摘要
利用高碘酸钠选择性氧化壳聚糖，将吡喃糖环上的C2C3位的共价键打开，引入醛基基团，制得的壳聚糖氧化产物（双醛壳聚糖D-CTS）在造纸工业及生物材料等领域，具有广阔的应用前景。D-CTS对金属离子具有螯合作用，含有的醛基可以进一步还原金属离子，因此可用D-CTS来稳定还原制备纳米粒子。由于在D-CTS中引入了醛基，醛基具有一定的交联作用，能提高D-CTS与其它造纸助剂（如阳离子淀粉和聚丙酰胺）的复配性能，降低壳聚糖使用成本。将D-CTS与阳离子淀粉（CS）复配制成D-CTS/CS复合施胶剂，用于纸张表面涂布，可以增强纸张力学性能、抗油脂性能及水蒸气阻隔性能，从而扩大纸张在食品包装中应用。D-CTS中的醛基是新生成的反应活性点，可以进一步进行接枝反应，制备性能各异的生物材料。
在高碘酸钠选择性氧化壳聚糖反应过程中，高碘酸根/壳聚糖（IO4/GlcN）摩尔比对壳聚糖氧化产物具有重要影响。反应过程中，高碘酸钠并不是完全消耗在壳聚糖C2C3开环氧化上，部分高碘酸钠消耗在壳聚糖主链降解上。当壳聚糖氧化度达到46.3%后，继续增加高碘酸钠含量，氧化度增长缓慢，这可能是由于此时生成了较多的醛基，醛基对附近糖单元的氨基和羟基存在键能影响，阻碍了反应的进一步进行。氧化反应较合适的pH值在3-4之间，pH值过低，引起氨基质子化，阻碍反应进行；pH值过高，壳聚糖呈溶胀状态，阻碍高碘酸根进入壳聚糖内部参与反应。壳聚糖的氨基基团乙酰化，对分子链降解具有一定的保护作用。XRD表明，随着氧化度提高，壳聚糖晶体Ⅰ、Ⅱ的衍射吸收峰（2θ=11.2，2θ=20.1）消失。壳聚糖的吡喃环的开环数目增加，削弱了壳聚糖分子之间氢键作用，壳聚糖结晶形态遭到破坏，呈弥散状态。DSC分析表明，随着D-CTS氧化度提高，壳聚糖的热稳定性降低，这主要是由于壳聚糖的吡喃糖环发生了开环反应，破坏分子结晶形态，使分解放热峰温度降低。TG分析表明，随着氧化度提高，D-CTS分解起始温度逐渐降低。
双醛壳聚糖中的氨基和醛基能提高壳聚糖对金属离子的螯合作用，同时醛基可以在纳米银制备过程中充当还原剂。利用D-CTS稳定还原纳米银粒子是一种绿色环保的纳米银制备方法。D-CTS的氧化度及pH值对纳米银颗粒的粒径和形态有重要影响。利用紫外光谱和动态光散射仪（DLS）对纳米银粒径和形态进行分析，当D-CTS的氧化度为32.3%，pH=3时，纳米银颗粒的粒径最小，在30-40nm之间。XRD显示纳米银颗粒具有37.45°和44.47°的晶体衍射峰，表明生成的纳米银颗粒具有(111)和(200)的面心立体结构。FTIR分析表明，稳定纳米银颗粒的主要基团是醛基和氨基，通过Job滴定法测试出纳米银颗粒与D-CTS之间形成了四配位的配合物。制备的纳米银颗粒具有较好的稳定性，常温条件下3个月内没出现明显的聚集现象。
利用高碘酸钠氧化壳聚糖制备D-CTS，由于D-CTS具有醛基，具有较好的交联复配性能，能提高D-CTS与其他造纸助剂的相容性。将双醛壳聚糖与阳离子淀粉复配来制备双醛壳聚糖/阳离子淀粉复合施胶剂（D-CTS/CS），并用于纸张涂布，可以改善涂布纸的包装性能。随着D-CTS氧化度增加，涂布纸的抗张强度、抗油脂性能明显增强。当施胶剂pH值在5.5-6.5、D-CTS含量为0.3wt%、干燥温度为80℃、环境温度为20-30℃及环境湿度为50-60%时，涂布纸具有最好的抗油脂性能，抗油脂指数达到10。随着D-CTS氧化度增加，涂布纸的水蒸气透过率有一定的降低。增加施胶剂涂布量能大幅提高涂布纸的水蒸气阻隔性能。阳离子淀粉经D-CTS复配后，ATR-FTIR中的C-ONR2、C-O-C的特征吸收峰明显增强，表明D-CTS与阳离子淀粉、纤维素之间的交联程度明显增加，相容性增强。对D-CTS/CS进行热力学分析，随着D-CTS的加入，TG失重起始温度降低至230℃；DSC放热区域（230-365℃）先增大后减小，热力学测试结果与纸张样品力学性能变化规律基本一致。随着D-CTS的加入，复合施胶剂的成膜性能增强，涂布纸的表面平滑度有了大幅的提高。SEM分析表明：D-CTS/CS施胶后，纸张纤维排列紧密、纤维之间孔隙缩小。D-CTS/CS施胶后的纸张水接触角减少，0.1-0.5s内接触角下降较缓慢，表明涂布后的纸张表面形成了一定的薄膜，纸张表面孔隙减小，水滴在纸张表面渗透减缓，纸张抗水性能提高。液体石蜡在涂布纸表面是铺展的，0.1-0.5s内接触角下降值较小，表明D-CTS的加入对纸张表面能影响不大，纸样的表面对油滴存在一定的附着作用，这种附着作用与施胶剂的静电作用力有关，它能阻碍油滴在纸张中渗透，从而提高纸张的抗油脂性能。
D-CTS在造纸中应用时，由于其分子量较小，架桥能力较差，限制了它作为浆内添加增强剂应用。为了提高D-CTS的架桥性能，利用乙酰肼三甲基氯化铵（GT）与醛基发生接枝反应，制备阳离子双醛壳聚糖（CDCTS）。GT中的叔氨基团可以提高D-CTS的阳离子强度及增强D-CTS的架桥性能，提高纸张留着率及纸张力学性能。另外，叔氨基团引入可以进一步提高纸张的抗菌性能。当GT/CHO（醛基）摩尔比为2：1、D-CTS醛基含量为92.6%、反应温度60℃及反应时间为30min时，接枝率可达52.1%。FTIR及1H-NMR分析表明，GT已经接枝到D-CTS上。随着CDCTS接枝率增加，支链数量及阳离子浓度增加，CDCTS架桥能力提高。CDCTS与纤维及填料等粒子之间形成了物理架桥作用，提高了细小纤维、纤维及填料之间的作用力。CDCTS加入提高了对填料CaCO3助留性能，使纸张灰分含量上升。Zeta分析表明，与D-CTS相比，有更多的CDCTS被吸附到纤维及细小纤维的表面，使纤维表面强度增加，从而使纸页的力学性能增强。随着CDCTS接枝率从0%增加到52.1%，纸张对对金黄色葡萄菌（S. aureus）抗菌指数从76.2%增加到98.3%，增长明显；纸张对大肠杆菌（E. coli）抗菌指数从67.4%增加到78.4%，接枝率在纸张对E. coli抗菌性能方面影响较小。随着CDCTS浓度增加，纸张S. aureus和E. coli的抗菌指数都增加，S. aureus的抗菌增强效果更明显。CDCTS拥有更多的支链，更容易在S. aureus细菌表面进行交错沉积，形成一个阻隔膜层，从而阻碍养分及其它物质在S. aureus细胞壁中通过，抑制细菌生长。拥有较多支链的CDCTS，不容易渗透进入E. coli细胞壁内部，使CDCTS增强效果趋缓。
In the selective oxidation of chitosan using sodium periodate, chemical bonds betweenC2and C3in the pyranose ring were opened. Meantime, the aldehyde group is introducedinto the oxidation products of chitosan. The oxidation products of chitosan by sodiumperiodate were always called dialdehyde chitosan (D-CTS), which shows broad applicationprospects in the paper industry and biomaterials science. D-CTS has the chelation capacity ofmetal ion. Aldehyde group in D-CTS can be further reduced. Therefore, D-CTS can be usedto stabilize and prepare nanoparticles. Aldehyde group in D-CTS plays a role in improvingcomplex formulation with other additives for papermaking (such as cationic starch andpolyacrylamide), which can lower cost of chitosan. The D-CTS and cationic starch (CS) couldbe mixed to prepare a sizing agent, which can enhance the mechanical properties, water vaporbarrier properties and grease resistance. The coated paper has broad application in thepackaging industry. The aldehyde D-CTS is a new active site, which can be grafted functiongroups to prepare biomaterials with different functions.
The molar ratio of IO4/GlcN has an important impact on the oxidation products in theselective oxidation of chitosan using sodium periodate. Sodium periodate is not entirelyconsumed in the ring-opening reaction between C2and C3in the pyranose. It is partiallyconsumed in degradation of backbone of chitosan. When the degree of oxidation (DO) isabove46.3%, DO continues to grow slowly as the content of sodium periodate increases. Thismay be due to more aldehyde generated in the oxidation reaction, which may influence theamino and hydroxyl groups in the sugar ring. The optimal pH value in the oxidation reactionis between3and4. Lower pH value would make amino groups to be protonated, which couldhinder the reaction process. Higher pH value would make chitosan swelling, which couldprevent periodate permeating into the internal of chitosan molecules. Amine groups inchitosan are acetylated, which can reduce degradation of the molecular chain. XRD analysisshows crystal diffraction peak absorption peak (2θ=11.2，2θ=20.1) disappears with thedegree of oxidation raised. With the improvement of DO, the number of open-loop ofpyranose increases, which would weaken the hydrogen bonding between molecules ofchitosan. Chitosan would be amorphous state after the oxidation. DSC analysis shows thethermal stability of D-CTS is reduced. This is mainly because of the ring-opening of chitosanand corrupted crystalline morphology. TG analysis indicates that the decompositiontemperature of D-CTS decreases as the degree of oxidation raised.
A simple, green method was developed for the synthesis of silver nanoparticles (AgNPs)by using dialdehyde Chitosan (D-CTS) as the reducing and stabilizing agents. Amino groupsand aldehyde groups in D-CTS can improve the chelation capacity of metal ion. Themorphology and size distribution of the AgNPs were found to vary with the dialdehydecontent of D-CTS and the pH value of the reaction solution. The synthesized AgNPs werecharacterized by UV-Vis spectroscopy and dynamic light scattering (DLS). When the degreeof oxidization was32.3%and the pH value of the reaction solution was3, AgNPs possessed aminimum size of30-40nm. XRD results indicated the presence of nano-silver had the face-centered cubic structure (111) and (200) corresponding to crystal diffraction peaks(37.45°and44.47°). SEM results showed that nano-silver particles of30to40nm in sizewere homogeneously dispersed in the solution. FT-IR spectra revealed that the aldehydegroups and the amino groups were the major agents that stabilizing the AgNPs. The possiblemechanism of D-CTS on the reduction and stabilization of AgNPs analyzed by Job methodmay be due to the formation of four coordinate complexes. The synthesized AgNPs remainedstable for more than three months.
Due to the aldehyde groups generated in the oxidation of chitosan using sodiumperiodate, D-CTS has a better cross-linking properties, which can improve the compatibilitywith other additives for papermaking. The sizing agent of D-CTS/CS was prepared by mixingcationic starch with D-CTS. The package performances of the paper coated by D-CTS/CScould be improved, which can broaden the applications in food packaging. As the DO ofD-CTS increases, the tensile strength and grease-resistance index of coated paper increasedramatically. When the concentration of D-CTS, pH values of the sizing agent, dryingtemperature, ambient temperatures and ambient humidity are0.3wt%,80℃,20-30℃and50-60%respectively, the coated paper possesses the best grease-resistance index (the greaseresistance index could reach10). As the DO of D-CTS increases, WVP of coated paperdecreases. The increasing coating weight can improve the water vapor barrier propertiesdramatically. ATR-FTIR analysis indicates: the absorption peaks of C-ONR2and C-O-Cgroups increase significantly. It shows that the degree of crosslinking among D-CTS, CS andfibers is raised. TG analysis of D-CTS/CS shows that the beginning temperature of weightloss is230℃. DSC analysis shows that the exothermic area (230-365℃) first increases andthen decreases, which is in accordance with the changes of mechanical properties of thecoated paper. With the addition of D-CTS, the surface smoothness of coated paper has beensubstantially improved. It shows that the film formation performance of the coated paper hasbeen greatly improved. SEM analyses show that the surface of the paper coated by D-CTS/CSbecomes smoother. The pores among the fibers become smaller. The contact angle of waterreduced after the paper coated by D-CTS/CS. When the contact time is between0.1s and0.5s,the contact angle decline more slowly. It shows that the film is formed on the surface of thepaper. Meantime, the paper surface porosity decreases. The water droplets on the surface ofpaper permeate slowly, which indicates the water resistance of the coated paper increases.Liquid paraffin spreads on the surface of coated paper. When the contact time is between0.1sand0.5s, the contact angle declines very slowly. It indicates that the sizing agent has slighteffect on the surface energy. There is an attraction between oil droplets and the surface of thecoated paper. This attraction may be related to the electrostatic force between oil droplets andthe sizing agent, which can impede the oil droplets penetrating into the inside of paper.
D-CTS has difficulty in wet-end as strengthening agents, due to its small molecularweight and poor bridging capabilities. In order to enhance the bridging properties of D-CTS,Girard's Reagents (GT) were used to graft onto the aldehyde groups in D-CTS to synthesis ofcationic dialdehyde chitosan (CDCTS). The tertiary amine groups in GT reagents can improvepositive ion intensity and mechanical properties of the paper. Meantime, the grafted polymerwith better bridging performance can improve retention properties. In addition, the tertiary amine groups in GT reagents can further enhance the antibacterial properties of the paper.When the molar ratio of GT/CHO, the aldehyde content of D-CTS, the reaction temperatureand the reaction time is2,92.6%,60℃and30min, the percentage of grafting (PG) can reach52.1%. FTIR and1H-NMR analysis show that GT reagents have been grafted onto the D-CTS.As the PG of CDCTS increases, the number of branched-chain and electrolyte concentrationsis raised. With the bridging performance of CDCTS improved, more physical bridges could beformed between fillers and fibers. The physical bridges can improve the force among finefibers, fibers and fillers. At the same time, the ash content of the paper went up for theimproved retention capacity of CaCO3. Zeta analysis showed that there was more CDCTSadsorbed to the surface of fibers compared with D-CTS. Because the wall surface of the fiberincreases, the mechanical properties of paper were improved. With the PG of CDCTSincreasing from0%to52.1%, the antibacterial index against S. aureus was raised from76.2%to98.3%. At the same time, the antibacterial index against E. Coli increased from67.4%to78.4%. Compared with S. aureus, the PG has less effect on the antibacterial index againstE. Coli. With the concentration of CDCTS increasing, the antibacterial indexs againstS. aureus and E. Coli were improved significantly. CDCTS with more branches is more likelyto form a barrier coating on the surface of S. aureus, which can prevent nutrients and othersubstances passing through the cell wall of S. aureus. It may inhibite the bacterial growing.On contrary, CDCTS with more branches has difficulty in penetrating into the cell wall ofE. Coli, which can slow growth of antibacterial properties against E. Coli.
引文
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