Broadband dielectric measurements on the (R)-1-methylheptyl-6-(4′-decyloxybenzoyloxy)-2-naphthalene carboxylate antiferroelectric liquid crystal

Abstract

We have studied by means of polarization, tilt and broadband dielectric spectroscopy the liquid crystal (R)-1-methylheptyl-6-͑4Ј-decyloxybenzoyloxy͒-2-naphthalene carboxylate. This compound together with a ferrielectric and two antiferroelectric phases shows a broad range, 6°C, of Sm-C ␣ * phase. The dielectric study shows the presence of a Goldstone mode in the latter mesophase, which in turn would confirm its tilted and helical nature. Other collective modes, such as the soft mode and a mode related to the antiferroelectric ordering and the helical structure, have also been characterized. With respect to the molecular modes, the rotation around the molecular long axis has been studied from the isotropic to the low temperature antiferroelectric phase. This study shows that the molecules are quite rigid and that the dynamics of the motions around the molecular long axis is not greatly influenced by the transitions among the different smectic mesophases. In particular, the thermal activation energy of the process is the same in the paraelectric Sm-A phase and in the complex Sm-C ␣ * phase. However, the frequency shows a jump at the Sm-C ␥ *→Sm-C A * phase transition, which could be due to an increase of the order of the molecular short axes.

The investigation of the relaxation processes in antiferroelectric liquid crystals by electro-optic spectroscopy

“…7 Nevertheless, the problem of finding the mechanisms governing the various relaxation processes in the antiferroelectric phase has not yet been solved and the aim of this letter is to unambiguously establish these mechanisms using electrooptic spectroscopy of a AFLC. Though the nature of the relaxation process in SmC A phase has somewhat been speculated in earlier experimental publications [3][4][5] and theoretical works, 8,9 nevertheless no clear evidence is given and theoretical investigations are divorced from the experiments. In this letter, the mechanism of the collective processes is found on comparing the results of the dielectric and electro-optic spectroscopy with those from the theoretical investigations.…”

“…The frequency of Process 0 is ϳ2.5 kHz, whereas that of Process 1 lies between 10 and 70 kHz depending on the temperature. 5 Hence it can only be concluded that these processes are specifically related to the common characteristics of antiferroelectric phases; however the origin of these processes is still to be explained. Four possible physical mechanisms, some of these already suggested [3][4][5] for the collective molecular reorientations in the antiferroelectric phase, are schematically presented in Fig.…”

“…5 Hence it can only be concluded that these processes are specifically related to the common characteristics of antiferroelectric phases; however the origin of these processes is still to be explained. Four possible physical mechanisms, some of these already suggested [3][4][5] for the collective molecular reorientations in the antiferroelectric phase, are schematically presented in Fig. 1: ͑i͒ A deviation from the antiferroelectric order by the azimuthal angle , such that changing in the opposite sense 3 in the adjacent layers ͓Fig.…”

“…[3][4][5][6] The dielectric spectra in the frequency range from 10 Hz to 1 GHz show the existence of two collective relaxation processes and two molecular relaxation processes in the SmC A phase and these are denoted as Processes 0,1,2,3 with increasing of the relaxation frequency. 6 The lower frequency processes ͑Process 0 and Process 1͒ are found to exist 6 only in the two antiferroelectric phases SmC A and AF phases.…”

“…A determination of the origin of the collective modes will allow for a further improvement in these materials to occur with regard to their switching speeds and hence increase their usefulness for displays and electro-optic devices. A study of the relaxation processes has been made using dielectric spectroscopy, [3][4][5][6] electro-optics, 4 and photon correlation spectroscopy. 7 Nevertheless, the problem of finding the mechanisms governing the various relaxation processes in the antiferroelectric phase has not yet been solved and the aim of this letter is to unambiguously establish these mechanisms using electrooptic spectroscopy of a AFLC.…”