Phase Behavior of Acetylated Cellulose Nanocrystals and Origins of the Cross-Hatch Birefringent Texture.

ABSTRACT

Cellulose nanocrystals hydrolyzed by hydrochloric acid and esterified by acetic acid produce acetylated cellulose nanocrystals (CNC-AA) with acetyl ester surface functional groups. While much attention has been paid to understanding the phase behavior (liquid crystal) of aqueous dispersions of sulfonated nanocrystals, relatively few studies have focused on CNC-AA dispersions. CNC-AA dispersions exhibit multiple phase regimes and markedly different phase behavior due to their lower surface charge. At concentrations above 5.0 × 10-4%vol, a decrease in the apparent diffusion coefficient indicates the onset of interparticle interactions and a transition from the dilute regime. From 0.003 to 0.31%vol, biphasic behavior is observed, consisting of a birefringent lower phase and disordered or isotropic upper phase. Small-angle neutron scattering was used to measure the growth of fractal structures with increasing concentration and indicates a two-dimensional assembly with short-range order in a plate-like assembled geometry. Above 0.31%vol, the dispersion transitions exhibit a cross-hatch birefringent texture, which is believed to exist as a fine-scale nematic that possesses frozen-in flow shear behavior, consistent with a glassy phase. This cross-hatch pattern is maintained in dried films, where atomic force microscopy and scanning electron microscopy reveal a layered sheet-like structure. Imaging also indicates that the basic unit of CNC-AA microstructure in the film consists of 0.5-1.5 µm scale aligned nanorod domains, which agrees with neutron scattering and the dimensions of each individual "hatch" in the birefringent texture observed by cross-polarized microscopy. The assembly of the nanorods into this layered structure and the fine-scale nematic birefringent cross-hatch texture is of significant fundamental interest, particularly since it differs greatly from cellulose nanocrystals with other surface chemistry and offers potential opportunities in other applications owing to the unique assembly.