X-RAY STUDY OF THE MESOPHASE-MESOPHASE TRANSITION OF OCTAALKYLOXYORTHOCYCLOPHANES

Previous studies of the mesomorphic octaether derivatives of cyclotetr averatrylene, CTTV-I-n (n is the number of carbon atoms per ether chai n), have shown two columnar mesophases for 9<n<16. In the higher tempe rature mesophase the columns are packed in a two-dimensional hexagonal unit cell, while in the lower phase the packing is two-dimensional ce ntered rectangular. DSC showed no heat of transition between the two m esophases. For n=13, the transition occurred over a wide temperature r ange. We find this wide transition region is a third mesophase, with a n oblique net intermediate between the other two. The upper boundary o f this phase marks breaking of hexagonal symmetry by the appearance of a unit cell having gamma=120 degrees, but with a not equal b. With de creasing temperature, b is essentially constant while a and gamma incr ease until -2b cos gamma=a, marking the transition to the primitive ce ll for the two dimensional centered rectangular net. Similar behavior is observed for n=14.