Proton NMR Investigations of the Smectic-C Phases of Three Alkyloxy-Azoxybenzenes

Abstract

Proton NMR investigations of the smectic-C phases of three 4,4'-bis-/?-alkyloxy-azoxybenzenes (C 7 , C 8 , C 9 ) with temperature independent tilt angles are presented.The behaviour of well-aligned samples (strong magnetic field ^ 1.5 T, slow cooling rate) under rotation in the field can be described by the models of Luz and Meiboom, and Wise, Smith, and Doane, (LM/WSD), resp. However, on polarization of the samples in presence of magnetic fields ^ 0.75 T the molecular directors are not arranged preferentially parallel to the direction of the polarizing magnetic field but are rather inclined, i.e., the layers are stacked preferentially perpendicular to the original magnetic field direction (PSL model). It is shown that all the angular dependences of NMR second moments can be interpreted in terms of a superposition of the LM/WSD and PSL models, or, on the other hand, by assuming totally disordered fractions of the samples together with portions that fully obey the behaviour demanded by one of the above models (LM/WSD or PSL). The tilt angles derived from the comparison of experimental and theoretical angular dependences of the second moments for well-aligned samples are applied to the explanation of the experiments at lower polarizing fields successfully, too.

Proton nmr studies of smectic phases of three n-(4-n-alkyloxybenzylidene)-4'-n-alkylanilines (n0 . m's)

“…In contrast to this, for the propyl member of the TBBA homologous series we observed a distinct change in the proton NMR lineshape at the C-G transition at 0 = 0 [13]. This is due to the fact that the molecular directors do not remain aligned to the original polarizing direction because there is a jump of the tilt angle at this transition of about 140 [14] not accomponied by a corresponding change in the inclination of the layers.…”

“…Distinct jumps are discernible at the transitions smectic-A-smectic-C (since the linewidth minimum in smectic-C is shifted towards greater rotation angles because of the possibility of nearly free rotation of the molecular directors on the tilt cone, cf. [10][11][12]), smectic-C-smectic-B, and smectic-B-smectic-G. Whereas in smectic-A the MAO-linewidth seems to be temperature independent (Av 1.4 kHz for 50.6 and 1.0 kHz for 70.5), it clearly grows with falling tem- Fig.…”

“…This is due to the fact that the molecular directors do not remain aligned to the original polarizing direction because there is a jump of the tilt angle at this transition of about 140 [14] not accomponied by a corresponding change in the inclination of the layers. Obviously the viscosity in smectic-G is too high in this substance to admit alterations in the layer distribution [13]. 4.…”

“…The smectic-C phases of 50.6 is so narrow that no angular dependences could be taken. For 70.6, however, an angular dependence at the end of the smectic-C phase has been recorded, yielding by comparison with theoretical angular dependences (detailed description in [10]) a tilt angle of about 12° (assuming the validity of the Luz-Meiboom/Wise-Smith-Doane model [11,12]).…”

“…figure 4 for 70.5. The crosses ( + ) in the smectic-C plot correspond to the theoretical dependence according to the Luz-Meiboom/Wise-Smith-Doane model [11,12] calculated as specified in [10] for a tilt angle 6 = 12.50.…”