壳聚糖的选择性氧化反应及其增强纸页机理研究

Periodate oxidation of chitosan leads to the open-ring reaction of the sugar ring between C2 and C3 and the formation of dialdehyde groups. This oxidized chitosan product has great potential applications as papermaking strength additives and biomaterials. However, excessive depolymerisation of chitosan can occur during the oxidation reaction, which affects the product properties negatively. Little has been known about the reaction mechanism of the periodate oxidation of chitosans and the depolymerisation of the glucosidic bond of the chitosan backbone during the oxidation reaction. In the proposed research, chitosan will be modified chemically by grafting different functional groups onto the chitosan backbone for better control of the oxidation and the depolymerisation reactions of chitosan. The amino-group of C3 and the hydroxyl group of C6 of the sugar unit of the chitosan macro molecular chain would be the target reaction sites for the grafting of the functional groups. In the first part of the proposed research, kinetic models for both the oxidation and depolymerisation reactions of chitosan during the periodate oxidation reaction will be developed, in order to better understand the reaction mechanism and optimize the process. In the second part of the proposed research, a novel paper strength additive will be synthesized by treating the oxidized chitosan with a Girard reagent to impart to it quaternary ammonium groups. This product will be applied to paper in the papermaking process and its effects on the paper properties will be investigated. Resdults of the proposed research will provide useful information on the production and applications of chitosan products.

利用高碘酸钠选择性氧化壳聚糖，将糖环C2C3键打开，产生醛基基团，制得的壳聚糖氧化产物在造纸工业和生物材料等领域具有广阔的应用前景。但反应过程中，高碘酸钠选择性氧化壳聚糖反应机制及分子主链糖苷键降解机理尚不明确，制约了对壳聚糖氧化产物结构性能的有效控制和产率的提高。本项目通过改变反应条件，在壳聚糖主链的糖环C3氨基及C6羟基上接上不同的功能基团，来控制壳聚糖氧化度及分子主链的降解。通过机理分析及反应条件优化，拟解决高氧化度下壳聚糖主链糖苷键降解问题，为缓解多糖分子链糖苷键在氧化过程中的降解提供理论依据。在此基础上，将壳聚糖氧化产物接枝吉拉德试剂制备出新型的壳聚糖季铵盐，分析壳聚糖氧化产物及壳聚糖季铵盐对纸页增强作用机理，为优化氧化壳聚糖制备造纸增强剂的工艺提供理论指导，从而促进壳聚糖类造纸助剂的生产技术和应用水平提高。研究成果也可为扩展壳聚糖在生物材料及水果和食品包装应用提供理论基础。

壳聚糖氧化产物和壳聚糖衍生物在造纸工业和生物材料等领域具有广阔的应用前景。但高碘酸钠选择性氧化壳聚糖反应机制及分子主链糖苷键降解机理尚不明确，制约了对壳聚糖氧化产物结构性能的有效控制和产率的提高。探讨了高碘酸钠氧化制备双醛壳聚糖（D-CTS）的影响因素和反应机理，结果表明D-CTS的氧化度和反应液pH值对纳米粒子粒径分布及微观结构有重要影响。通过选择性氧化改性制备双醛羧甲基壳聚糖(DCMCS)，发现在DCMCS的C2、C3位均有醛基生成，且氧化中伴随着C6位部分羧基的断裂，DCMCS的结晶度也随之明显下降。分子颗粒尺寸减小且趋于球形。用绿色环保的碳酸二甲酯作为甲基化试剂两步法可得到N,N,N-三甲基壳聚糖（TMC）,解决了目前大多采用毒性较大的碘甲烷或硫酸二甲酯试剂进行季铵化。利用D-CTS稳定还原纳米银粒子的方法制备纳米银,结果表明纳米银颗粒与D-CTS之间形成了四配位的配合物，具有较好面心立体晶体结构，具有良好的稳定性。D-CTS中的醛基和氨基是稳定纳米银粒子的主要基团。利用乙酰肼三甲基氯化铵（GT）与醛基发生接枝反应，可制备高阳离子双醛壳聚糖（CDCTS）。阐释了改性壳聚糖及其复配物提高纸页强度的作用机制。增强机理主要是通过提高壳聚糖类增强剂所含的羟基、氨基与纤维之间的氢键结合，使氢键结合增加同时也增加了纤维间的结合面积；二是壳聚糖类增强剂所带的阳电荷能够提高浆料细小纤维的留着，提高了纸页的匀度，从而也提高了纸强度。将改性壳聚糖涂布在纸页表面，纸页表面形成了致密的疏水膜层，可大幅提高涂布纸的水蒸气阻隔性能，纸页强度亦有大幅增加。壳聚糖的成膜微观结构是影响纸张力学性能和抗油脂性能的重要因素。醛基是新生成的反应活性点，能提高D-CTS与其它造纸助剂复配性能，改善强度和阻隔性能并降低壳聚糖使用成本，也可制备性能各异的生物材料。因此利用高碘酸钠选择性氧化壳聚糖制备的双醛壳聚糖（D-CTS）在造纸工业和其他领域中都具有重大的应用前景。