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KERR EFFECT AND PRETRANSITIONAL PHENOMENA IN NEMATIC LIQUID CRYSTALS
J. Filippini, Y. Poggi
To cite this version:
J. Filippini, Y. Poggi. KERR EFFECT AND PRETRANSITIONAL PHENOMENA IN NE- MATIC LIQUID CRYSTALS. Journal de Physique Colloques, 1975, 36 (C1), pp.C1-137-C1-139.
�10.1051/jphyscol:1975126�. �jpa-00215903�
Classification Physics Abstracts
7.130 — 8.813
KERR EFFECT AND PRETRANSITIONAL PHENOMENA IN NEMATTC LIQUID CRYSTALS
J. C. FILIPPINI and Y. POGGI
Abstract. — The Kerr constant B of some nematic liquid crystals (MBBA, ABABN, MPT, etc..) has been measured in the isotropic phase, near transition, by a precise pulse method. Contrary t o certain published results, our measurements prove that D e Gennes' theory on nematic-isotropic transition holds good for the Kerr effect of these substances : B oc (T— r * )- 1 ; T* is a temperature slightly below the transition temperature Te. For each substance, difference To — T* has been deduced from Kerr effect measurements, and the same values Te — T* have been found from magnetic birefringence measurements on the same samples.
Various experimental results have been published t V e s u for two years concerning t h e behaviour of t h e Kerr B constant of nematic liquid crystals in the isotropic
phase near transition [1,2,3], but no general conclusion
could have been drawn about the law of its temperature " / dependence. , /
The short range order effects in the isotropic phase /
above the transition point TQ have been theoretically g - /
discussed by De Gennes [4a] in terms of a Landau 2 , /
. , ~ ,, . . / . , ... ,. "2 p-methoxybenzylidene p-n-butylaniltne /
model for t h e nematic-isotropic transition : according g - / t o this treatment, t h e magnetic a n d electric birefrin- ° TC=461°C /
gence were expected t o be proportional t o (T — T*)~y j | - „ / where T * is a temperature slightly below Tc a n d y is a n * ~ / u n k n o w n exponent, y = 1 in a mean field theory such J " / as Maier-Saupe's [4b]. £ /
Whereas experimental researches on magnetic bire- m /
fringence [5] were giving such a law, with y = 1, those 2 - / on electric birefringence were unconclusive or seemed > / to prove that the previous law was not obeyed. Some of ~ - /
the results were given as a function of T — Tc and no / temperature T* was found [1, 2] : for ABABN, the ~ /
temperature dependence of the Kerr effect was found to / obey (T — Tc)~v in a larger range of temperature with " ,<
0.5 < y < 0.7 ; the difference Tc — T* was estimated | / ! , ] ] , t o b e very small ( < 0.2 °C) so t h a t these results were A5T* te 47 49 _51 53 T°C interpreted as is the K e r r effect were obeying (T— T*)~7 TEMPERATU RE
FIG. 1. — The inverse of the Kerr constant of MBBA as a (*) Centre National de la Recherche Scientiflque. function of temperature.
JOURNAL DE PHYSIQUE Colloque C l , supplément au n° 3, Tome 36, Mars 1975, page Cl-137
Laboratoire d'Electrostatique (*), B. P. n° 166, Centre de Tri, 38042 Grenoble-Cedex, France
Résumé. — On a mesuré la constante de Kerr B de quelques cristaux liquides nématiques (MBBA, ABABN, MPT, etc..) en phase isotrope près de la transition. Les mesures prouvent que, contrairement à certains résultats publiés, la théorie de De Gennes sur la transition nématique- isotrope s'applique à l'effet Kerr de ces substances : B oc (T— r * )_ 1 ; T* est une température légè- rement inférieure à la température de transition Tc. On a déterminé, pour chaque substance, à partir des mesures d'effet Kerr, les différences Tc — T* et on a retrouvé les mêmes valeurs à partir de mesures de biréfringence magnétique sur les mêmes échantillons.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975126
Cl-138 J. C. FILIPPINI AND K. POGGI
t-
p-ethoxybenzylidene p - n - b u t y l a n i l i n e
K ' Tc = 75.4 O C
E 1 TN=74.6"C
TEMPERATURE
FIG. 2. -The inverse of the Kerr constant of EBBA as a function of temperature.
I
i MPT: p-methoxy p e n t y l p'- t o l a n e
L ' /
TEMPERATURE
with 0.5 < y < 0.7. For the optical Kerr effect, there are conflicting reports : a(T - T*)-? law, with y = 0.6, was found for MBBA by Prost and Lalanne [3a] but Wong and Shen [3b] find y = 1.
The origin of these incoherent results lies in experi- mental difficulties concerning both the chemical stability of the substance and the method of measure- ment. During our first attempt on MBBA [2], the purity of the sample was not good : the transition temperature T, was changing continuously and we had to measure it at each measurement of the Kerr effect :
FIG. 4. -The inverse of the Kerr constant of ABABN as a function of temperature.
b i p h e n y l n i t r i t e
TEMPERATURE
FIG. 3. - The inverse of the Kerr constant of MPT as a function FIG. 5. - The inverse of the Kerr constant of HBN as a
of temperature. function of temperature.
KERR EFFECT IN NEMATIC LIQUID CRYSTALS Cl-139
it was impossible to give the Kerr constant as a function of temperature T, but only as a function of T - Tc ; so, T* could not have been found. In the following experiments, we had very pure samples and the cells were clean and dehydrated so that Tc was constant (change less than 0.1 OC) during the experiments. Great care was taken during the measurements : a pulse method was preferred to methods using alternating voltages : the voltage was applied to the substance for a short time (- l PS) ; SO, neither heating, nor electro- chemical reaction - which could change T, - took place, and, above all, space charges had no time to move in the substance and the electric field was uni- form ; high voltages (up to 20 1V applied on 2 mm) were used, which increased the precision of the bire- fringence measurements.
Figures 1 to 5 show the inverse of the Kerr constant as a function of temperature. In a range of temperature depending on the substance, the curve is linear, and, by extrapolation, we can get a temperature T*, proving that a ( T - T*)-* law is obeyed with y = 1.
It has seemed to us of great importance to verify the consistence of the results by measuring the-tempera-
ture dependence of the magnetic birefringence of the same substances ; so, differences T, - T* for the same samples were determined by two ways as shown on the following table.
Tc - T* determined by Substance electric birefringence
- -
MBB A 0.7 4 0.1 OC EBBA 0.8 f 0.1 OC
MPT 1.9 f 0.1 OC
ABABN 1.3 + 0.1 OC
HBN 0.9 & 0.1 OC
Tc - T* determined by magnetic birefringence
- 0.7 f 0.1 OC 0.8 4 0.1 OC 1.8 f 0.1 OC not measured
0.9 f 0.1 OC
One can see that differences Tc - T" are the same.
For MBBA, T, - T* had already been measured by light scattering and magnetic birefringence and values different from ours had been obtained 15, 6, 71 ; the differences might be explained by differences in purity of the samples ; nevertheless our value agrees with a recent one by Johnston from electric birefringence [S].
So, it is proved, by experiments on five different substances, that De Gennes' theory holds for the Kerr effect of nematic liquid crystals in the isotropic phase.
References
[ l ] SCHADT, M. and HELFRICH, W . , Mol. Cryst. Liq~i. Cryst. 17 [4b] MEIER, W . , SAUPE, A., 2. Naturforsch. 13a (1958) 564 ;
(1972) 335. > , 14a (1959) 882 : 15a (1960) 287. . .
121 FILIPPINI, J . C., C . R. Hebd. S a n . Acad. Sci. B 275 (1972) STINsoN, '. W. LmsTERy J. D., Phys. Rev. 25
349. (1970) 503.
161 CHU, B., BAK, C . S., LIN, F. L., Phys. Rev. Lett. 28 (1970) [3a] PROST, J. and LALANNE, J. R., Phys. Rev. A 8 (1973) 2090. 1 1 1 1 .
[3bl WONG, K. L. and SHEN, Y . R., Phys. Rev. Lett. 30 (1973) [7] LITSTER; J . D. and STINSON, T. W., J. Appl. Phys. 41 (1970)
895. 996.
[4a] DE GENNES, P . G., Mol. Cryst. 12 (1971) 193. [8] JOHNSTON, A. R., J. Appl. Phys. 44 (1973) 2971.
