氨基改性纤维素纳米晶体的自组装行为研究

Self-assembly Behavior of Amino Modified Cellulose Nanocrystals

采用硫酸水解法制备纤维素纳米晶体(CNC),然后用环氧氯丙烷、氨水依次对其表面改性得到氨基改性的CNC (CNC-NH2),进一步采用蒸发诱导自组装(EISA)法制备CNC-NH2薄膜.通过偏光显微镜(POM)、原子力显微镜(AFM)、扫描电子显微镜(SEM)和流变测试对CNC-NH2悬浮液和薄膜的自组装行为进行研究.结果表明,CNC-NH2为棒状粒子,氨基的引入提高了其热稳定性.随着CNC-NH2悬浮液浓度增大,手性向列型液晶相逐渐形成,且干燥成膜后仍然能保持织态结构.流变测试表明,CNCNH2悬浮液显示独特的流变行为,可以观察到明显的相转变浓度和相转变温度.此外,考察了超声时间对CNC-NH2悬浮液成膜的影响.随着超声时间的延长,CNC-NH2膜由无色透明向虹彩色转变,且具有虹彩现象的膜对光的特殊波段产生吸收.

Cellulose nanocrystals(CNCs) are a class of sustainable renewable nanomaterials with many advanced features, such as biodegradability, biocompatibility, large specific surface area, and outstanding mechanical properties. They have attracted significant interest due to their fascinating self-assembly behavior. In this work, amino modified CNCs(CNC-NH2) were prepared by reacting CNCs with epichlorohydrin and then with ammonia. The structure, morphology and self-assembly behavior of CNC-NH2 in aqueous suspensions and solid films were investigated by UV-Vis spectroscopy, DLS, XRD, XPS, POM, AFM, SEM, TEM and rheology testing. The degree of substitution of amino groups was deternmined to be 0.048. CNC-NH2 showed a rodlike morphology with the length(L) of(180 ± 75) nm and width(W) of(13 ± 5) nm, and the average aspect ratio(L/W)was nearly 13.8. CNC-NH2 could be well dispersed in water and the ζ-potential was determined to be-28.0 mV,which could be stored for a long time due to the strong electrostatic repulsion. The chiral nematic liquid crystal was observed in the CNC-NH2 suspensions as the solid content increased up to 1.75 wt%. The CNC-NH2 suspensions displayed unique rheological properties, which were divided into three regions: isotropic, biphase and liquid crystal, with increasing solid content. Moreover, the evaporation-induced self-assembly(EISA) method was used to prepare CNC-NH2 films. It was found that the liquid crystalline structure of CNC-NH2 in colloidal suspension still maintained in the solid films. The iridescent color of the films could be tuned by the initial content of the suspension and the ultrasonic time. The iridescence and colored films were obtained only if the pitch length in the solid film was approximately that of the wavelength of visible light. This work provides a new way for the preparation of cellulose liquid crystal materials, and helps to understand the formation mechanism of the liquid crystal of the rodlike nanoparticles.