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

Abstract

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, 13 C 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 co-products 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 by-product 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 T 10% and T 50% values above 256 °C and 323 °C, respectively. Thus, they can be used for melt-processing operations performed at moderately high temperatures without thermal decomposition. Nanocomposites of polyvinyl alcohol with modified CNCs (1 wt% malonate-, malate-, citrate and unmodified CNCs) wereprepared. An increase in the thermal decomposition temperature by almost 40 °C was obtained for PVOH-citrate modified CNC nanocomposites.Furthermore, since TGA determined weight loss up to 150°C is attributed to loss of bound water, this provides a tool to determine CNC water affinity. By 150°C, the weight loss of modified and non-modified CNCs is 6% and 2%, respectively. Consequently, modified CNCs with surface carboxylate groups have higher water affinity than non-modified CNCs. Moreover, significant differences in the thermal stability are observed as a function of the di-or triacid used for CNC modification. Based on the peaks of the derivative thermogravimetric curve (DTG, T 50% ), modified CNCs have the following thermal stability: malonate = HCl > malate > citrate CNCs.Corresponding values for T 50% are 366°C, 365°C, 350°C and 345°C, respectively. The amount of residual char at 600°C for HCl, malonate, malate and citrate CNCs is 8, 13, 16 and 20%, respectively. Hence, it follows that increasing the T 50% for di-and tri-acid modified CNCs results in correspondingly lower char formation. Increased char amounts is likely due to CNC functionalizations that lead to relatively larger number of cross-linking events at elevated temperatures.Since the exclusion of air during melt processing is generally not practical, the effect of CNC modification on thermal stability in air was also determined and the correspo...

One-Pot Water-Based Hydrophobic Surface Modification of Cellulose Nanocrystals Using Plant Polyphenols

“…The unmodified CNCs showed typical 2θ peaks at 17.3°, 19.4°, 26.2° and 40.5° that correspond to the crystal planes of 101, 10ī, 002, and 040 in the crystal structure of cellulose I. 12,54 Though these 2θ peak values are relatively higher than the ones commonly reported in literature, we attribute this to the fact that we used Co radiation instead of Cu. Since d-spacing is radiation source independent, the d-spacing values at these 2θ peaks were calculated to be 0.60, 0.53, 0.39, and 0.26 nm, respectively, which agrees well with the literature.…”

Functional nanomaterials through esterification of cellulose: a review of chemistry and application

“…The other example of in situ esterification is the production of surface modified CNCs via one-pot reaction methodology, which combines organic acidcatalyzed Fischer esterification and concurrent cellulose acid hydrolysis of amorphous cellulose chains (Fig. 3a) Braun et al 2012;Sobkowicz et al 2009;Spinella et al 2016;Yu et al 2016). For example, acetylated and butylated CNCs were synthesized using acetic or butyric acid that both serves as the reaction solvent and reagent for the esterification at 105°C with the presence of hydrochloric acid as catalyst and hydrolyzing agent .…”

“…The resulting acetylated and butylated CNCs with size of 200-300 nm in length and about 20-50 nm in width are of similar dimensions compared to those obtained by hydrochloric acid hydrolysis alone. Meanwhile, via this one-pot technique using a catalytic quantity of hydrochloric acid and a bio-based organic acid, such as, citric, malic or malonic acid, various anionic carboxylated CNCs were extracted (Spinella et al 2016). Thus, this onepot reaction methodology is quite versatile because the organic acids used for the Fischer esterification can be selected to introduce the desired functionalities.…”

Poly(ε-caprolactone) Biocomposites Based on Acetylated Cellulose Fibers and Wet Compounding for Improved Mechanical Performance

“…Acetylation has been considered as a scalable method practiced in industry to improve cellulose adhesion to hydrophobic matrices, 13,14 and for its promising potential as a new green emerging method. 15,16 Degree of substitution (DS) values below 0.48 are recommended for composite applications because higher DS values result in decreased cellulose crystallinity and degree of polymerization. 17,18 In addition to compatibility issues, there are other processing challenges with cellulosic fibers related to drying.…”