The mesomorphic phase transition temperatures of the unsymmetrical 1,4-phenylene esters of the 4-n-alkoxybenzoic acids

The normal melting process occurs at a specific temperature, characteristic of every substance, when the constituent molecules of the solid have sufficient vibrational kinetic energy to overcome the net attractive forces binding the crystal lattice. The resulting phase change involves a transition from the highly ordered environment of the solid to the isotropic liquid. From X-ray diffraction studies1, the isotropic liquid has been found to possess some short range molecular ordering. The absence of any long range order prevents the liquid from exhibiting properties of the solid state, particularly those properties which are characteristically associated with crystalline anisotropy. However birefringence and some other anisotropic properties are shown by the melts of certain organic compounds. The phenomenon was first observed by Reinitzer2 in 1888 with esters of cholesterol and Lehmann3 in 1890 with ammonium oleate and p-azoxyphenetole. Both men found that the substances melted sharply at specific temperatures to give cloudy liquids which, when heated further, cleared suddenly at characteristic temperatures to yield the normal liquid. The melts were also found to be birefringent in thin films when viewed with crossed polaroids - a property previously found only in crystalline solids