(266a) Cellulose Nanocrystal: Synthesis, Characterization, Dispersion in Organic Media and Surface Modification

Abstract: The Cellulose Nanocrystal (CNC) surface is highly hydrophilic, which hinders its dispersion in organic polymers. However, successful modification of CNCs often requires reactions in apolar organic medium. Here, different polar aprotic solvents, such as acetonitrile, dimethyl sulfoxide, propylene carbonate and acetone, were examined to disperse CNC. Aqueous CNC suspensions at different pHs were freeze dried and redispersed in the organic solvents. From the visual inspection of vials and UV-vis spectra of each CNC suspension of different solvents it was found that dimethyl sulfoxide was the best solvent among them to disperse the CNC and make a stable suspension. Moreover, dimethyl sulfoxide was later used as a reaction medium for the oxa-Michael addition of the primary hydroxyl groups on the CNC surface to N-Benzylmaleimide. Herein, two reaction routes were explored. First, an acid catalyzed method was used where the sulfate ester groups in freeze dried CNC themselves were used as acid catalyst at 100°C. In the second case, the reaction was run in basic medium catalyzed by triethylamine at 70°C. The reaction samples were examined in NMR and FTIR before and after the reaction indicating successful modification in both cases; however the TEA catalyzed reaction resulted in a higher degree of modification. No efforts were made to further remove water other than freeze drying the CNC and redispersing in DMSO.

CNCs for this work were isolated in-house from southern bleached softwood kraft (SBSK) pulp from Mississippi woods by sulfuric acid hydrolysis, as it provides highly stable negatively charged and acidic nanocrystal aqueous dispersions. The size of CNC was measured using AFM and light scattering resulting in an average length of 122 ± 70 nm and height of 6.1 ± 3 nm. Quantification of superficial sulfate groups was performed by conductometric titration with sodium hydroxide; the sulfur content in CNC was found to be 0.41 wt%. The CNC concentration was measured by total organic carbon (TOC), UV-vis spectroscopy and gravimetric techniques.

Results from this work are expected to allow further chemical modifications of CNC with the goal of introducing them in thermoset nanocomposite resins. To the knowledge of the authors, this report is the first time an oxa-Michael addition has been used to modify the surface of CNC.