A low power beam of He-Ne laser light is scattered from the sample of liquid crystal at various temperatures and is made incident on a fixed cavity Fabry-Perot etalon. Analysis of the change in the diameter of the Fabry Perot rings is indicative of the change in the mesophase of the LC. This spectroscopic technique of FPSS has enabled us to determine the mesophase transition temperatures of a wide range of liquid crystals. The technique has now been applied successfully to detect the phase transition temperatures of the biphenyl-based ferroelectric liquid crystals. These findings have been corroborated by the analysis of the thermal runs of the same samples using differential scanning calorimetery. A comparison of the observations and the results obtained by the two techniques is presented. This study clearly indicates the success of the Fabry-Perot technique in determining the PTTs. The FTIR analysis of the same samples is also presented here.
Spectroscopic techniques have been the most sensitive ones to register data with an unparalleled accuracy. The technique of Fabry-Perot Scattering Studies using a FP etalon coupled with a He-Ne laser and a spectrometer is used for characterization of thermotropic liquid crystals. Mesophase transition temperatures have been determined for a wide range of liquid crystals[LCs] which includes nematic, smectic, cholesteric as well as ferroelectric and antiferroelectric LCs. New mesophase transitions have also been detected and classified in addition to those reported by earlier workers. These findings have been corroborated for the same samples using Differential scanning calorimetery [DSC]. A comparison of the observations and the results obtained by FPSS and those obtained by DSC clearly indicates the success of the Fabry-Perot technique in determining the transition temperatures. It is also a clear pointer to the superior sensitivity of the FPSS method. The importance of the FPSS technique lies in its ability to determine with ease the transition temperatures more accurately and with a better sensitivity because it enables us to register even those mesophase transitions which require very small heats of transitions endothermic /exothermic) for their occurrence. The mesophase transitions involving such small heats of transitions tend to be missed out in DSC. This paper gives a complete review of the samples investigated to date using this technique.
The spectroscopic technique using the Fabry-Perot [FPSS] etalon coupled with a spectrometer has been used to determine the mesophase transition temperatures of the samples (1) MHPB(H)PBC (2) MHPB(Cl)PBC. Earlier FPSS work on the sample MHPOBC(R) is also substantiated here with results from Differential Scanning Calorimetery [DSC] and Polarization Microscopy Studies [PMS]. A comparison of the results obtained with those of DSC clearly indicates the success of the FPSS technique in determining the transition temperatures of the above samples belonging to the FLCs and AFLCs groups. The photographs of the textures using PMS also support the above results. This novel technique has led to the detection of new mesophase transitions in addition to those reported by DSC and PMS. These new findings are also corroborated by the analysis of the thermal runs using DSC and the textures from PMS.
Change in the phase of thermotropic liquid crystals occurs at definite phase transition temperatures. A spectroscopic technique using the Fabry-Perot etalon coupled with a spectrometer has been used to determine the mesophase transition temperatures of the liquid crystal-Cholesteryl myristate. New mesophase transitions have been detected and classified at the temperatures of 52 degree(s)C, 74.5 degree(s)C and 87 degree(s)C in addition to those reported by earlier workers at 71 degree(s)C, 81 degree(s)C and 90 degree(s)C. These findings have been corroborated by the analysis of the thermal runs of the same samples using differential scanning calorimetry.
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