Measurements are presented for the diamagnetic anisotropy of thirteen nematic liquid crystals. As any bias in the rotation around the long molecular axis is most probably small, the results probe accurately the temperature variations of S, the degree of orientational order. The critical behaviour of S near the nematic-isotropic phase transition can be described by an exponent β of the order of 0.25. Procedures are considered to obtain the molecular magnetic anisotropy. By a systematic variation of the structure of the molecules an impression of the contributions of the various molecular groups can be obtained It turns out to be rather difficult to obtain accurate data for the molecular magnetic anisotropy. As a consequence the absolute scale of S can only be fixed with an accuracy of the order of 10 %
2014 On a construit un appareil pour mesurer les viscosités de cisaillement. ~1, ~2, ~3 et ~12 de Miesowicz. Celles-ci sont déterminées lors de l'ecoulement d'un liquide nématique dans un capillaire rectangulaire qui fait partie d'un appareil thermostatisé. L'ensemble est situé entre les pôles d'un électro-aimant pour contrôler le directeur n. De plus, on a déterminé l'angle d'alignement par écoulement en mesurant la variation de la biréfringence dans l'échantillon entre la situation où n est fixé parallèle à l'écoulement et celle où il est aligné par l'écoulement. On donne des résultats pour des ensembles complets de coefficients de viscosité de p-methoxy-p'-butylazoxybenzène et de p, p'-dibutylazoxybenzene. Les résultats sont bien en accord avec des rapports visco-élastiques obtenus par des techniques de diffusion de la lumière. La relation d'Onsager-Parodi est bien vérifiée compte tenu de la précision expérimentale. Abstract. 2014 To determine the Miesowicz viscosity coefficients a shear-flow setup has been built, which allows the measurement of ~1, ~2, ~3 and ~12. These coefficients are determined by pressing the liquid crystal through a rectangular capillary which forms part of a thermostated setup placed between the poles of a magnet in order to control the director n. In addition, the flow-alignment angle 03B80 is determined from the change in optical path difference between the ordinary and extraordinary components of polarized light, comparing the situation with n aligned along the flow-direction and with flow-alignment, respectively. Results are given for the complete set of viscosity coefficients of p-methoxy-p'-butylazoxybenzene and p, p'-dibutylazoxybenzene. The results show good agreement with visco-elastic ratios found with light-scattering techniques. Within the experimental accuracy the Onsager-Parodi relation is well fulfilled. Tome 44 N° 2 FEVRIER 1983
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