Effect of Ionic Strength on Cellulose Nanocrystal Dispersion Viscosity and Surface Modification of Thin Film
Type of DegreeMaster's Thesis
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The primary aim of this research was to explore the effect ionic strength on aqueous cellulose nanocrystal (CNC) dispersions with a range of concentrations spanning isotropic, biphasic, and liquid crystalline regimes. The CNCs used in this research were extracted from woody biomass using sulfuric acid hydrolysis and obtained from the US Forest Products Lab. This research was partially motivated by an unusual rheological characteristic of lyotropic CNC dispersions. When viscosity is plotted as a function of mesogen concertation, lyotropic liquid crystals typically exhibit a relative maximum in the middle of the biphasic concentration regime and a relative minimum at the beginning of the liquid crystal phase. However, CNCs do not exhibit this typical rheological behavior, instead CNC dispersion viscosity simply increases with concentration. This has been attributed to the charge on the CNC and change in the overall ionic strength, and pH, with increasing concentration. This research explored the hypothesis that holding ionic strength constant across a range of CNC concentrations would result in the typical non-monotonic viscosity versus concentration curve. Solutions of 0.01 M hydrochloric acid (HCl) and 0.1 M sulfuric acid (H2SO4) were added to CNC dispersions until a target ionic strength was achieved for all the dispersion concentrations. Steady shear rheology and cross-polarized optical microscopy were used to monitor the changes of the dispersion microstructure after acid addition. In general, acid addition was found to affect cholesteric pitch and viscosity. It was also found that liquid crystal dispersions became biphasic after each acid addition; this suggests that the acid addition reduced the solvent quality. The studies using HCl addition suffered from experimental errors and were inconclusive. The sulfuric acid addition studies had less error, but failed to support the hypothesis since they did not result in the expected rheological behavior. Obtaining conclusive results was impeded by the complexity of the ionic interactions between the CNC and the added acid, and the discovery that the CNC dispersions used in this study were much more sensitive to rheological testing protocols than the CNC dispersions used in previous studies. The secondary aim of this research was to test the feasibility of producing hydrolytically stable cellulose nanocrystals films for use in microelectromechanical systems (MEMS). Surface modification of CNC thin films included the adsorption of various chlorosilane derivatives which produced a two dimensional, self-assembled monolayer (SAM) on top of the film. Water contact angle measurements along with the time-lapsed microscopy were used to determine the hydrophobicity and hydrolytic stability of CNC thin films. The results suggest that chlorosilane, namely octyltrichlorosilane (OTS), produced water resistant CNC films when tested using a solution of OTS and hexane. However, further optimization of the application protocol is needed.
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