| Nanocellulose is one of the most celebrated biomaterials in the materials community for a plethora of reasons that include their renewability, environmental benignity, vast abundance, biocompatibility, excellent physical strength and wide chemical-modification capacity. Thus, the preparation and application of nanocellulose are drawing a tremendous level of attention in both academia and industry in these decades. 2,2,6,6-tetramethylpiperidine-1-oxyl(TEMPO)-mediated oxidation with post-mechanical treatment has been proposed as a promising and widely-used method for the preparation of nanocellulose. Bleached kraft pulp has often been used as the starting material for research on production of TEMPO-oxidized cellulose nanofibrils(TOCNs). Although the lignin content is extremely small, there is still a significant amount of hemicellulose present in the bleached pulp, which may influence the nanocellulose production. The present work is therefore predicated on the effect of hemicellulose on TOCNs production and the properties of oxidized pulps, TOCNs and cellulosic films, in order to provide the theory foundation and technical support for application.The type and content of hemicellulose vary greatly in different materials such as softwood, hardwood and non-woody plants. In this study, bleached kraft pulps from different materials including pine, eucalyptus, bamboo and bagasse, were used to prepare TOCNs. Firstly, the influence of hemicellulose on the TEMPO-mediated oxidation and properties of oxidized pulps were investigated. It was found that most mannans were removed during TEMPO/Na Br/Na Cl O system oxidation because they contain the C6 primary hydroxyls and were easier to be oxidized. However, most of xylans remained in oxidized pulps. The presence of hemicelluloses, especially for xylans, has adverse impact on the reaction rate and formation of carboxylate groups acting as a physical barrier against cellulose oxidation because of the lower accessibility of cellulose to chemicals. Besides, mannans presented in softwood can result in non-beneficial consumption of the oxidant(Na Cl O). Under the same condition, the order of carboxylate content in oxidized pulps from different materials was: eucalyptus > pine > bamboo > bagasse. Under the same condition,pine had highest reaction rate of TEMPO-mediated oxidation whereas bagasse exhibited lowest reaction rate.Secondly, the properties of TOCNs and their suspensions from different materials were investigated. The results showed that the order of thermal stability was: pine > eucalyptus > bamboo > bagasse, regardless of original pulps and TOCNs. The presence of hemicellulose reduced the thermal stability of cellulosic materials. All TOCNs had the similar average widths of 3~5nm. Pine TOCNs had highest the average length of 370 nm approximately, while the eucalyptus TOCNs was the shortest with average length of 290 nm. The average length of obtained bamboo(350nm) and bagasse(360nm) TOCNs were similar. Eucalyptus exhibited higher degree of nanofibrillation than other three materials. The presence of hemicellulose further influenced on the properties of TOCNs by affecting the carboxylate content. The higher carboxylate content may result in shorter TOCNs. Under the close carboxylate content, higher hemicellulose content may promote the nanofibrillation at the stage of mechanical treatment. The light transmittances of different TOCN suspension were: eucalyptus > pine > bamboo > bagasse, while the viscosity of TOCN suspension showed the opposite trend. The higher carboxylate content was beneficial to obtained more transparent and flowable TOCN suspension. The presense of hemicellulose can reduce the transmittance of suspension. The higher xylans content may cause higer viscosity of suspensions.Furthermore, TEMPO-oxidized pulps(TOP) and TOCNs from different materials were used to prepare two types of cellulosic films vacuum filtration. With the same material, TOCN films, having higher Young’s modulus and tensile strength, exhibited better physical strength than corresponding TOP films. The Young’s moduli of TOCN films from different materials were similar around 6.5GPa. The eucalyptus TOCN films had highest tensile strength of 118 MPa. The fiber length, carboxylate content and hemicellulose content can affect the physical strength of TOP films to different extent. The TOCN films with denser structure between nanofibers had smoother surface than TOP films without observing any fibers. Both TOCN films and TOP films had ultrahigh transmittance. The transmittances of all TOCN films were above 90%, especially for eucalyptus(96%). And that of TOP films also around 85% without any obvious differences between materials. However, the TOP films cause severer light scattering than TOCN films due to the diameter within micron-range.Finaly, the TOCNs were preparrd and characterized from bleached eucalyptus and pine kraft pulp that possess varying hemicellulose contents. Alkali(Na OH) and xylanase treatments were used to remove hemicellulose to different degree. Alkali extraction is able to swell the fiber and remove the intra-microfibrillar xylan and that which is on the surface of fiber, while xylanase hydrolysis can only remove the xylan that is on the surface of the fiber. Therefore, alkali pretreatment of pulp can increase the carboxylate content more remarkably than xylanase, especially at an alkali concentration of 2.0mol/L Na OH. The results showed that the removal of hemicellulose(especially xylans) from the original feedstock facilitated not only an increase in both the carboxylate content of the TEMPO-oxidized fibers, but also enhanced oxidation. It was also determined that the presence of xylans in the cellulosic feedstock hindered chemical accessibility through a barrier mechanism in which the TEMPO-mediated oxidation reaction rate was reduced. The enhancement of xylanase pretreatment on TEMPO-mediated oxidation of eucalyptus pulps was more remarkably than that of pine due to higher xylans in eucalyptus. Alkali pretreatment decreases the thermal stability of TOCNs. It appears that alkali pretreatment prepares shorter TOCNs, while xylanase treatment had less obvious effect on the fiber length. The more xylans removed the higher light transmittance of the obtained TOCNs suspension. |
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