Disorder isotropic

The types of liquid-crystalline phases of interest can be defined by the direction describing the preferred orientation (see Chapter 8). The limits are the usual completely ordered (crystalline) and completely disordered (isotropic) phases, with a nematic liquid-crystalline phase ("mesophase") between them. 

Unlike ferromagnets, where G((p) = G(-(f), the free energy of nematic liquid crystals exhibits an asymmetry G(S) G(-5), as discussed previously and shown in Fig. 3b. To reproduce this behavior, a term that is an odd power of the order parameter S is needed in the Landau expression for the free energy of a nematic liquid crystal. A term linear in 5 is not allowed since the equilibrium condition dQdS = 0 could not then be satisfied in the disordered isotropic phase where 5=0. The presence of a cubic term will lead to the desired asymmetry in G as a function of 5 and to the emergence on cooling of a second minimum in G at a finite 5 value. 

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

Such a description encompasses the whole field of liquid crystals, where it is non-covalent, intermolecular interactions which determine the molecular organization leading to the various liquid crystalline phases or, in the extreme, to a totally disordered isotropic phase [2], The science of liquid crystals is really the art of balancing the various intermolecular interactions to achieve a desired liquid crystal phase rather than an ordered crystalline phase. Nature demonstrates this art in its highest form in the self-organization of lipids to produce liposomes and cellular membranes. 

The influence of chain packing (Le. free volume) on solubility, diffusivity and permeability in liquid crystalline polymers can be studied by comparing properties of LCPs in the disordered, isotropic state with those in the ordered, liquid crystalline state. HIQ-40 is a random, glassy, thermotropic, nematogenic terpolymer synthesized from 40 mole percent p-hydroxybenzoic acid and 30 mole percent each of isophthalic acid and hydroquinone. The chemical structures of the constituent monomers for fflQ-40are ... 

By comparing the sorption and transport behavior of small molecules in an as-cast, disordered, isotropic sample with those of an annealed, ordered, frozen liquid crystalline sample, the effect of axial ordering on sorption and transport properties may be determined unambiguously. Moreover, the influence of axial ordering on other properties (e.g. density, fractional free volume, glass transition temperature, and free volume accessible to orthoPositronium) may be determined. 

Figure 2. Structure of as-cast and annealed HIQ-40 films. In the as-cast sample, the mesogenic units of the polymer chains are kinetically trapped in a disordered, isotropic arrangement. In the annealed sample, arrows in the domains represent the director, n, and point in the direction of orientation of the domains.

Generally, the photoisomerization of azobenzenes is both thermally and optically reversible cis-io-trans isomerization can be driven with irradiation at visible wavelengths or by thermal relaxation. In a bulk LC material, this can result in an isotropic-to-liquid crystalline phase transition. Thus, light can be applied at different wavelengths to control the switching between the ordered LC and disordered isotropic states in both directions, as shown in Figure 7.4. This is very important for photonic applications of PLCP materials. 

The limits of deformed LC networks are the usual completely ordered (crystalline) and completely disordered (isotropic) phases. The nematic LC phase is a mesophase between them. The oriented parts are the sequences along the chain backbone. In this case, the disorder is the already-mentioned sliding of segment relative to one another to place them out of register. There are also a variety of smectic LC phases, in which layers of molecules or chain sequences occur in layers that are disordered relative to one another. In contrast, cholesteric phases have layers of nematic arrangements that are stacked in rotated arrangements, and a similar stacking occurs in the case of the discotics. ... 

During fabrication the orientation occurs on the surface, which depends on chemical nature of LC polyester. The molecular weight also affects surface orientation the short chains being rapidly oriented. The low viscosity of liquid crystals monomer/polymer as compared to isotropic fluid is due to their ready local alignment. The rheology is complex in nature. The viscosity in the anisotropic phase is much lower as compared to that at disordered isotropic state. In the anisotropic solution phase the director readily aligns in the shear direction and lower viscosity results. 

Phase transitions in condensed phases are characterized by symmetry changes, i.e. by transformations in orientational and translational ordering in the system. Many soft materials form a disordered (isotropic) phase at high temperatures but adopt ordered structures, with different degrees of translational and orientational order, at low temperatures. The transition from the isotropic phase to ordered phase is said to be a symmetry breaking transition, because the symmetry of the isotropic phase (with full rotational and translational symmetry) is broken at low temperatures. Examples of symmetry breaking transitions include the isotropic-nematic phase transition in hquid crystals (Section 5.5.2) and the isotropic-lamellar phase transition observed for amphiphiles (Section 4.10.2) or block copolymers (Section 2.11). 

Polymer crystallization can be described as a phase transition process from the disorder isotropic melt to the order semicrystalline one. The disorder state is characterized by the randomly coiled chains, while the order state is complex because it is formed by crystalline chain-folded lamellae surrounded by the amorphous chains that constitute the fold surfaces and the interlamellar regions. The amorphous interfaces are formed by entanglements, end groups, bulky substituent groups, and chain defects, all of which cannot be included into the crystalline lattice. Polymers form metastable (thin) lamella separated by intervening amorphous layers and are nearly always semicrystalline. 

Cooling down from the disordered isotropic phase to the ordered blue phase an enhancement of the molecular optical rotation occurs as a result of short-range ordering. The chirality of the cholesteric liquid crystal gives rise to complex pretransitional phenomena. 

The number of polymers which form structures intermediate between a three-dimensionally ordered crystalline phase and a disordered isotropic phase is... 

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