Structure Assembly And Performance Study Of Xylan Based Composites

The exploiting and utilization of renewable resources has attracted much attention because of the energy crisis and environmental pollution. Hemicelluloses, which come from plant cell wall, are abundant and cheap natural polysaccharides with great potential application values. It has been widely used in fields of papermaking, energy source, food industry, biomedicine and so on. Owing to its replaceable of petrochemicals, non-toxic, biodegradability and environmental friendliness, hemicelluloses and its derivatives have become attractive advanced materials. However, the high-valued utilization of hemicelluloses has been limited due to its source-dependent diversity, high branching and amorphous molecular structure. To solve this problem, a hemicelluloses model compound—xylan was used as substrate, and the high-valued utilization of hemicelluloses discussed from two aspects in this research. Firstly, structure assembly based on xylan and another natural polysaccharide(chitosan) was carried out. Secondly, combination between xylan and inorganic nanometal materials and their properties were discussed, which developed a xylan based composite material. The main researches are described as follows: 1. The preparation and properties of xylan/chitosan compositesXylan can be soluble in alkaline solution, while chitosan can be soluble in some specific organic acid, precipitation appeared when they were mixed together and the reaction was hard to control. To solve this problem, maillard reaction between the aldehyde groups of xylan and the amino groups of chitosan was carried out in cosolvent ionic liquid in this research, and finally got melanoidin, which is the xylan-chitosan maillard product. When the mass ratio of chitosan to xylan was 1:1, the maillard reaction proceeded easily, and relatively high antioxidant property was also noted for the xylan-chitosan conjugate with this mass ratio. So the obtained xylan-chitosan maillard reaction product is a promising antioxidant agent for food industry.To improve the properties of pure chitosan film, chitosan films were immersed in Na OH solution with different concentrations xylan to simply prepare active xylan/chitosan film. In this process, the xylan chains entered into the gap of chitosan film and became nodules, then the nodules growing lager with the increase of xylan concentrations, leading to strong hydrogen bonds and electrostatic interactions between xylan and chitosan. And finally a tight, smooth xylan/chitosan film prepared at the xylan concentration of 3%. The results showed that the tensile strength, breaking elongation and anti-ultraviolet performance of the xylan/chitosan films were improved greatly with the increasing concentration of xylan; water vapor transmission rate, water absorption rate and oxygen barrier property of xylan/chitosan composite films were higher than those of chitosan film. The xylan/chitosan composite film has more potential to be used as food package than pure chitosan film. 2. Green synthesis of nanometal materials and their propertiesAg nanoparticles(Ag NPs) were prepared by adopting biopolymer xylan as stabilizing and reducing agent via Tollens reaction under microwave irradiation. Au nanoparticles(Au NPs) were also prepared using biopolymer xylan as stabilizing and reducing agent under heating condition. The optimal condition for Ag NPs was as follows: microwave irradiation temperature was 60~70 °C, microwave power was 800 W, microwave time was 30 min, the ratio of xylan to Ag NO3 was 50 mg: 0.13 mmol, and ammonia concentration was 2%. Under this reaction condition, the Ag NPs were well dispersed with the diameters of 20~35 nm due to the package of xylan. The optimal condition for Au NPs was as follows: the ratio of xylan to HAu Cl4 was 150 mg: 15 mg, reaction temperature was 80 °C and reaction time was 40 min. Under this reaction condition, the Au NPs were well dispersed with the diameters of 10~30 nm. The mechanism of green synthesis of nanometal materials by xylan is as follow: the reducing aldehyde groups of xylan reduced the Ag+/Au3+ to Ag0/Au0, as a result, these reducing aldehyde groups were oxidized to carbonyl groups. Part of the side chains dropped and slightly hydrolysis of backbone happened during this process. Then further reaction promoted the growth of Ag/Au clusters, leading to the formation of Ag NPs/Au NPs. The formed Ag NPs/Au NPs were capped and stabilized by xylan matrix through the high electronegativity of the hydroxyl groups. In addition, the xylan/Ag NPs composites showed high selectivity and sensitivity for Hg2+ detection, and the limit of detection(LOD) was 4.6 n M, and the xylan-Ag NPs composites can also be applied for Hg2+ detection in real water samples. Moreover, the xylan/Au NPs composite exhibited highly selective and sensitive sensing of cysteine in aqueous solution, and the limit of detection(LOD) for cysteine was calculated as 0.57 μM, and the xylan/Au NPs composite can also be applied for Cys detection in human serum. This study not only provides a new way for green synthesis of nanometal material, but also develops cheap, rapid and simple methods for Hg2+ and Cys detection, which achieved the high-valued utilization of hemicelluloses. 3. Synthesis of xylan-click-quaternized chitosan(Xylan-click-QCS) via click chemistry and its application in the preparation of nanometal materialsBased on the above research, to make further high-valued utilization of hemicelluloses, this paper is aim at combining xylan, chitosan and nanometal materials at the same time. In this research, firstly, propargyl xylan was synthesized through nucleophilic substitution reaction between xylan and propargyl bromide in Na OH solution. On the other hand, the tosyl group was introduced onto the 6 position of synthesized quaternized chitosan(QCS), and the azide group replaced the tosyl group to get 6-amido-QCS(QCS-N3). The synthesis conditions of the above reactions were optimized, respectively. Subsequently, the novel Xylan-click-QCS polymer was obtained via click reaction between the terminal alkyne groups on xylan chains and the azide groups of QCS. Then Ag NPs and Au NPs were synthesized by adopting Xylan-click-QCS polymer as reducing and stabilizing agent, and optimized the reaction conditions to get well-dispersed and highly stable nanoparticles. There were two kinds of Ag nanomaterials with diameters of 10~20 nm and 2~5 nm respectively, indicating the formation of Ag nanoclusters except for Ag nanoparticles in this reaction. After separation, the Ag nanomaterials with different diameters can be used in different areas. The diameter of the synthesized Au NPs was 20~30 nm, which possess a more uniform size distribution.