2010 Volume 67 Issue 4 Pages 254-262
The spacer conformations of two main-chain liquid crystals (LCs), CNφφ-OCH2CH2-X-CH2CH2O-φφCN (X=O; CBE2, X=CH2; CBA5), have been investigated on the basis of conformational energies derived from density functional calculations and the orientational order parameters from 2H NMR experiments under the rotational isomeric state approximation. Polarization microscopic and differential calorimetric observations revealed that CBA5 and CBE2 form an enantiotropic and a monotropic nematic LC phase, respectively. The intramolecular orientational correlation P2(cos ψ) of CBA5 was found to remain invariant over the entire range of the LC phase, whereas P2(cos ψ) of CBE2 definitely decreases with decreasing temperature. Here, ψ stands for the angle between mesogenic core and molecular axis. These results suggest that LC molecules will show the so-called nematic conformation to adapt themselves to the uniaxial potential field, and that the stability of nematic conformation attributed to the bond rotational characteristic of chain segment (CO-C-C-X) strongly correlates with the stability of the nematic LC phase.
