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One-Pot Preparation of Carboxylated Cellulose Nanocrystals and Their Liquid Crystalline Behaviors

Zhou, Lijuan, Li, Na, Shu, Jie, Liu, Yunxiao, Wang, Kuntao, Cui, Xiang, Yuan, Yuan, Ding, BeiBei, Geng, Yong, Wang, Zhaolu, Duan, Yongxin, Zhang, Jianming
ACS sustainable chemistry & engineering 2018 v.6 no.9 pp. 12403-12410
atomic force microscopy, carboxylation, cellulose, cost effectiveness, crystal structure, hydrolysis, liquids, manganese, nanocrystals, oxalates, oxalic acid, oxidants, polymers, potassium permanganate, pulp, reducing agents, sulfates, sulfuric acid
Carboxylated cellulose nanocrystals (CNCs-COOH) have attracted great attention for their potential applications in reinforcing polymer materials and surface modification. Herein, we developed a low-cost approach to prepare CNCs-COOH from pulp with high yield at mild reaction conditions (50 °C, 1 wt % sulfuric acid medium) using potassium permanganate (KMnO₄) and oxalic acid (OA, H₂C₂O₄) as the oxidizing and reducing agents, respectively. The oxidant dosage in this strategy is much lower than that in a conventional TEMPO method, and the yield of CNCs-COOH can reach as high as 68.0%, with a carboxylate content of 1.58 mmol/g. In this reaction system, the presence of the OA can complex with Mn³⁺ to form [Mn(C₂O₄²–)]⁺ and prevent the Mn³⁺ from being reduced to Mn²⁺, leading to the strong oxidizing capacity of the reaction system maintained for a longer time. Atomic force microscopy analysis showed that rod-like CNCs were obtained with an average size of 10–22 nm in diameter and 150–300 nm in length. The crystal structure of as-prepared CNCs-COOH was nearly unchanged, and the crystallinity was 89.2% based on WAXD analysis. Of particular interest, CNCs-COOH suspension with high concentration (>6 wt %) also exhibited the same intriguing chiral nematic liquid crystalline self-assembly behaviors as sulfate CNCs prepared by traditional H₂SO₄ hydrolysis method. This study provides an efficient and cost-effective way to fabricate CNCs-COOH, leading to great potential applications in constructing advanced functional material.