X-Ray Study of the Mesophase-Mesophase Transition of Octaalkyloxyorthocyclophanes

Abstract

Previous studies of the mesomorphic octaether derivatives of cyclotetraveratrylene, CTTV-I- n (n is the number of carbon atoms per ether chain), have shown two columnar mesophases for 9<n<16. In the higher temperature mesophase the columns are packed in a two-dimensional hexagonal unit cell, while in the lower phase the packing is two-dimensional centered rectangular. DSC showed no heat of transition between the two mesophases. For n=13, the transition occurred over a wide temperature range. We find this wide transition region is a third mesophase, with an oblique net intermediate between the other two. The upper boundary of this phase marks breaking of hexagonal symmetry by the appearance of a unit cell having γ=120°, but with a≠b. With decreasing temperature, b is essentially constant while a and γ increase until -2b cosγ=a, marking the transition to the primitive cell for the two dimensional centered rectangular net. Similar behavior is observed for n=14.