Concurrent Cellulose Hydrolysis and Esterification to Prepare a Surface-Modified Cellulose Nanocrystal Decorated with Carboxylic Acid Moieties

Cellulose nanocrystals (CNCs) were modified with natural di- and tricarboxylic acids using two concurrent acid-catalyzed reactions including hydrolysis of amorphous cellulose segments and Fischer esterification, resulting in the introduction of free carboxylic acid functionality onto CNC surfaces. CNC esterification was characterized by Fourier transform infrared spectroscopy, 13C solid state magic-angle spinning (MAS), and conductometric titration experiments. Average degree of substitution values for malonate, malate, and citrate CNCs are 0.16, 0.22, and 0.18, respectively. Despite differences in organic acid pKa, optimal HCl cocatalyst concentrations were similar for malonic, malic, and citric acids. After isolation of modified CNCs, residual cellulose coproducts were identified that are similar to microcrystalline cellulose based on SEM and XRD analysis. As proof of concept, recycling experiments were carried to increase the yield of citrate CNCs. The byproduct was then recycled by subsequent citric acid/HCl treatments that resulted in 55% total yield of citrate CNCs. The crystallinity, morphology, and substitution of citrate CNCs from recycled cellulose coproduct is similar to modified citrate CNCs formed in the first reaction cycle. Thermal stability of all modified CNCs under air and nitrogen resulted in T10% and T50% values above 256 and 323 °C, respectively. Thus, they can be used for melt-processing operations performed at moderately high temperatures without thermal decomposition. Nanocomposites of poly(vinyl alcohol) with modified CNCs (1 wt % malonate, malate, citrate, and unmodified CNCs) were prepared. An increase in the thermal decomposition temperature by almost 40 °C was obtained for PVOH-citrate-modified CNC nanocomposites.