Paranematic phase

Director modes are, as opposed to biaxial fluctuations, excited very easily in the nematic phase, where their Hamiltonian is purely elastic, whereas in the isotropic phase they are characterized by a finite correlation length. This implies that their wetting-induced behavior should be quite the inverse of that of biaxial modes. Thus, in the disordering geometry, the director modes are forced out of the substrate-induced isotropic boundary layer into the nematic core (see Fig. 8.6 bottom). The lowest mode is a Goldstone mode. In the paranematic phase a few lowest director modes are confined to the nematic boundary layer, whereas the upper ones extend over the whole sample and are more or less the same as in the perfectly isotropic phase. 

Interaction between phase boundaries gives rise to an attractive fluctuation-induced force which is proportional to exp —2 d — 2dw)/ i)- The attraction is due to the identical boundary conditions. Except in the vicinity of the metastability limit of the paranematic phase, the distance between the substrate and the phase boundary is much smaller than the distance between the two boundaries, and the interaction between the phase boundaries is very weak. 

The effective interaction between solid substrates is a superposition of the two contributions discussed above. In the range of stable paranematic phase (0 > 1, d v d) the fluctuation-induced force between the two substrates is governed by the interaction between the solid substrate and the phase boimdary. It is mediated by the structural interaction which determines the functional dependence of the wetting layer thickness on the sample thickness... 

Fig. 8.11. (a) Structural force per unit area in a heterophase paranematic (thick lines) and nematic system with molten boundary layers (thin lines). Solid lines correspond to the force in the nematic phase and dashed lines to the force in the isotropic phase. For the thicknesses above the corresponding verticals the isotropic (paranematic) or nematic phase is stable, respectively. The force is short-range and attractive, (b) Structural presure in the hybrid nematic system in a biaxial structure (solid line) and bent-director structure (dashed line). In both cases the interaction is long-range and repulsive. 

Smeared Paranematic-to-Nematic Phase Transition in LCEs. 163... 

Apart from the nonuniform director alignment, the inhomogeneity of the local order parameter is also encountered in LCEs. This is best observed in a H-NMR spectrum (Fig. 9b) recorded in the vicinity of the phase transition. The spread 5v of the spectral intensity between 0 and 20 kHz in this spectrum corresponds to a spread of the local order parameter 85 in the range between approximately 0 and 0.45. Two pronounced peaks can be noted in each half-spectrum, corresponding to the coexisting paranematic (lower S, inner peak at Vpn) and nematic components (higher S, outer peak at v ). 

The broadening of the spectral line is very moderate for the Xsc = 0.15 sample and much more pronounced for the LCEs with lower crosslinker concentrations. This pronounced broadening arises primarily from the distribution of the local order parameter S, which indicates either a higher heterogeneity or phase-transition behaviour closer to below-critical in less crosslinked LCEs, or both. This is particularly obvious for the Xsc = 0.075, for which one can observe a coexisting nematic and paranematic component (inner and outer peak) at Tpn.n. 

Fig. 24 Selection of H-NMR spectra of side-chain LCEs doped with (a) 8 and (b) 28 wt% of aD2-8CB. Arrows denote spectral features attributed to paranematic (PN) and nematic (N) phases. Data taken from [3]...

Ordered Network in an Isotropic Liquid Crystal. The strong pretransitional increase of the effective birefringence (Figure 12.25(a)) for all examined concentrations of the polymer suggests that in addition to the direct contribution of the polymer network, there is a temperature-dependent contribution from the paranematic order induced in the isotropic liquid crystal phase by internal surfaces of the network. 

In a recent work, isotropic-nematic-smectic A phase transitions in thermotropic liquid crystals were also induced by applying an electric field [140]. The liquid crystal investigated (a mixture of 8CB and lOCB) showed a first order isotropic to smectic A transition. When in the isotropic phase and near the spontaneous transition temperature, a field-induced first order transition was observed from a paranematic to a nonspontaneous nematic phase. For higher values of the applied electric field, another first order transition occurred from the nonspontaneous nematic to a phase exhibiting the same order as a smectic A phase. A phenomenological Landau-de Gennes model has been developed to describe these transitions [141],... 

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