Department of Physics, North Bengal University, Siliguri, India.
J Phys Condens Matter. 2010 May 19;22(19):195106. doi: 10.1088/0953-8984/22/19/195106. Epub 2010 Apr 16.
We report the measurements of birefringence as a function of temperature of a binary system 4-cyanophenyl [4'(4''-n-heptylphenyl)]benzoate (7CPB) + 4-cyanophenyl 4-nonylbenzoate (9.CN) showing a nematic-smectic A(d)-re-entrant nematic phase sequence by means of the optical transmission method. The temperature dependence of the birefringence has been determined from the transmitted intensity data and the orientational order parameters have been calculated. These observations indicate that re-entrant nematic to induced smectic A(d) and induced smectic A(d) to nematic phase transitions for all the mixtures are of second order. There is a continuous change in the Δn values at the nematic-smectic A(d) and smectic A(d)-re-entrant nematic phase transitions. However, for some mixtures a slight increase in birefringence on cooling in the vicinity of the smectic A(d)-re-entrant nematic transition has been observed. We have also fitted our experimental results with those calculated from the modified McMillan theory as proposed by Luckhurst and Timimi.
我们报告了二元体系 4-氰基苯基[4'（4''-正庚基苯基）]苯甲酸酯（7CPB）+4-氰基苯基 4-壬基苯甲酸酯（9.CN）的双折射随温度的测量结果，该体系通过光学传输方法显示出向列-层状 A(d)-向列相的顺序反转相序列。通过透射强度数据确定了双折射的温度依赖性，并计算了取向有序参数。这些观察表明，所有混合物的向列相到诱导层状 A(d)和诱导层状 A(d)到向列相的反转相转变均为二级相变。在向列-层状 A(d)和层状 A(d)-向列相反转相转变处，Δn 值连续变化。然而，对于一些混合物，在层状 A(d)-向列相反转相转变附近的冷却过程中，双折射略有增加。我们还将实验结果拟合到 Luckhurst 和 Timimi 提出的修正 McMillan 理论计算的结果中。
