ResearchNote: On the Activation Energy of a Friction Coefficient of Liquid Crystals

Abstract

Temperature dependence of the friction coefficient -λ₁/p of a nematic liquid crystal in a range well below its clearing point is expressible in the usual exponential form. Examining data for 24 nematic liquid crystal compounds and mixtures, Flanders¹ observed that they exhibit a common activation energy value 10.9 kcal/mole. Earlier² we obtained values of the friction coefficient for 8 nematic compounds as a function of temperature from a set of old experimental data published by Zwetkoff and Sosnovsky.³ Following Flanders, we have prepared a plot of these data on a semi-log paper to confirm his finding for p-azoxyanisole (PAA), p-azoxyphenetole (PAP), p-methoxycinnamic acid, di-p-acetoxybenzalazine, anisaldazine and di-benzalbenzidine (DBB). However, we find that the activation energy for anisal-p-aminoazobenzene (APAAB) and di-anisalbenzidine (DAB) is 5.5 kcal/mole, only a half of the more common value. As shown in Figure 1, the smaller value for APAAB could be due to a large experimental error (∼15%) at 140.5[ddot]C because the slope coincides with DBB and others in a higher temperature range. The slope for DAB is definitely exceptional, though. We note in addition that, at least for PAA adn PAP, the coefficients - λ₁/p and - λ₁ share practically the same activation energy becuase of the negligible variation of the mass density p with temperature.