Dielectric behaviour of two dimeric liquid crystalline siloxanes

Abstract

The temperature and frequency dependence of their dielectric constants have been investigated for two dimers consisting of swallow-tailed terminal groups connected via siloxane central parts of di erent length. The results have been compared with those of the respective monomer. The central part contributes in di erent ways to the static dielectric constants in the isotropic phase. From relaxation measurements, it can be concluded that the reorientation process about the short axis of the terminal groups is comparable with the respective process in monomers. The relaxation frequencies as well as intensities are almost unin¯uenced by the siloxane part which suggests a very¯exible structure of such dimers. Unfortunately, the maintenance of the antiparallel arrangement of swallow-tailed parts of the dimers could not be proved because of insu cient material for experimental work on the SA phase.

Study of dielectric parameters of liquid crystal elastomer

“…The corresponding loss peaks are usually extended over several frequency decades. [33][34][35] Similar behaviour is observed in Figure 3 for NLCE where high-frequency relaxation peak spread over frequency range of 10 kHz to 1 MHz.…”

“…In polymers, these α and δ relaxations are also observable with decreasing temperature. [33][34][35][36][37] For further justification, we studied the temperature dependence of permittivity for NLCE ( Figure 8) and compared it with that of polymers. We observed a remarkable similarity of the relaxation behaviour of two materials.…”

“…[37] In polysiloxanes, starting isothermally at low frequencies, generally, the α (alpha) relaxation is found, which accompanies the glass transition in the amorphous part of the material, which is the onset of segmental movements of the polymer chains. [33][34][35][36][37] The α relaxation has the highest strength, and therefore, it is often called the primary or main relaxation. At higher frequencies, the δ (delta) relaxation is present, which involves reorientational movements of dipoles carried by side group or liquid crystalline group [33][34][35] or is associated with molecular mobility within mesogenic moieties activated by the longitudinal component of the dipole moment of mesogenic group.…”

On the molecular theory of dimeric liquid crystals

“…Thus in the present, fully uniaxial case, all the dipolar correlation factors are conveyed by the pair averages <cos h ij > and <cos h i cos h j >. These can be evaluated according to equations ( 21) and (22). For the monomeric system there is just one pair average, pertaining to non-bonded mesogenic cores.…”

“…This reflects the favouring of parallel dipolar association, indicating that the dimer tends towards U-shaped conformations. The interplay between bonded and non-bonded correlation factors, as well as the qualitative dependence of that interplay on the spacer parity, can account for the diverse trends observed in experimental dielectric studies for the permittivity of nematic dimers [22][23][24][25] and some of the marked differences from the behaviour of the monomer compounds. Thus, for example, the experimentally observed decrease of the parallel permittivity component e // with decreasing temperature (increasing orientational order) and, more notably, its initial increase and subsequent decrease, are readily accounted for by the competing tendencies in the temperature dependence of the order parameters and correlation factors of bonded and non-bonded pairs shown in figures 4 and 5.…”