Field-Induced Order in the Isotropic Phase

Recently, deuterium NMR has been used to monitor orientational order [6.71, 6.72] induced in the isotropic phase of liquid crystals by an external field. This may be a magnetic field, an electric field, or a shear field produced by flowing the liquid [6.73]. The anisotropic magnetic energy of a particle in a magnetic field B is [6.73] 

FIGURE 6.10. Plot of the inverse field-induced quadrupolar splittings, Ai/i, versus temperature for the chain deuterons of 5CB-di5 measured in a magnetic field 

FIGURE 6.11. Plot of the biaxial order parameter Sxx Syy vs. Szz for p-xylene dissolved in the isotropic phase of a nematogen 60CB. The magnetic field strength is 11.6 T (after Ref. [6.2]). 

Void and R.L. Void, The Molecular Dynamics of Liquid Crystals, NATO Advanced Study Institute, II Ciocco (1989). 

15 Orsay Liquid Crystal Group, Phys. Rev. Lett. 22, 1361 (1969). 

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The first observation of optical-field induced ordering in the isotropic phase of a nematic liquid crystal was reported by Wong and Shen [74]. They have used a linearly polarized Q-switched, 50 kW ruby laser pulse to induce molecular ordering in the isotropic phase of a nematic liquid crystal. The in-... 

Externally applied fields that conjugate to the nematic order parameter can induce some orientational order in the isotropic phase (which is called the parane-matic order, in analogy with, e.g. paramagnetism) and suppress or even eliminate the I-N discontinuity for strong field values. An additional linear term —GS must be added to the free-energy expansion to account for the coupling between the applied field G and the order parameter S ... 

Shear flow-induced birefringence measurements of an isotropic solution of PpPTA in concentrated sulfuric acid with a clearing point of 45 °C were also performed by Picken [77]. As shown in Fig. 5 the flow-induced birefringence increases strongly when the isotropic-nematic transition is approached. The results demonstrate that the application of a relatively small shear rate already leads to a degree of orientational order, in the initially isotropic solution, that is comparable with the order in the nematic phase. This points to a strong coupling between the orientation and the external flow field, and to the occurrence of a shear-induced phase transition. 

At extremely high fields the isotropic phase should be indistinguishable from the nematic one, even well above the zero field transition temperature, since the uniaxial order induced by a magnetic or ac electric field in the isotropic phase will be comparable with the nematic orientational order. However, such fields are hardly accessible, even with the pulse technique. Much stronger changes in order parameter may be achieved with ferroelectric transitions (see below). 

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],... 

If the director is already parallel to the electric field, the free energy decreases as increases. This means an ordering of the anisotropic state, which in the isotropic phase is equivalent to a field-induced transition to the nematic liquid crystal phase. This is similar to the case when the pressure induces a transition to the lower symmetric phase. Such a situation is described by the Clausius-Clapeiron equation that relates the increment of the phase transition temperature, to the pressure. In our case this is equivalent to the field-induced increase of the isotropic-nematic transition as ... 

Orientational order plays an important role in solid polymers. It is often induced by industrial processing, for example in fibers and injection- or compression-modulated parts. In polymers with liquid-crystalline properties of the melt or solution, the anisotropies generated by the flow pattern are particularly pronounced. In order to improve the mechanical properties of polymer fibers or films, the degree of orientation is intentionally enhanced by drawing. At the same time, anisotropy of mechanical properties can result in low tolerance to unfavourably directed loads. In many liquid-crystalline polymers, in the mesophase near the transition to the isotropic phase, electric or magnetic fields can induce macroscopic orientational order [1]. Natural polymers such as silk protein fibers, which are biosynthesized and spun under biological condition, also have good mechanical properties because of their ordered structure [2]. 

In isotropic media, only the odd-order susceptibilities are nonzero because of inversion symmetry, hence the lowest order, nonlinear term is the third-order susceptibility. If (as in the gas phase) the number density N is low enough so that local field effects are small, then the macroscopic nonlinear polarization induced by the laser field is simply and this... 

The growth of the Kerr constant is accounted for by the considerable contribution of fluctuations of the orientational order parameter to dielectric properties of the isotropic phase. This contribution can be calculated in the framework of the Landau theory [218]. Field E induces the orientational order [219]... 

In Chapter 8 we will present a detailed discussion of the isotropic phase molecular orientation effects by an applied optical field from a short intense laser pulse. It is shown that both the response time and the induced order Q depend on the temperature vicinity (T-in a critical way they both vary as (r- which becomes very large near T. This near-r critical slowing down behavior of the order parameter Q of the isotropic phase is similar to the slowing down behavior of the order parameter S of the nematic phase discussed in the previous section. Besides the nematic isotropic phase transition, which is the most prominent order disorder transition exhibited by Uquid ciystals, there are other equally interesting phase transition processes among the various mesophases, such as smectic-A smectic-C, which will be discussed in Chapter 4. 

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