STUDIES ON THE MICROSTRUCTURE OF PHASE-TRANSITIONS IN 2 KINDS OF LIQUID-CRYSTALS BY PAT AND DSC

The positron lifetime and DSC measerments for EBBA and DOBAMBC have be en made with heating and cooling clycles. The experimental results sho w that a shorter lifetime (tau1) is essentially independent of tempera ture while the longer lifetime (tau2) and the intensity (I2) change ab ruptly double or triple with temperature. Consequently, the EBBA has o nly nematic phases while the DOBAMBC has two liquid-crystalline phases (smectic C and smectic A) with transition temperatures as follows: f or EBBA, solid --> nematic (304.5 K), nematic --> isotropic (356.5 K), isotropic --> nematic (356.5 K), nematic --> solid (301 K); and for D OBAMBC, solid --> smectic C (346 K), smectic C* --> smectic A (357.5 K), smectic A --> isotropic (389 K). These critical temperatures are i n accordance with the transition temperatures measured by DSC. In addi tion, the difference in the solid-nematic transition temperature in th e heating and cooling cycles was also observed. A discussion about the correlation of the observed changes in lifetime (tau2) with the chang es in molecular orientational order (S) and dielectric anisotropy (DEL TAepsilon) is presented.