rate also showed upraised trend when higher Na Cl O dosage was incorporated. SEM images of oxidized cellulose fibers showed that the fiber surface was corroded and this corrosion intensified with the increase of oxidation degree. The exfoliation of the primary wall or even the S1 sublayer of the fibers in the alkaline TEMPO-mediated oxidation resulted in a strong fibrillation of cellulose fibers, especially under higher temperature. XRD patterns of oxidized cellulose fibers indicated that the cellulose crystal type would stay unchanged even under severe conditions. T here were minor changes in the crystalline regions of the inner layers of the cellulose fibers. Nevertheless, the reaction temperature still had negative effect on crystallinity of cellulose. The crystallinity of cellulose reduced to 40.8% from 56.4% as the temperat ure increased from 25°C to 40°C.This study certificated that PVA, WPU and TOCNs could be successfully electrospun into nanofibers. TOCNs were well dispersed in the polymer matrix and they had good compatibility with other components. This study discussed the influences of needle diameter, solution concentration and TOCNs amount on electrospinning. Comparing with needle of larger diameter(0.84 mm), the needle with diameter of 0.51 mm produced rougher nanofibers, and there existed beads and ultrathin nanofibers. The morphology of electrospun nanofibers changed when the concentration of PVA/WPU solution increased from 10 wt.% to 15 wt.%. Nanofibers electrospun from PVA/WPU solution with concentration of 15 wt.% had poor smoothness and large diameter fluctuation. Additionally, some nanofibers with large diameters emerged and the nanofibers cohered together. The electrospun nanofibers had a smooth morphology when the TOCNs dosage was 5%, and the mean diameter of nanofibers was 512 nm. However, when the addition of TOCNs increased to 10%, the regularity deteriorated and the mean diameter decreased to 333 nm. The addition of negative charged TOCNs, leading to changes in solution viscosity, ionic strength and conductivity, resulted in change of morphology and diameters of electrospun nanofibers. The IR characterization certificated the existence of hydrogen bond among TOCNs, PVA and WPU. The incorporation of TOCNs could effectively improve the tensile strength of electrospun nanofibers. An increase of 55% in tensile strength was achieved with 5% TOCNs addition. On the other hand, the adoption of TOCNs was harmful to electrospun nanofibers’ elongation at break, which decreased by 42%. Furthermore, the well distribution of TOCNs in polymer matrix hindered the motion of polymer chains and inhibited the decomposition. Thus, the addition of TOCNs had positive effect on thermal stability of PVA/WPU electrospun nanofibers. Finally, the XRD characterization of electrospun nanofibers indicated that TOCNs’ addition could increase the crystallinity of PVA/WPU electrospun nanofibers.TOCNs/HPG composite films could be prepared through a solution casting method. The resulting films were transparent, smooth and flexibility. They also had a good mechanical strength. The influence of bending mass ratio of HPG to TOCNs on film properties was investigated. The tensile strength of films improved with the increasing mass ratio of TOCNs, and the tensile strength of composite films increased from 19 MPa to around 45 MPa when the TOCNs dosage varied from 0 to 50%. The nano-effect of TOCNs has remarkable strengthening performance in composite films. T he increased intensity of O-H absorption peak and the peak shift of sodium carboxylate group in IR spectra indicated that HPG and TOCNs were cross-linked. SEM images revealed that there existing good interactions between components. And TOCNs and HPG had good miscibility. Moreover, the film thickness, porosity and cracks reduced with the increasing addition of TOCNs. Water contact angle of composite films increased as the HPG dosage increased, and HPG could enhance the surface water resistance of composite films. The oxygen permeability of TOCNs/HPG composite films was in the range of 5.67-7.62 cm3·m-2·day-1·0.1MPa-1 and was increased with the increase of HPG content. However, the water vapor barrier property was improved with the addition of HPG. Besides, the thermal stability of composite films was also enhanced with the increase of HPG mass ratio.