Dielectric and DSC investigations of 4-n-substituted benzoic and cyclohexane carboxylic acids .1. Textural changes in homologous 4-n-alkoxybenzoic acids

We present the results of dielectric- and differential scanning calorimetry investigations on several homologous of 4-n- alkoxybenzoic acids: in particular the 4-n-heptyl-, -octyl-, -nonyloxybenzoic acids are studied. Our measurements confirm the existence in the nematic phase of a textural transition, which on cooling changes a well ordered state, f.i. a twisted state, to a highly distorted one, characterized by the appearance of a multidomain structure. Such a transition turns out to be reversible, with a small hysteresis. The transition exhibits a clear evidence for the 4-n-octyl- and the 4-n-nonyloxybenzoic acids. On the contrary, the presence of this transition in the 4-n-heptyloxybenzoic acid is suggested by a small peak only in the curve of the dielectric permittivity. The effect of the simultaneous presence in the nematic phase of open and closed dimers and of cybotactic clusters can explain the observed dielectric behavior.