Gelation of smectic liquid crystal phases with photosensitive gel forming agents

This paper studies the potential of 3 newly synthesized low molecular weight gelling agents (2 of them with azo groups) with the semicarbazide group as a H-bonding motif to gel the smectic phase of a commercial chiral smectic C* material. To detect gelation two methods were used: the stabilization of the director pattern, which measures the interaction of the physical gel network with the LC-director, and the suppression of the onset of electrohydrodynamic instabilities, which measures the increase of viscosity. Special emphasis was put on the influence of the photochemical trans-cis isomerization of the gelling agents on gelation. From the stabilization of the director pattern an elastic component can be seen by the memory effect (two different switching times). Photochemical trans-cis isomerization destroys this memory effect. The increase of the threshold field for the onset of electrohydrodynamic instabilities in free-standing films measures the increase of viscosity and the supression of flow. Both methods detect the same melting temperature of the gel and the same reduction of the gelling ability upon trans-cis isomerization. This proves that the gelling abilities of the new azo-containing gelling agents can be reversibly switched on and off by photoisomerization.