Nematic blend

On the other hand, miscibility is, in one sense, one of the characteristic properties in liquid crystals. Miscibility in liquid crystals is a well-known macroscopic property where one can see two mesogens exhibiting a thermodynamically identical liquid crystalline phase are mixed to show its phase at arbitrary component ratio. This has already been applied to liquid crystals for LCDs to control some properties such as temperature range of nematic phase. A diversity of functional properties such as temperature range, anisotropic electrical permittivity, viscosity etc. can be controlled in nematic blends and non-mesogenic molecules also can be a component which contributes to the resultant properties as they behave like a solute in liquid solution. However, charge transport property has not yet been well studied in terms of molecular blends with liquid crystalline materials, while thin film organic photovoltaics have been so extensively studied in recent years as molecular blends. 

Fig. 8.13. Temperature dependence of the forp-azoxyanisole (1) and its nematic blends...

Solution blending Polar as well as nonpolar solvents can be used in this method. The polymer is solubilized in a proper solvent and then mixed with the filler dispersion. In solution, the chains are well separated and easily enter the galleries or the layers of the fillers. After the clay gets dispersed and exfoliated, the solvent is evaporated usually under vacuum. High-density polyethylene [24], polyimide (PI) [25], and nematic hquid crystal [26] polymers have been synthesized by this method. The schematic presentation is given in Scheme 2.2. 

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... 

Polarized optical photographs of the blends are shown in Figure 20.3. The spherical LCP domains are irregularly dispersed in the PEN and PET phases below 20 mol % PHB content (Figure 20.3(a)). The results observed from 30 mol % PHB reveal a continuous co-existence of the PHB phase and the PEN/PET matrix in the blended polymers (Figure 20.3(b)). However, the blend with 40 mol % PHB shows a nematic LC phase. This result is similar to that found for the copolyesters synthesized by Chen and Zachmann [26], who found... 

As sparse as the dataset describing mainchain nematic LCP blends with conventional polymers is, it is rich compared to the almost non-existent data on the blending of other types of LCPs-side chain polymers, flexible spacer polymers, smectics, etc. 

As in the case of LCP/conventional polymer blending, little data exists on the blending of LCPs of different inherent chain architecture or mesophase symmetry. Publications from the laboratories of Ringsdorf [80] and Finkelmann [81] show phase separation in blends of sidechain nematics with other similar polymers or small molecule analogs. It is now established that, in contrast to the behavior of low molecular weight LCs, LCPs are often immiscible. 

Consider a number n of stiff polymer components (here stiff is used to mean semiflexible ) and define orientation-dependent ideal and interacting response (n x n) matrices X0(Q, u, u ) and X(Q, u, u ) respectively. In this case, orientational correlations have to be included in addition to the usual isotropic ones. Doi et al. [36-38] have developed the theory for solutions of stiff homopolymers. Their formalism is applied in Appendices C and D to multicomponent blend mixtures of stiff polymers without and with the incompressibility condition respectively. The interaction potentials comprise anisotropic (also called nematic) contributions as well as the usual isotropic ones ... 

The isotropic-to-nematic transition is defined by the characteristic equation Det M = 0 (where Det represents the determinant of a matrix). If the Van der Waals interactions were turned off (W0 = 0) so that only nematic interactions are left, then M would be the denominator of X so that X would blow up for this condition (Det M = 0). Above certain critical values of Wj s the blend forms the nematic phase. As in the case of purely flexible mixtures, the spinodal condition is ... 

Influence of chain length and spacer length on Tg, on the heat capacity increment at Tg (ACp), and the shape of the Cp(T) curve win be presented in the first paper (preliminary results can be found in (3)). Subsequently, we will address influence of thermal history in the isotropic phase and N+I biphase, physical aging below Tg and Tg in blends of LCPs. Finally, an interpretation of the macroscopic data in terms of molecular organization in these and other nematic LCP glasses will be attempted. 

Figures 4 and 5 contain the DSC traces which are obtained upon second heating for each of the component materials. The small endotherm observed is typical of these materials. This is attributed to the small entropy change which occurs at the crystal-nematic transition temperature (15). It should also be noted that the two polymers display transition temperatures which differ by about forty degrees. Thus, in the blend samples, if two endotherms are present, there should be no problem in discerning them.

Figure 14 shows the phase diagram of a flexible-semiflexible polymer blend at a constant pressure. Theoretical calculations and experimental results show that such mixtures can exhibit an isotropic-isotropic and isotropic-nematic phase separation. Our calculations are able to capture the isotropic-isotropic phase separation and serve to show that the origin of such a transition can be purely entropic. 

The finding that the PEIs of 27b and monosubstituted hydro quinones form broad nematic phases, but show little propensity to crystallize, has prompted various modifications of their structures and properties. In this connection it should be stated that non-crystalline LC-polymers have found little interest in the past decades, but they may be attractive for various applications provided that the Tg can be varied between 90 and 250 °C. For instance, the absence of crystallinity has the advantage that the mechanical properties do not depend on the thermal history, and thus on the processing conditions. The temperature allowing a convenient processing may be reduced below 200 °C, which is of interest for the processing of LC-polymer reinforced blends and composites. Furthermore, non-crystalline nematic FC-polyesters are a useful basis for the synthesis of cholesteric lacquers, films or pigments (Sect. 5). 

In contrast, the transitions of most well-defined SCLCPs prepared by controlled polymerizations are relatively narrow. The effect of polydispersity was therefore investigated by blending well-defined (pdi < 1.28) poly 5- [6 -[4"-(4 "-methoxy-phenyl)phenoxy]alkyl]carbonyl bicy-clo[2.2.1]hept-2-ene)s of varying molecular weights (DP =5, 10, 15, 20, 50, 100) to create poly disperse samples (pdi = 2.50-4.78) [22]. In this case, both monodisperse samples and multimodal blends underwent the nematic-isotropic transition over a narrow temperature range. Polydispersity also had no effect on the temperatures of transi-... 

Furthermore, the blends with more than 3 wt% of PLA exhibits large haze values, indicating the existence of phase-separated morphology as previously reported by Tatsushima and co-workers [56]. As a result, the blends lose transparency and show similar level of orientation birefringence to the blend with 1 wt% of PLA. This is reasonable because PLA chains in the dispersed phase have no nematic interaction with CAP chains, thus the orientation will relax immediately as compared to the CAP. As demonstrated, much attention has to be focused on the miscibility for this method. 

Figure 9.29 Nematic interaction parameters between CAP and PET or PEN, eCAP, add in CAP/PET and CAP/PEN blends. Reproduced with permission from S. Nobukawa, H. Hayashi, H. Shimada, A. Kiyama, H. Yoshimura, Y. Tachikawa, and M. Yamaguchi,/. Appl. Polym. Set, 2014, in press. 2014, Wiley Online Library [61].

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