High-resolution x-ray measurements of the smectic phases of terephthal-bis-(4n)-alkylanilines

Abstract

High-resolution x-ray measurements (error ∼200-400 ppm) of smectic layer spacing $d$ of six homologs (TB3A-TB8A) of terephthal-bis-($4n$)-butylaniline (TBBA) have been made as a function of temperature. In the smectic-$C$ phase, the exponent of the tilt angle for TB3A and TBBA has a value close to $\frac{1}{3}$, implying critical behavior. For higher homologs the smectic-$A$ to -$C$ transition becomes weakly first order. Two new effects, asymmetry of the Bragg reflections in the smectic-$C$ phase due to layer undulations in the bulk sample, and existence of domains with different values of $d$ in smectic-$H$ and lower-temperature phases, were observed. The jump in $d$ at the smectic-$C$ to -$\frac{F}{H}$ transition and its variation in the $H$ phase with temperature are strongly dependent on the size of the molecule. In the smectic-$H$ phase the molecular tilt (also measured by conoscopy) for TB8A decreases with decreasing temperature in contrast to other homologs. However, the (110) spacing (measured with error less than 50 ppm) of the smectic-$H$ phase increases with temperature for both TB5A and TB8A. The smectic-$F$ phase of TB8A was, for the first time, detected by miscibility with TB7A.