Liquid-crystalline polymers nematic-isotropic transition

Liquid crystalline solutions as such have not yet found any commercial uses, but highly orientated liquid crystal polymer films are used to store information. The liquid crystal melt is held between two conductive glass plates and the side chains are oriented by an electric field to produce a transparent film. The electric field is turned off and the information inscribed on to the film using a laser. The laser has the effect of heating selected areas of the film above the nematic-isotropic transition temperature. These areas thus become isotropic and scatter light when the film is viewed. Such images remain stable below the glass transition temperature of the polymer. 

The SD is a phase separation process usually occurring in systems consisting of more than two components such as in solutions or blends. However, in the present case the system employed is composed of one component of pure PET. In this case, what triggers such an SD type phase separation Doi et al. [24, 25] proposed a dynamic theory for the isotropic-nematic phase transition for liquid crystalline polymers in which they showed that the orientation process... 

It is not necessary to carry out synthesis, if the triggering photochromic compound has good affinity to the polymer matrix. A mixture of the polyacrylate with BMAB which exhibits an excellent function as trigger is equally photoresponsive. While the monomer model compound (i.e. the acrylate before polymerization) does not provide a liquid crystalline phase, the polymer shows a clear nematic - isotropic transition at ca. 61 °C and the glass transition temperature at 24 °C as shown in Fig. 4. Tj j depends very much on the length of the alkyl spacer. In comparison with the... 

The Onsager and Flory theories are both statistical theories on rigid rod liquid crystalline polymers, but the former is a virial approximation while the latter is a lattice model. The first is more applicable to dilute solutions while the latter works especially well for high concentrations and a highly ordered phase. These theories with experiments, especially critical volume fractions 4>i and critical order parameter Sc at nematic-isotropic transition are made below. 

Many liquid-crystalline polymers are semi-crystalline and have only one mesophase that is nematic. The DSC thermograms of such polymers are very similar in appearance with Figure 4.22. Many other polymers are noncrystalline but have one and only one mesophase. The DSC will then show a glass transition and a transition from the mesophase to the isotropic liquid phase. Yet many other polymers may have more than one liquid crystalline phase so that their DSC curves should also include the transitions from one mesophase to another. DSC is obviously a very useful and very convenient technique for the characterization of liquid crystalline polymers. Nevertheless, the following points should be emphasized in order to interpret the DSC results with less ambiguity. 

In the latter system, there appears to be competition between alignment and thermal motion, so the best results were obtained when the poling was carried out close to the 298 K rather than at higher temperatnres the nematic to isotropic transition was = 373 K. More recent attempts have been made to improve the poled systems by incorporating the nonlinear, optically active molecnles into the polymer chain structure and comb-branch liquid crystalline polymers with R and the new group... 

The thermotropic behavior of liquid crystalline polynorbornenes also reach their limiting values at 50 repeat units or less [22, 182, 188-190]. For example, Fig. 9 demonstrates that the glass and nematic-isotropic transitions of both terminally and laterally attached systems level off at 25-50 repeat units, and correspond to the transition temperatures of the infinite molecular weight polymers. The same is true of the crystalline melting and smectic-isotropic transition temperatures of poly ( )-endo, exo-5,6-di [ -[4 -(4"-methoxyphenyl)-phenoxy]hexyl]carbonyl bicyclo[2.2.1]-hept-2-ene) [190]. 

In SMPs with a liquid crystalline transition the specific heat capacity increases significantly up to the transition point due to long range fluctuations of the order parameter near the transition [49]. At the transition temperature, a first-order phase transition occurs. The recorded DSC peak of the liquid crystalline transition will be the mixture of these two contributions. A schematic example for a liquid crystalline polymer is shown in Fig. 2 (diagram e), showing transitions in the form of sharp endothermic peaks, Tc-n for the crystal-nematic transition and for the nematic-isotropic transition. 

Still in the isotropic phase, but closer to the phase transition temperature, a shear induced transition to the nematic phase occurs, see Fig. 6. Based on the equations presented here, such a behavior has been predicted theoretically quite some time ago [20, 21]. This phenomenon has been observed in lyotropic liquid crystals, in particular with wormlike micelles [5] and in side-chain liquid-crystalline polymers [35]. In Fig. 6, results are presented for = 1.3. For comparison, the highest temperature for which a metastable nematic phase exists, -d = 9/8 = 1.125, is included. For imposed shear rates, shear stress and consequently the viscosity jump smaller values at the induced phase transition. For imposed shear stress there is a jump to higher shear rates. 

The review is organized as follows Sect. 2 deals with the shear-thickening behavior found in dilute and very dilute surfactant solutions. Section 3 examines the shear-banding instability and the isotropic-to-nematic transition revealed in the semidilute and concentrated regimes, respectively. The last part focuses on the wormlike micellar nematics under shear, and emphasizes the analogy with liquid-crystalline polymers. 

---