Effects of Molecular Length on Nematic Mixtures — IV: Structure Effects on Viscosity of Ester Mixtures

Abstract

The flow viscosity (η) of nematic ester mixtures is studied as a function of average molecular length (\(\bar L\)), chemical structure, and temperature. When \(\bar L\) is increased by use of longer alkyl end groups, the η of less polar mixtures increases while the η of more polar mixtures changes slightly or even decreases. However, when cybotactic nematic characteristics occur with increased \(\bar L\), then η increases sharply. Studies of η are made for eighteen different classes of ester structures used as additives in 4-alkoxyphenyl 4-alkylbenzoate (R0-R’) mixtures at fixed values of \(\bar L\) for both additive components and mixtures. Many interesting effects of structure on η are observed, and approximate class viscosities (ηclass) at 25°C are assigned to each of the 18 classes. The η25° of other ester mixtures is estimated by summing ηclass times the mole fraction of that class present in the mixture. These ηcalc values are generally within 10% of the actual η25° for multicomponent ester mixtures containing some RO-R’ components. Temperature variations often result in a non-linear plot for log η vs T-1 of ester mixtures. The apparent activation energy between 25° and 40°C generally increases strongly with the η of the mixture, ranging from 6.7 to 12.0 kcal/mole between η25° values of 16 and 188 cP.