Abstract

Acetylation of cellulose nanocrystals (CNCs) is one of the most available routes to enhance hydrophobicity of CNCs. This study investigated the impact of CNC allomorphs on the acetylation. CNC samples with different allomorphs (cellulose I, II and III) were prepared and surface modified by acetylation in heterogeneous system. Then, CNCs and acetylated CNCs (ACNCs) were characterized by solid-state CP/MAS ¹³C NMR spectroscopy, X-ray diffraction, atomic force microscope and thermogravimetric analysis. It was found that all obtained ACNC allomorphs kept their original intact crystal forms and rod-like structure. The degree of substitution of ACNC I was highest and up to 1.98, followed by ACNC III′, and ACNC II′ was lowest which only came to 0.32 after the reaction of 3 h. In the process of acetylation, the crystallinities of CNC allomorphs decreased, in which CNC II dropped least and CNC III dropped most. Both allomorphic transformation and acetylation improved the thermal stability of native CNC I. This comparative study about CNC allomorphs can offer a new sight for their morphologies and internal properties and also guide for applications like composite materials.

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摘要

纤维素纳米晶体（CNC）的乙酰化是增强CNC疏水性的最可行途径之一。这项研究调查了CNC同种异形体对乙酰化的影响。制备了具有不同同素异形体（纤维素I，II和III）的CNC样品，并在异质系统中通过乙酰化对其进行了表面改性。然后，通过固态CP / MAS ^13对CNC和乙酰化CNC（ACNC）进行表征C NMR光谱，X射线衍射，原子力显微镜和热重分析。发现所有获得的ACNC同种异形体均保持其原始完整的晶体形式和棒状结构。ACNC I的取代度最高，最高为1.98，其次是ACNC III'，ACNC II'最低，反应3 h后仅达到0.32。在乙酰化过程中，CNC同种异形体的结晶度下降，其中CNC II下降最少，CNC III下降最多。同种异形转化和乙酰化都改善了天然CNC I的热稳定性。这项关于CNC同种异形的比较研究可以为其形貌和内部特性提供新的视角，并为复合材料等应用提供指导。

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