Paranematic ordering

The last issue considered in this Chapter will be pretransitional ordering in a nematic sample with dispersed polymer networks above the NI transition T j = 1.1232). As seen already in studying a regular fiber array, some surface-induced paranematic ordering may persist in the vicinity of fibers. This ordering can be detected by optical means [2] alternatively, it can be detected also by NMR [23]. 

Figure 13. Pretransitioiial ordering in sample C at T = 1.2, for planeur anchoring with w = 1-. S x, y) order parameter map cross section at z/a = 25- The paranematic order decays to zero over the correlation length f k 5a, except in polymer-rich areas where nematic bridges Can form in between fibers (as here in the left lower corner with S < 0.2). Note that while the columns denoting fiber positions are vertical (with S = 1), the fibers themselves can be tilted with respect to the rry-plane (Fig. 9).

Even at high temperatures there is a nonzero splitting of the spectral doublet, which indicates a residual nonzero paranematic order... 

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. 

Fig. 14 shows the simulated transmitted light intensity (/) curves versus reduced temperature (above Tjyj = 1.1232) for different anchoring strengths w. As expected, the intensity I is nonzero due to paranematic surface-induced ordering, S) 0, and decreases with increasing... 

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. 

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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