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Light Scattering and Microscopic Investigations of Mesophase Transitions of Cholesteryl Myristate.

The solid state of cholesteryl myristate may exist in a negatively birefringent spherul itic form of randomly and nonrandomly correlated aggregates of crystals best characterized by an orientation correlation distance which was found to be in the order of 1 micron. Cholesteryl myristate exhibits a smectic and cholesteric mesophase both when heating from the solid and when cooling from the isotropic melt. The morphology of the smectic phase is the same upon heating and cooling. It is best described as consisting of regions having crystal orientation randomly correlated. For the cholesteric phase, however, non-randomness leads to correlated regions having definite shape. On heating to the cholesteric phase there is a transition from the solid state or the smectic phase to a cholesteric phase having disclike non-random orientation correlation. The isotropic cholosteric phase transition, occuring on cooling, on the other hand, occurs in two steps, The first is a rapid transformation to a turbid "blue" homeotropic state consisting of particles of sizes less than one micron, while the second is a much slower transition to a more macroscopically ordered focalconic spherulitic state. The latter transformation may be described by nucleation and growth kinetics similar to those obeyed by crystalline polymers. The process can be followed by direct microscopic observation, by the depolarized light transmission technique or' by the low angle light scattering procedure. The light scattering is similar to that observed for a spherulitic polymer and is shown to arise principally from fluctuation in the orientation direction of anisotropic structures. Spherulite sizes, orientation of the optic axes, and kinetic parameters are determined. The kinetics of transformation fit the Avrami equation giving an n value of 3. The analysis of the temperature dependence of the growth and nucleation rates yields very small small interfacial energy products, δ1 δ2, of 0.1 and 0.2 erg2/cm4 for growth and nucleation respectively.
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