摘要
本研究利用碱木质素作为生物模板诱导剂,通过水热法成功合成了一种具有良好紫外屏蔽性能的木质素/TiO2复合纳米颗粒。结果表明,木质素/TiO2复合纳米颗粒具有锐钛矿晶型、良好的纳米粒径和热稳定性。同时,证实了木质素的含氧官能团和TiO2表面羟基发生类酯化反应,从而使木质素与TiO2形成包覆嵌顿式结构的复合材料。在此基础上,研究了木质素/TiO2复合纳米颗粒与纳米纤维素水凝胶制备薄膜材料的抗紫外性能。结果表明,添加质量分数10% 的木质素/TiO2复合纳米颗粒的纳米纤维素薄膜在全紫外线波段(200 ~ 400 nm)吸收了约90% 的紫外线,表明木质素/TiO2复合纳米颗粒具有良好的紫外屏蔽能力。
近年来,随着臭氧层的破坏,过度的紫外线辐射对人类健康和物质稳定性都产生了负面影
TiO2表面改性主要为无机涂覆和有机包覆改
木质素是自然界中储量最丰富的芳香族聚合
因此,本研究尝试采用碱木质素为原料,采用水热法制备木质素/TiO2复合纳米颗粒,考察了复合纳米颗粒的晶型结构、粒径分布、表面形貌及热稳定性等性质;利用复合纳米颗粒作为唯一紫外屏蔽组分制备纳米纤维素水凝胶复合薄膜,研究复合薄膜材料的紫外屏蔽性能;以期为利用木质素/TiO2复合纳米颗粒制备紫外屏蔽膜材料提供理论指导。
碱木质素,西格玛奥德里奇(上海)贸易有限公司;钛酸丁酯、无水乙醇,分析纯,杭州高晶精细化工有限公司;浓硫酸,质量分数98%,杭州高晶精细化工有限公司;TEMPO氧化纳米纤维素水凝胶(TOCNF),购自天津科技大学;去离子水,实验室自制。
首先,将30 mL无水乙醇与10 mL钛酸丁酯进行混合,加入2 mL质量分数20%的H2SO4溶液,继续搅拌30 min;取0.5 g碱木质素加入6 mL去离子水中搅拌均匀,将碱木质素水溶液缓慢加入搅拌的钛前驱体溶液中,继续搅拌至混合溶液呈溶胶乳胶状,移至不锈钢内衬双联平行高温高压反应釜中,温度120℃,反应时间2 h。反应结束后,将反应釜中液体倒出,静置24 h后在真空干燥箱中50℃干燥,轻轻研磨至没有硬块得到固体粉末状样品,即为木质素/TiO2复合纳米颗粒,样品标记为lignin/TiO2。设置对照组,上述实验过程中不加入木质素进行平行实验,即得到纳米TiO2颗粒,样品标记为TiO2。
采用激光粒度仪(英国Malvern Zetasizer Nano S型)对lignin/TiO2和TiO2的粒径进行分析。将待测样品颗粒配制成0.01 g/L的乙醇分散液,超声分散后转移至测量池中,每个样品粒径测试3次取平均值。
采用热场发射扫描电子显微镜(FE-SEM,德国蔡司Vitra 55)对样品表面微观形貌进行分析。将检测样品放在导电胶上,抽真空镀金10 min,在分辨率1.7 nm、电压1 kV下对样品进行表面形貌观察。
采用热重分析仪(TGA,美国Perkin Elmer Pyris I型)对制备的lignin/TiO2进行热稳定性分析,称取4 mg的样品于铝制坩埚中,设置升温区间25~800℃,升温速率10℃/min,采用氮气作为保护气体,流速40 mL/min。
将制备的纳米颗粒作为唯一紫外屏蔽组分制备纳米纤维素水凝胶复合薄膜。通过纳米颗粒在纳米纤维素水凝胶基材中的分散情况及复合薄膜的紫外线透过率,综合评定纳米纤维素基紫外屏蔽膜的紫外屏蔽性能。
称取15 g TOCNF,加入15 mL的去离子水稀释,制备若干组,备用;称取10%(相对TOCNF质量)碱木质素、TiO2及lignin/TiO2,分别将不同的材料与TOCNF混合,在磁力搅拌器下常温搅拌30 min,转速300 r/min;将混合搅拌后的液体在300 W功率下超声分散5 min;用溶液浇铸法将混合液体倒入9 cm×9 cm的玻璃皿中,将玻璃皿放入恒温恒湿培育箱,温度30℃,相对湿度80%,放置36 h后取出玻璃皿中薄膜。不加入紫外屏蔽材料的空白水凝胶薄膜命名为TOCNF,其余样品分别命名为lignin/TOCNF、TiO2/TOCNF、lignin/TiO2/TOCNF。
采用紫外可见光分光光度计(Cary 60型,美国安捷伦)对制备的复合薄膜进行紫外线透过率测试,得到不同波长下的透过率曲线,测试波长范围为200~800 nm,扫描速率为240 nm/min。
为探究制备TiO2的晶型及复合前后样品晶型是否发生变化,对制备的TiO2和lignin/TiO2颗粒进行XRD表征,结果如

图1 TiO2和lignin/TiO2的XRD谱图
Fig. 1 XRD spectra of TiO2 and lignin/TiO2
复合材料的粒径大小也是复合材料作为紫外屏蔽剂的重要应用指标,较小的粒径有利于其在基体中的分散,为探究木质素复合后粒径变化,对TiO2、lignin/TiO2和碱木质素进行了粒径分析,结果如

图2 TiO2和lignin/TiO2的粒径分布
Fig. 2 Particle size distribution of TiO2 and lignin/TiO2
由
采用FE-SEM表征了样品的微观形貌,TiO2和lignin/TiO2的FE-SEM图如

图3 TiO2和lignin/TiO2的FE-SEM图
Fig. 3 FE-SEM images of TiO2 and lignin/TiO2
为了明确TiO2与复合材料lignin/TiO2的热稳定性差异,研究了TiO2和lignin/TiO2的TG和DTG曲线,如


图4 TiO2和lignin/TiO2的TG和DTG曲线
Fig. 4 TG and DTG curves of TiO2 and lignin/TiO2




图5 lignin/TiO2的XPS谱图
Fig. 5 XPS spectra of lignin/TiO2




图6 紫外屏蔽膜外观图片
Fig. 6 UV-shielding film appearance pictures
制备的4种复合薄膜的紫外屏蔽效果如

图7 紫外屏蔽膜的紫外透过曲线
Fig. 7 UV transmittance curve of UV-shielding film
本研究利用碱木质素为诱导剂,通过水热法合成了一种具有良好紫外屏蔽性能的木质素/TiO2(lignin/TiO2)复合纳米颗粒,表征了其物理化学性能;并利用其制备了纳米纤维素水凝胶复合薄膜,研究了其紫外屏蔽性能。
3.1 由碱木质素为模板合成的lignin/TiO2复合纳米颗粒实现了TiO2被木质素以化学键连接形式包覆,且复合纳米颗粒具有良好的热稳定性。
3.2 lignin/TiO2复合纳米颗粒具有片层状微观形貌,为锐钛矿型TiO2改性复合材料,粒径分布均匀,与生物基材具有较好的相容性,可以作为紫外屏蔽组分加入有机高分子材料中,实现紫外屏蔽功能。
3.3 lignin/TiO2复合纳米颗粒作为唯一紫外屏蔽组分的lignin/TiO2/TOCNF纳米纤维素水凝胶膜体现了较好的紫外屏蔽性能,在可见光区透光性良好,且紫外屏蔽作用强,可吸收近紫外区90%的紫外线。
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