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Concentrated solutions of hydroxypropylcellulose in aqueous and in polar organic solvents form lyotropic liquid crystalline phases. The critical concentration of hydroxypropylcellulose needed for anisotropic phase separation is dependent upon the solvent and is much greater than that expected for rod-like species in solution. The flexibility of the cellulosic chains in a particular solvent is believed to be the main factor governing the critical hydroxypropylcellulose volume fraction at phase separation. The anisotropic phase exhibits birefringence, iridescence, and very high optical activity, properties characteristic of cholesteric liquid crystals. The helicoidal pitch for most of these cholesteric samples in water and organic solvents varies from 280 to 700 nm, but in certain organic solvents the helicoidal pitch ranges up to 6000 nm. The latter samples exhibit fingerprint-like periodicity lines in the light microscope and distinctive "shimmering" colors. For both types of samples the helicoidal pitch is found to vary inversely with the third power of the hydroxypropylcellulose volume fraction. A model is proposed for the cholesteric structure in which the average separation, d, between the chain molecules varies with (phi)(,2)('- 1/2), where (phi)(,2) is the polymer volume fraction. The experimental x-ray diffraction data are in accord with this relationship. The angular twist between the molecules in adjacent cholesteric layers is calculated to vary from 0.30 to 1.8(DEGREES) over the mesophase concentration range investigated. / Light scattering measurements give twice the weight average molar mass for hydroxypropylcellulose as do sedimentation equilibrium measurements. Viscosity measurements in organic solvents show that the hydroxypropylcellulose conformation in dilute solution is neither that of a random coil nor that of a rigid rod. All aqueous hydroxypropylcellulose solutions show a lower consolute temperature that varies more with the sample molar substitution than with its molar mass. A qualitative "phase" diagram for aqueous hydroxypropylcellulose solutions is also included.

This thesis investigates the applications of liquid crystals in three areas of aerodynamic testing, namely temperature, shear stress and pressure measurement over surfaces. The use of the selective reflection colours from encapsulated chiral nematic materials to map surface temperatures is assessed. This method is very successful in a wide range of applications for determining heat transfer rates, but has limitations where high heat transfer rates are present, due to the thermal response time of the material structure, and the effects of temperature gradients. The thermal time constant is determined as a function of material viscosity. It is typically 10ms for a chiral nematic material at room temperature. The effect of a temperature gradient on the selective reflection is studied in terms of the pitch gradient produced in the material structure. Two improved methods are then proposed. The first makes use of the cholesteric to isotropic transition to indicate an isotherm on a heated surface, while the second uses changes in the birefringence colours produced by an aligned nematic layer applied to a surface and viewed between crossed poiarisers. Changes in the selective reflection colours from cholesteric liquid crystals can be used to assess surface shear stress levels. This method is assessed and determined to be too problematical for accurate measurements. A new method is proposed, based on the shear induced texture change from the focal-conic to the Grandjean texture in the cholesteric phase. This method may be used to quantify surface shear stress levels as well as to provide excellent flow visualisation once flow alignment has occurred. Tests have been conducted on the pressure sensitivity of chiral nematic liquid crystal materials. The results indicate that the change in transition temperature with pressure is of the order of 40°/kbar. which may not be sensitive enough for wind tunnel purposes.

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