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Increased Nematic-Isotropic Transition Temperature on Doping a Liquid Crystal with Molecularly Rigid Carboxylic Acids.

A thermotropic nematic liquid crystal (LC) becomes an isotropic liquid at the nematic-isotropic transition temperature (TNI), which depends on the molecular order of the mesophase. By means of a polarized optical microscope and a differential scanning calorimeter, it was found that doping the nematic liquid crystal 4-n-pentyl- 4'-cyanobiphenyl (5CB) with molecularly rigid carboxylic acids (benzoic, 1-naphthoic, 2-naphthoic and biphenyl-4-carboxylic acids) increases TNI without modification of the nematic-isotropic transition enthalpy. This increment in TNI is due to the increased order caused by the formation of molecularly rigid and elongated dimers of carboxylic acids in the nematic LC, as confirmed with infrared spectra. Furthermore, TNI increased with the length of the molecularly rigid dimers at the same concentration level. Conversely, doping the LC with molecularly flexible acids caused lowering of TNI. A quantitative correlation was established between the TNI increase of the rigid carboxylic acids and the length of the dimers of these acids; a predictive model for these ΔTNI values as a function of acid molar fraction was developed. It was also demonstrated that the doping of 5CB with rigid carboxylic acids increases the rotational viscosity of the liquid crystal.