Small Angle X-Ray Studies of Liquid-Crystal Phase Transitions I.p-Azoxyanisole

Abstract

Small angle X-ray scattering experiments have been performed on p-azoxyanisole in the isotropic liquid, nematic liquid, and solid regions. Measurements were carried out in a Hart-Bonse type small angle diffracto-meter with the sample maintained at temperatures controlled to ±0.003°C. Premonitory phenomena were observed in the isotropic liquid at temperatures considerably above the nematic-isotropic transition, temperature, T_c. As the nematic-isotropic transition was approached from above, the correlation length for molecular ordering increased from 300 Å at 15°C above T_c to 1900 Å at T_c. Within the nematic liquid region the correlation length was found to be essentially constant down to several degrees above the nematic-solid transition, followed by an additional increase in correlation near the transition to the solid phase. The experimental results indicate that neither the Ornstein-Zernike nor Fisher theories of critical point behavior are adequate to explain the phase transition. The Frenkel heterophase fluctuation theory is in qualitative agreement with the temperature dependence of the correlation length.