Dielectric measurements of liquid crystals Z. Belarbi, G. Guillaud, M. Maitrot, J. Huck, J. Simon, F. Tournilhac
To cite this version: Z. Belarbi, G. Guillaud, M. Maitrot, J. Huck, J. Simon, et al.. Dielectric measurements of liquid crystals. Revue de Physique Appliquee, 1988, 23 (2), pp.143-147. �10.1051/rphysap:01988002302014300�. �jpa-00245757�
HAL Id: jpa-00245757 https://hal.archives-ouvertes.fr/jpa-00245757 Submitted on 1 Jan 1988
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Classification Abstracts 61.30 - 77.20
Dielectric measurements of Z.
Belarbi, G. Guillaud, M. Maitrot, J. Huck, J. Simon (1) and F. Tournilhac (1)
Laboratoire Electronique des Solides, Université Claude Bernard, Lyon I, 43 bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France (1) Laboratoire de Chimie Inorganique, CNRS (UA 429) E.S.P.C.I., 10 rue Vauquelin, 75005 Paris, France
juillet 1987, accepté
le 19 novembre
Nous avons utilisé une nouvelle cellule permettant des mesures diélectriques sur de faibles quantités de cristal liquide. Cette cellule a été utilisée pour des mesures diélectriques du cyanobenzylidène p’ octyloxyaniline (CBOOA). Nous avons déterminé la permittivité complexe sous l’action d’un champ électrique continu et alternatif, ainsi que sous champ magnétique et ceci dans un grand intervalle de fréquences et de températures en phase nématique et smectique. Résumé.
A new measurement cell for very small quantities of liquid crystals is used for the determination Abstract. of the complex permittivity of the cyanobenzylidene p’ octyloxyaniline (CBOOA). The dielectric behaviour in the nematic and smectic-A phase is given as a function of an applied electric field (static and alternative) as well as of a magnetic one within a large frequency and temperature range. 2014
The electrical behaviour is relatively scarcely studied in the numerous papers relative to liquid crystals. Maier and Meier were the pioneers in the domain . Nematic compounds with cyano substituents along their long molecular axis were studied in order to have twisted nematic displays with low threshold voltages, and other electronics or non linear optics applications [2-4]. The organic molecules have the advantage to possess very high polarizabilities. Barnik et al.  were the first to study the effect of second harmonic generation with a liquid crystal. The molecules of cyanobenzylidene p’ octyloxyaniline (CBOOA) possess a high permanent dipole moment due to the presence of a cyano group at the edge of the chain. This compound can be oriented by an electric or magnetic field and become non-centrosymmetric in the nematic and smectic phases. It is therefore necessary to determine the optimum values of electric field frequency and intensity to be used for second harmonic generation. Dielectric measurements reflect both electronic and orientational polarizations. The knowledge of
dielectric constants EN and 03B5’~ gives fruitful informations about the structure of the mesophase. It can be very interesting to get special cells in order to be able to have precise measurements on some liquid crystals with a very small amount of material (some mg). The compound CBOOA is one of the most studied material for the smectic A - nematic transition : the phase is smectic A between 73° and 83 °C and nematic between 83° and 109 °C. This material presents a quasi second order transition  to the nematic phase and has a partially bilayered structure [7, 8]. Many studies have been made with this compound : thermomechanical , elastic constant , transition nematic smectic and ultrasonic absorption [10, 11], mechanical intramolecular relaxation time, calorimetric and magnetic anisotropy , viscosity and magnetic measurements for N - SA transition , viscosity by NMR and DMR studies [13, 14]. The DC conductivity versus temperature was given by Mircea-Roussel et al. . The value of dielectric anisotropy of CBOOA is positive
(039403B5 > 0). In the present
work, the dielectric and elastic
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01988002302014300
constants, and their variations with AC
voltage amplitude, magnetic temperature are given. 2.
or DC and with
Cells are constituted of two microscope glass plates. These plates are attacked by fluorhydric acid, the depth depends on the processing duration and the acid concentration. The glass plates are then coated with chromium and gold thin films evaporated successively. The adherence of these electrodes is
3. Results and discussion. The transition temperatures were determined from capacitance measurements for frequencies between 10 kHz and 2 MHz with heating and cooling rates of 0.5 °C/mn and from differential scanning calorimetry (D.S.C.). Capacity versus temperature is given in figure 2 for f 10 kHz. =
very satisfactory. The cell thickness is determined from capacity measurements without liquid crystal. Sometimes a glass lens of large focal distance (600 mm) is used as lower electrode. A small quantity of studied material is adequate : a thickness of about 5 jim is easily obtained (Fig. 1).
Fig. 1. - Measuring cells a) with two plates, b) with an attacked glass plate and plate, 2. liquid crystal, 3. lens.
attacked a lens, 1.
The sample cell is fixed in a flat copper box heated with paste fixed resistances (Foil Minco). The whole system is of reduced dimensions and can be introduced between the poles of an electromagnet. The temperature of the cell is thermostated to ± 0.2 °C and a platine resistance is used to test the liquid crystal temperature. The electrical connections are carefully realized to avoid parasitic impedances. Dielectric measurements are performed using the cell as a capacitor of area S 7.06 mm2 and of 30 03BCm in a large frequency range thickness d (10- 3 to 2 x 10b Hz) on cooling the sample. The complex permittivity is given by : =
Fig. f =
2. - Capacity 10 kHz.
function of temperature at
correspondence between satisfactory. Moreover
both methods is the very capacitance measurements allow the detection of the smectic A - nematic transition. The variation of the conductance G(03C9) and capacity C (w ) are given versus frequency for some temperatures in figure 3. The amplitude of the measurement electric field was about 100 mV in order to avoid any orientation of the sample. ir’ and à (úJ ) are given for the different phases in table 1 at a frequency f 10 kHz. =
Measurements of the components of the complex permittivity in nematic and smectic phases are performed in two geometries : E, H//n and H 1 n. In the lower frequency range (10- 3 to 10 Hz) the capacitance and conductance are measured with a semi-automatic apparatus achieved in our laboratory
. From 100 Hz to 100 kHz, we used a General Radio 1610 bridge and from 10 kHz to 2 MHz an automatic HP 4275 bridge controlled by a microcomputer HP 85F.
We observe two distinct domains in the curves : the high frequency range (103-105 Hz) where the capacitance and conductance are constant, which reflects the bulk properties and the lower frequency (10- 3-103 Hz) corresponding to ionic range phenomena. In fact, the important increase of the capacitance at low frequencies is due to the presence of ionic impurities near the electrodes (blocking effect). To avoid this blocking effect, the dielectric
145 measurements were performed in the following work at f = 10 kHz. The variations of ei and 03B5"~ are given in figures 4, 5, 6 and 7 versus the amplitudes of DC and AC applied electric fields respectively at T 105 °C and f = 10 kHz. The variation of the two dielectric constant components e’ 1 and 03B5’~ versus magnetic field intensity are given in figure 8 at T = 105 °C and f = 10 kHz. =
Fig. 3. - Dielectric spectra (conductance and capacitance) for various temperatures a) T = 130 °C ; b) T 105 °C ; c) T 95 °C ; d) T 80 °C ; e) T 60 °C. =
8. 03B5’~ and 03B5’~ as a function of 105 °C and f 10 kHz.
at T = 105
The variation of constants yj and 03B5’~ are given temperature at f 10 kHz under a magnetic field of H 9 kGauss in figure 9. versus
(VDC)-1 with a much larger linearity range. This allows us to determine with accuracy the asymptotic 4.8 at 8.8 and to obtain 039403B5 value 03B5’~(V ~ oo ) 10 kHz. 105 °C and f H -+ 10 kG, T With an applied magnetic field, the experimental results show that 03B5’~ is almost constant and very near e (without orientation). It can be deduced that the molecules are in the planar alignment (director parallel to the electrodes) in the nematic phase without external force (see Fig. 8). The value of the threshold voltage for the 0.9 V and is rather Freedericksz transition is Vth weak for nematic compounds with cyano substituents along their long molecular axis. The Freederiksz transition allows the calculation of the splay elastic constant K11 from the threshold potential, with 039403B5 > 0 =
9. 03B5’~ and sl as 10 kHz and H 9 kG.
function temperature at
and These dielectrical measurements allow the determination of the values of dielectric anisotropy Ase, elastic constants and transport properties. The parallel component of conductivity o-jj 03B5"~03C9 decreases up to voltages higher than the Freederiksz transition when a DC electric field or magnetic field induces a déformation in liquid crystal, and then increases slowly above a threshold field. A different behaviour is observed with AC voltage in this case, 03C3~ increases whatever this voltage may be. The values of the dielectric constant e’ in the isotropic phase and the dielectric anisotropy Ae one in the nematic phase are similar to those found by 5.5 in the Billard et al.  (at T’ 85 °C, 039403B5
In the smectic range,
however, thèse authors did
not detect any dielectric anisotropy ; this fact was probably due to desorientation in the sample. In our
case, we observe continuity of El at the SA ~ N transition while El’ decreases strongly. The same behaviour has been observed by Nagabhushan et al.  in the compound 4 OBCAB. The sharp drop in constant ei is due to the effect of dimerisation  : in the smectic phase of CBOOA, the antiparallel correlation is stronger than in the nematic phase. In fact, a fraction of the molecules form quadripolar dimers by head to head association and cannot participate to dipolar effects. Engelen et al.  estimated the fraction of dipolar correlation at 0.5. It can be noticed that the slope of 03B5’~ versus (VAC)-1is linear and close to the slope of 03B5’~ versus
This value is close to that of Cheung et al.  obtained by optical measurements with an homeotropic orientation. These authors found K11 K33 (bend constant) at temperatures from 93 °C to the nematic-isotrope transition (T 109 °C). Finally, from our measurements, we can deduce that the compound CBOOA possesses a high polarizability (As = 4.8), it can be completely oriented with an electric field strength higher than 3 000 V/cm (AC or DC) or by a magnetic field. On the other hand, we can avoid ionic phenomena with frequencies higher than = 10 kHz. =
in this paper the results of dielectric of cyanobenzylidene p’ octyloxyaniline obtained with a new measurement cell, which allows us to determine dielectric constants with satisfactory precision. Our data for the nematic phase are in good agreement with these reported by Billard  but we observe a different behaviour in the smectic-A phase which may be explained by the partial dimerisation of the molecules. We have determined the optimum conditions of orientation and utilisation of the compound CBOOA as an organic material suitable for second harmonic generation.
Acknowledgments. Z. Belarbi wishes to thank Mr. G. Massouras for precious discussions and help.
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