Aqueous dispersions of sulfonated cellulose nanocrystals (CNC) were investigated using cross-polarized optical microscopy both prior to shear and during post-shear relaxation. Sulfonated CNC produced via the sulfuric acid hydrolysis of biomass are an intriguing nanomaterial for several reasons: high specific strength, greater than 100 GPa Young’s modulus, renewability, increasing commercial availability, and easy processability. Aqueous dispersions of these CNC readily form cholesteric (chiral nematic) liquid crystals (LCs); retention of this microstructure in solidified films enables them to have photonic properties. In contrast, sheared dispersions can be cast to form films with nematic-like microstructures that enables mechanical anisotropy. Similar behavior exists for model cholesterogenic mesogens such as hydroxypropyl cellulose (HPC). Understanding relative balance of chiral and shear forces on CNC dispersion microstructure is key to achieving the desired microstructure in films produced from these dispersions. This research compares the quiescent microstructures and relaxation dynamics of HPC and CNC dispersions using a Linkam shear cell and cross-polarized optical microscopy. A range of gap heights were used to evaluate the effects of confinement on the formation of the cholesteric helix after shear cessation. In addition to qualitative comparisons, changes in the degree of alignment relative to flow direction were quantified optical contrast measurements. Differences in the time scale of post-shear relaxation were evaluated based on the differences in rheological properties, mesogen size, and cholesteric pitch. These results were used to aid the development of a robust model for predicting the three dimensional microstructure of cholesteric liquid crystals during post-shear relaxation.

Comparing the rheo-optics of aqueous sulfonated cellulose nanocrystal and model cholesteric dispersions

255th National Meeting (2018)

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