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DIELECTRIC STUDY OF A MESOMORPHOUS SUBSTANCE IN GLASSY NEMATIC PHASE
D. Lippens, C. Druon, J. Wacrenier
To cite this version:
D. Lippens, C. Druon, J. Wacrenier. DIELECTRIC STUDY OF A MESOMORPHOUS SUBSTANCE IN GLASSY NEMATIC PHASE. Journal de Physique Colloques, 1979, 40 (C3), pp.C3-306-C3-309.
�10.1051/jphyscol:1979358�. �jpa-00218754�
JOURNAL DE PHYSIQUE Colloque C3, s u p p l k m e n t au no 4, Tome 40, A v r i l 1979, page C3-306
DIELECTRIC STUDY OF A MESOMORPHOUS SUBSTANCE IN GLASSY NEMATIC PHASE
D. LIPPENS, C. D R U O N and J. M. WACRENIER
Centre Hyperfrtquences et Semi-Conducteurs (*), Universitt de Lille-I, B.P. 36, 59650 Villeneuve d'Ascq, France
RCsumC. - Les auteurs donnent les rksultats de la mesure du tenseur permittivitk complexe du melange eutectique E, dans les phases nkmatique et nkmatiquegelke.
L'kvolution de ce tenseur en fonction de la tempkrature montre I'existence en phase nbmatique gelke d'une absorption dipolaire importante trks diffkrente suivant la direction de mesure.
Le relevk des spectres diklectriques met en kvidence une trks grande similitude entre les deux phases ktudikes. Une premi$re interprktation est proposke, en particulier elle montre la possibilitk d'une reorientation de la molkcule autour d'un de ses axes transversaux en phase nkmatique gelke.
Abstract. - In this paper, the authors report the results of the measurement of the complex permittivity tensor on the eutectic mixture E, in nematic and glassy nematic phases.
The evolution of this tensor versus temperature shows the existence in glassy nematic phase of an important dipolar absorption which is very different according to the measurement direction.
The plot of dielectric spectra puts forward a great similarity between the two phases studied. A first interpretation is proposed and, in particular it shows the possibility of a reorientation motion of the molecule around one of its transverse axes in glassy nematic phase.
1. Introduction. - The domain of existence for a mesophase is for most thermotropic liquid crystals relatively limited. Therefore, the study of dielectric spectra versus temperature is difficult. The putting forward of a phase often called glassy nematic phase can solve this problem. Recently, measurements of the permittivity versus temperature have been performed on a sample of M.B.B.A. [I]. They showed the exis- tence of an orientational polarization in the glassy nematic phase of this compound ( I ) . However, these studies have been carried out on a non oriented nema- tic. Thus, they don't allow the study of the anisotropy of the permittivity tensor.
We report here the dielectric measurements on a cyanobiphenyl mixture supplied by B.D. H. under reference E,.
E, is an eutectic mixture of five components whose gezeral formula is given in figure 1 :
P.C.B. in proportion of 55
%
with Ri = C,H,, P.O.C.B. - 15 %with Ri = C 5 H , , 0 H.O.C.B. - 13%
with Ri = C,H,,O O.O.C.B. - 17%
with Ri = C,H,,O.
The quoted nematic range is - 2 O C to 54 OC. E, has also the particularity of presenting a glassy nematic phase [2]. This phase is a metastable state of matter obtained by rapidly cooling a uniformely oriented sample of nematic. A long range orientational order is thus retained for temperatures very inferior to the cristal to nematic transition temperature.
Measurements of the components of complex per- mittivity tensor in nematic and glassy nematic phases and for the two orientations of the mesophase optical axis parallel (E
//
n) and perpendicular (E l n) to the electric field E were performed. The medium is anisotropic and dielectric spectra are very different with the direction of measurement.- - R i -0- 0-CN-- (1)
2. Experimental. - The simultaneous plot of realand imaginary parts of complex permittivity
FIG. 1 . - General formula o f the different components o f E,. E* = E' - j ~ "
is carried out from 1 Hz to 1 GHz.
(*) L.A. C.N.R.S. NO 287. The cell used for the study in nematic phase was
( I ) Similar results have been found by M . Jaffrain (Laboratoire described previousl~ [3]. ''I'he cell used for the dielectric
de Physique Exptrimentale MolCculaire, UniversitC Paris VI). measurements in glassy nematic phase consists of
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979358
DIELECTRIC STUDY O F A MESOMORPHOUS SUBSTANCE IN GLASSY NEMATIC PHASE C3-307
three plates, 1 mm thick and 14 mm in diameter, separated by 300 pm mica spacers. The use of mica allows the limitation of the active capacitance varia- tions due to thermal dilatation effects. With its very small dimensions, the cell presents a weak heat capa- city. Thus, it allows fast variations of temperatures.
The nematic sample is oriented by means of a magnetic field. The orientation effects due to walls are negligible for a magnetic induction superior to 4 kG.
The orientation in glassy nematic phase is obtained by rapidly dipping the sample, previously placed in the gap of an electro-magnet, in liquid nitrogen. For very low temperatures, the magnetic field has no influence on the orientation of the molecules.
Measurements were performed either for fixed frequency and variable temperature or for fixed temperature and variable measurement frequency.
3. Experimental results. - 3.1 MEASUREMENTS
VERSUS TEMPERATURE. - Measurements of the per- mittivity for the two orientations E'/ n and E I n at liquid nitrogen temperature for a frequency of
10 kHz were carried out.
The values obtained are as follows : E ; , .= 2.9, e l = 2.5. They are of the same order as the permittivity
FIG. 2. -Variations of the components of permittivity tensor versus temperature : a) variations of real part; b) variations of
imaginary part.
which are measured at infinite frequencies. Therefore, the value of the permittivity anisotropy is 0.4 for this temperature.
In glassy nematic phase, we plotted variations of permittivity tensor components versus temperature (Figs. 2a et b).
It can be noted the existence of a very important orientational polarization for the direction of the electric field parallel to the optical axis of the meso- phase. For the perpendicular direction a weak absorp- tion at lower temperatures is noticed (Fig. 2b).
In figure 2a, we can observe a change of the permit- tivity anisotropy sign within a temperature range from
- 45 OC to - 58 OC.
In solid phase, the value of the permittivity at liquid nitrogen temperature is about 2.7. No orientational polarization is noticed in the range of existence of this phase (Fig. 2a).
3 . 2 MEASUREMENTS VERSUS FREQUENCY. - We report the Cole and Cole diagrams obtained in the following measurement conditions.
T = 22 OC in nematic phase
for the two measurement directions (Fig. 3) T = - 25 OC in glassy phase
in the parallel direction (Fig. 4a) T = - 50 OC in glassy phase
in the perpendicular direction (Fig. 4b)
.
FIG. 3. - Cole and Cole plots in nematic phase for T = 22 OC : a) E // n ; b) E I n. Frequencies are given in MHz.
The different measurement temperatures were select- ed in order to observe the dipolar absorption in a frequency range from 100 Hz to 100 kHz for which the precision is the best.
For E
//
n the Cole and Cole diagram in glassy nematic phase is slightly distributed(f,
= 7 kHz, amplitude 7.4, T = - 25 OC). This absorption isC3-308 D. LIPPENS, C. DRUON AND J. M. WACRENIER
- ,up carried by the pentyl group of module 0.4 D.
10 Its direction is defined by the angle b p L ) equal to 1800;
- pA carried by the different alkoxy groups whose characteristics do not vary sensibly with the number of carbon. Its module is about 1.3 D and its direction is defined by the angle pAL 1.720.
Thus, the molecule constituting E, have a very
4 8 12 16 20 &;I important longitudinal dipole component, mainly
resulting from the CN group and a transversal component introduced by alkoxy groups. These two
FIG. 4. - Cole and Cole plots in glassy nematic phase : a) E // n for T = - 25 OC ; b) E l n for T = - 50 OC. Frequencies are
given in kHz.
0.5
0
very similar to the absorption observed in nematic phase.
For the E
I
n direction, a very important distribu- tion in glassy phase can be observed (Fapparent = 5 kHz pour T = - 50 O C ) . It is analogous to the distribution observed in nematic phase (FaPparent = 40 MHz for T = 22 oC).We also plotted the permittivity anisotropy
AE' = E ; , - E; versus frequency in nematic phase
(Fig. 5). The dielectric anisotropy is shown to be negative in a frequency band located near 3 MHz and lying over one decade.
- components play a part in the orientational polariza-
-
/:<
O5 0.2 tion of Cole and Cole diagram of mixture E,, observed in E,.200 . nematic phase for the parallel direction, has an impor-
tant amplitude. The critical frequency is low.
FIG. 5. - Variation of anisotropy versus frequency in nematic phase.
3 3.5 4 4.5 5 61 Maier and Meier [5] pointed out that this absorp-
4. Discussion. - The analysis of the molecular structure of the different components of E, puts forward three elementary dipole moments [4].
- pc carried by the CN group of module 4.3 D, directed along the long molecular axis (L) ;
tion is due to the reorientation of the longitudinal component of the dipole moment in the two equili- brium positions of the nematic potential. This orienta- tion results from a rotation movement of the molecule around one of its transverse axes.
A dipolar absorption similar to the one plotted in nematic phase was shown in glassy nematic phase.
Thus, the molecular movement responsible for the parallel absorption domain in glassy phase is probably the same as the one intervening in nematic phase.
To our knowledge, the putting forward of a reorien- tation movement of the molecule in glassy phase is new. In fact, interpretations [I] of the polarization in this phase have excluded this type of movement up to now and they have only involved intramolecular motions.
The existence of a reorientation of the molecule, therefore of a low frequency mechanism is effectively very difficult to imagine in a solid of molecules oriented. However, there probably exists a very great number of faults in the sample. It can therefore be assumed that the migration of these imperfections throughout the sample makes the reorientation of molecules possible. In particular, this mechanism was already observed in ice [6].
Cole and Cole diagrams observed in nematic and glassy phase, for the direction perpendicular to n are widely distributed. They are due to complex mecha- nisms. These latter can correspond :
- to association phenomena. We have recently shown 17, 81 that two compounds of alkylcyanobi- phenyl series presented very important molecular associations and we have precised the part played by the associations in low frequency mechanisms for E l n ;
- to libration motions of the molecule these movements are located, in the case of very elongated molecules, in mean frequencies ;
- to rotation motions of the transversal compo- nent of the dipole moment which can produce a distribution of the Cole and Cole diagram in high frequencies.
DIELECTRIC STUDY OF A MESOMORPHOUS SUBSTANCE IN GLASSY NEMATIC PHASE C3-309
Thus, the dielectric properties of glassy nematic phase are very similar to those of nematic phase.
5. Conclusion. - We reported the results of the measurement of the permittivity tensor of mixture E, in the nematic and glassy phases. In this latter phase, variations versus temperature of the components of this tensor showed the existence of an important polarization.
The study of the dielectric relaxation puts forward a very high similitude between the absorption spectra plotted in the two phases studied. In particular, we pointed out the existence in glassy nematic phase of a reorientation movement of the molecule around one of its transverse axes. This motion is similar to the one observed for the first time by Maier and Meier in the nematic phases. We also showed a change of permittivity anisotropy in the study versus tempera- ture.
References
[I] MOSCICKI, J. K., NGUYEN, X. P., URBAN, S., WROBEL, S., [5] MAIER, W., MEIER, G., Z. Naturforsch. 16a (1961) 1200.
RACHWALSKA, M., JANIK, J. A,, Mol. Cryst. Liq. Cryst. [6] ONSAGER, L., RUNNELS, L. K., J. Chem. Phys. 50 (1969) 1089.
40 (1977) 177. [7] LIPPENS, D., PARNEIX, J. P., CHAPOTON, A., J. Physique 38 [2] KESLER, J. O., RAYNES, E. P., Phys. Lett. 50A (1974) 335. (1977) 1465.
[3] DRUON, C., WACRENIER, J. M., J. Physique 38 (1977) 47. [8] DRUON, C., WACRENIER, J. M., Results presented at the confe- [4] MINKIN, V. I., OSIPOV, D. A., ZHDANOV, Y. A., Dipole moments rence Physic and applications of smectics und lyotropic in Orgunic Chemistry (Plenum press, New York) 1970. crystals (Madonna di Campiglio), Ann. Phys. 3 (1978).
