Refractive index measurements at a 2nd order smectic A to C phase transition

2014 A measurement of the 3 principal refractive indices of azoxy-4,4’ (di-undecyl 03B1-methyl cinnamate) is made in the vicinity of the 2nd order smectic-A to smectic-C phase transition. The experimental procedure is : first the index differences are measured using the conoscopic method, then the absolute value of one of the indices is determined using an Abbe refractometer. In the smectic-A phase, a slow quasi-linear temperature dependence of the indices is found. In the smectic-C phase a faster effect adds to the smectic-A background. This effect is studied in more detail, and discussed in terms of angular fluctuations of the molecules. LE JOURNAL DE PHYSIQUE TOME 39, DÉCEMBRE 1978, Classification Physics Abstracts 61.30 64 . 70E 2013 42.70 The smectic-C liquid crystal phase is known to have monoclinic symmetry [1] i.e. to be biaxial [2, 3] ; so there exist 3 principal refractive indices in the C phase. Here we present measurements of these 3 indices versus temperature in AMC-11 (azoxy-4,4’ (di-undecyl a-methylcinnamate)) through the A to C phase transition. This phase transition appears to be a good second-order one since neither differential scanning calorimeter measurements (on PerkinElmer DSC-2) nor precise tilt measurements [4] show any jump at the transition. This compound also has the big advantage of not degrading too much with time, because of the relative stability of the azoxy bridge. Another advantage of AMC-11 is that good homeotropic alignment can be obtained using polymerized silane coated plates [5] and this point is very important for precise optical measure-