D-NMR Study on Orientation of D-Labeled Benzyl Group in PBLG Liquid Crystal

Abstract

D-NMR was clearly observed from the concentrated dichloromethane solution of PBLG (0.346 gr/ml), in which helical PBLG polymers form a cholesteric state in case of no magnetic field. The observed spectrum was decomposed into a singlet and five doublets, among which the doublet having smallest separation was stronger than the singlet and the rests. Since the doublets originate from the quadrupolar splitting of the Zeeman level of deuteron, the observed doublets mean the partial alignments of the benzyl groups in the liquid crystalline state. The order parameters were estimated as 0.0040, 0.025, 0.072, 0.10, and 0.13 from the observed splittings of the five doublets and e²qQ = 167 KHz for a static C-D bond. The singlet means complete averaging of the quadrupolar splitting due to a rapid motion. By rotating the sample about the axis perpendicular to the magnetic field, the observed separation of the strongest doublet was changed immediately after the rotation and reached to the former equilibrium value after ca. several minutes of the residence time. A dependence of the instantaneous separation upon the rotation angle, θ, was well approximated by |3 cos² θ − 1|. The spin-lattice relaxation times of both the singlet and the strongest doublet were found to be same. This fact suggests that rates of the molecular motions are almost same in the states corresponding these peaks. A model for the motion of the side chain end is proposed to explain the results consistently.