Studies on the microstructure of phase transitions in two kinds of liquid crystals by PAT and DSC

Abstract

The positron lifetime and DSC measerments for EBBA and DOBAMBC have been made with heating and cooling clycles. The experimental results show that a shorter lifetime (τ₁) is essentially independent of temperature while the longer lifetime (τ₂) and the intensity (I ₂) change abruptly double or triple with temperature. Consequently, the EBBA has only nematic phases while the DOBAMBC has two liquid-crystalline phases (smectic C* and smectic A) with transition temperatures as follows: for EBBA, solid → nematic (304.5 K), nematic → isotropic (356.5 K), isotropic → nematic (356.5 K), nematic → solid (301 K); and for DOBAMBC, solid → smectic C* (346 K), smectic C* → smectic A (357.5 K), smectic A → isotropic (389 K). These critical temperatures are in accordance with the transition temperatures measured by DSC. In addition, the difference in the solid-nematic transition temperature in the heating and cooling cycles was also observed. A discussion about the correlation of the observed changes in lifetime (τ₂) with the changes in molecular orientational order (S) and dielectric anisotropy (Δɛ) is presented.