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Study of blue phases transition kinetics by thermal lens spectroscopy in cholesteryl nonanoate

Abstract:

The thermal lens signal distortion recorded on the BPI supercooled blue phase of a cholesteryl nonanoate thin sample was studied. Results showed that the distortion develops as the induced thermal gradient generated by the excitation beam incidence promotes local transitions to the BPII and BPIII blue phases and to the isotropic liquid. BPII Bragg scattering of the excitation beam wavelength along with a thermo-optic coefficient change produces a thermal lens effect decay. As this phase disappears the signal recovers strength and reaches a steady state. On lowering the sample temperature, the peak value of the distortion shifts to longer times. Considering Johnson-Mehl-Avrami-Kolmogorov- type kinetics, the effective rate constants, the preexponential factor, and the overall effective activation energy for the transformations BPI to BPII and BPII to isotropic liquid were determined. A mathematical model was developed to represent the thermal lens signal variation produced during each transformation. This article shows an interesting application of thermal lens spectroscopy to the determination of phase transition kinetic parameters of cholesteric liquid crystals. © 2006 American Institute of Physics.