Studies on the microstructure of phase transitions in two kinds of liquid crystals by PAT and DSC |
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Authors: | Chuan-Yuan YIN Qing-Chao GU |
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Institution: | (1) Department of Physics and Department of Chemistry, Nanjing University, 210008 Nanjing, People's Republic of China |
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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 ( 1) is essentially independent of temperature while the longer lifetime ( 2) 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 ( 2) with the changes in molecular orientational order (S) and dielectric anisotropy (![Delta](/content/u36l2226476303g5/xxlarge916.gif) ) is presented. |
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Keywords: | 78 70 Bj 64 70 Ew |
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