Agarwal, V K

Arora, V P

Kumar, A

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Indian J. Phys. 69A (6), 635-638 (1995)■ N0T8T-IJP A- an international journalDielectric behaviour of cholesteryl oleyl carbonate and cholesteryl linoleate in dilute solutions of CCI4 at a single microwave frequencyV P Arora and A KumarDeportment of Physics, Vardhman (PG) College, Bijnor-246 701, India andV K AgarwalDepartment of Physics, Ch C S University, Meerut-250 004, India Received 7*6 May 1995, accepted 1 August 1995Abstract : In order to study the dielectric behaviour of isolated molecules, measurements of dielectric parameters permittivity (£') and loss (£*), have been made on cholesteryl oleyl carbonate and cholesteryl linoleate in dilute solutions of CCI4 at a single microwave frequency of 4.0 GHz: at 30°C. The results indicated that the dielectric relaxation time (r) is larger for longer molecule cholesteryl oleyl carbonate (52.2 p.s) compared to that for cholesteiyl linoleate (45.2 ps), os expectedKeywords : Cholesteryl liquid crystals, dielectric properties PACS No. : 77 84.NhCholesteryl liquid crystals find commercial use in both twisted nematic (TN) and guest host (GH) electrooptic displays and also in many thermographic applications [1-3]. Understanding of the dielectric behaviour of liquid crystal continues to provide a considerable research challenge. The dilute solutions of polar substance in non-polar solvent allows the solute molecule to be examined in a quasi-isolated state and the behaviour is relatively less affected by the dipolar field. In the present work, we have reported the dielectric behaviour of cholesteryl oleyl carbonate and cholesteryl linoleate in dilute solutions of CC14 at a single microwave frequency of 4.0 GHz and at room temperature 30°C. The two cholesteric liquid crystals were procured from M/S Eastman Kodak Co. and were used without further purification.Four dilute concentrations having weight fractions less than 0.04 were prepared with each liquid crystal and the measurements were made at 30°C. The coaxial line bench set up© 1995 IACS636 V PArora, A Kumar and V K Agarwalis shown in Figure 1. A gold plated co-axial line of 30 cm in length, was used as a liquid cell. A teflon plug was used to support the central conductor and to allow for a verticalcolumn of the liquid, which was terminated by a movable s i^orl circuiting plunger. The length of the liquid column in the cell could be adjusted to an accuracy of 0.001 cm by means of a micrometer plunger drive. 1 N 21 crystals were used for detection. The temperature was maintained within ± 0.5° by circulating water around the cell by a thermostat-type NBE.The inverse VSWR (p„) were determined for each concentration of the two liquid crystals for different liquid lengths by measuring tfie width AXn at double minimum power points at n \i/2, for n = 0, 1,2, 3, 4 and 5 and using the relations :Kp = -----Ax .r n  -3 nThe value of p„ were then corrected for the approximations involved in deriving the above relation [4] and p ( (corrected) arc obtained.The values of £ and e" were calculated from the equations :and/V2i K f f v dp^ _dn 'For coaxial bench (at 4.0 GHz), k L = °o and k^ = so that' O 2£ =U z- f dn( 1 )and (2)Dielectric behaviour of cholesteryl oleyl carbonate etc 637Here pc is the corrected inverse VSWR which takes into account the contributions from the losses in the wave guide walls and the imperfection of the short in the measured values of the inverse VSWR.The accuracy of measurements for e and e" is ± 2% and ± 5% respectively.The values of pc as functions of n for different concentrations of cholesteryl oleyl carbonate and cholesteryl linoleate in dilute solutions of CC14 are given in Table 1. The straight line plots of pc vs n yielded dpc/dn. The values of f'and e" obtained using eqs. (1) and (2), for the two mesogens at different concentrations are reported in Table 2.Table 1. Corrected VS WR (p c) as a function of n for different concentrations of cholesteric materials in dilute solutions of CCI4 at 30°C and at 4.0 GHzConcentration n 0 1 2 3 4 5Cholesteryl linoleate.0051 .0323 .0381 .0402 .0419 0419 .0452.0148 .0339 .0411 0411 0457 .0498 .0578.0212 .0335 0350 0364 .0406 .0469 .0586.0292 .0330 .0360 .0469 .0524 0662 0741Cholesteryl oleyl carbonate.0143 * .0335 0419 .0436 0398 ,0356 0486.0209 0368 .0369 .0423 0465 ,0473 0478.0299 0364 .0452 ,0452 .0461 .0423 0436.0391 .0373 0377 0436 0436 0482 .0524Table 2. Measured values of f  'and c" for two cholesteric materials in dilute solutions ot CCI4 and 4.0 GHz and at 30°CCholesteryl oleyl Cholesteryl linoleatecarbonateWt. fractions £' ff" Wt. fractions £ f"0.0143 2.231 0.0037 00051 2.231 0.00350.0209 2.235 0 0053 0.0148 2.241 0.00930.0299 2.236 0.0065 0 0212 2.246 0.01510.0391 2.239 0.0081 0.0292 2.250 0.0176Assuming that a single Debye type dispersion exists, the dielectric relaxation times (r) at 30°C are obtained using Gopal Krishna expression [5]whereX = my+ C,£' + £" + £"2-  2 X =  £*2 + (£' + 2)J ’I t"Y "  £"2 + (f' + 2)21ani*nd slope m =638 V P Arora, A Kumar and V K AgarwalThus, the plot of X against Y will give a straight line with slope (co t1). Figure 2 shows the plots for two cholesteric liquid crystals and these are reasonably straight lines within theY.W4F igu re  2 . Plots to deteim ine or  for cholesteryl ole^l carbonate (COC) and cholesteryl linoleate (CL).errors. From the slopes of lines, the dielectric relaxation times at 30°C are evaluated as52.2 ps for cholesteryl oleyl carbonate and 45.2 ps for cholesteryl linoleate respectively. It can be seen that there is a reasonable agreement between the two relaxation times, indicating that the assumption concerning simple Debye type dispersion is justified. Further, the cholesteryl oleyl carbonate is a slightly longer molecule, it is expected to exhibit larger relaxation time, as observed in the present study. 'R eferencesf 1 ] D L Whire and G N Taylor /  Appl Phys. 45 4718 ( 1974)[2] A Gobi. A Wunsch, G Heppke and F Oestencher J. Phys. (Pans) 40  773 (1979)[3J B Bahadur (Editor) Liquid Crystals : Applications and Uses Vql. 1 (Singapore : World Scientific) (1991) [4j A R Von Hippel Dielectric Materials and Applications (New York : John W iley) (1954)[5J K V Gopal Krishna Trans. Faraday Soc. 53 767 (1957)

Dielectric Behaviour of Cholesteryl Oleyl Carbonate and Cholesteryl Linoleate in Dilute Solutions of CCl4 at a Single Microwave Frequency

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Dielectric Behaviour of Cholesteryl Oleyl Carbonate and Cholesteryl Linoleate in Dilute Solutions of CCl4 at a Single Microwave Frequency

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