Temperature dependence of the splay and bend viscosities of PAA

Quasielastic laser light scattering is used to study the splay and bend modes of p-azoxyanisole (PAA) as a function of temperature. At values of the scattering wave vector, where more than one mode is present, extraction of single mode parameters is complicated by the need to fit the data to sums of exponentials. At the relatively large scattering angles at which the modes are isolated, the scattered signals are usually weak. By using the 647.1 nm line of a Krypton Ion laser, significant improvement in signal-to-noise ratios are achieved and by experimentally isolating a single mode, more reliable fits are obtained. The two viscosities, ηsplay and ηbend, are obtained from the decay rates of the director fluctuations using the K1 and K3 data from Freedricksz transition experiments performed at Cambridge [J. Phys. 40, 1513 (1985)]. An empirical approach is used to describe the temperature dependence of the viscosities. The nature of the coupling of the order parameter to the viscosities is believed to be responsible for the behavior of the two viscosities.