Cholesterics nematic-isotropic transition temperatures

Observed structures of a lyotropic material are classified into three categories nematic, smectic, and cholesteric. Nematic and cholesteric mesophases can be readily identified by microscopic examination. The existence of a smectic mesophase is not well defined and is only suggested in some cases. Solvent, solution concentration, polymer molecular weight, and temperature all affect the phase behavior of lyotropic polymer solutions. In general, the phase transition temperature of a lyotropic solution increases with increasing polymer molecular weight and concentration. It is often difficult to determine the critical concentration or transition temperature of a lyotropic polymer solution precisely. Some polymers even degrade below the nematic isotropic transition temperature so that it is impossible to determine the transition temperatures. Phase behavior is also affected by the polymer molecular conformation and intermolecular interactions. 

Chiral dithienylcyclopentene compounds can also be used to induce the cholesteric-isotropic phase transition [36]. A cholesteric polygonal fingerprint texture was exhibited by 10 wt% of 2 as a mesogenic dopant in a conventional achiral nematic 5CB as shown in Figure 5.6a. The cholesteric phase to isotropic transition temperature for the doped 5CB was 42 C. With UV irradiation at 310 nm (30 mW cm ) for 30 s, the sample went into the isotropic phase (Fig. 5.6b) whereas upon visible fight irradiation at 670 nm a reverse process was reached within 30 min (Fig. 5.6c). 

We made C15 8CB cholesteric nematic mixtures where the concentration, Cqo, of C15 in 8CB lowered the cholesteric-isotropic transition temperature by Tch (°C) = 40.36 0.22cqo (%). The pitch decreased with Coo as qo... 

Figure 13.13 Phase transition temperatures from isotropic phase to various structures as a function of chirahty. (a) isotropic-uniaxial nematic transition, (b) isotropic-uniaxial cholesteric transition, (c) isotropic-biaxial cholesteric transition, (d) isotropic-cubic transition.

Menczel and Leslie (1990,1993) indicated that the temperature gradient in the sample even at 80 C/min does not exceed 0.1 °C if the mass of the standard is not higher than 4 mg. Also, they determined that, in addition to the nematic (cholesteric) isotropic transition, other liquid crystal liquid crystal transi-... 

Pawloswski et al. described the s5mthesis of acetoacetoxypropyl cellulose (AAPC), formed by the acetoacetylation of hydroxypropyl cellulose using a diketene/acetone adduct (2,2,6-trimethyl-4-H-l,3-dioxin-4-one) in A/-methyl-2-pyrrolidinone (NMP) at elevated temperature. The authors showed that as with APC, AAPC also forms both thermotropic and lyotropic liquid-crystalline phases. It is noteworthy to mention that the nematic to isotropic transition occurs at 174 °C and thin films of cholesteric thermotropic AAPC show green reflection colors (Pawlowski et al. 1986, 1987). 

LCs were the earliest studied structures, in which polypeptide homopolymer rods pack in an ordered manner to form smectic, nematic, and cholesteric phases. The smectic LCs are mainly formed by polypeptide homopolymers with identical polymer length. The cholesteric phase can be prepared by synthetic polypeptides with polydisperse chain length. The nematic phase can be regarded as a special example of the cholesteric phase with an infinite cholesteric pitch. The cholesteric pitch and chirahty in the polypeptide LCs are dependent on many factors, such as temperature, polymer concentration, solvent nature, and polypeptide cOTiformation. Deep understanding of such phenomena is necessary for preparation of ordered polypeptide assembles with delicate stmctures. The addition of denaturing solvent to polypeptide solution can lead to an anisotropic-isotropic reentrant transition at low temperatures where the intramolecular helix-coil transformation occurs. However, the helical structure is more stable in LC phase than in dilute solution due to the conformational ordering effect. 

Figures 3 a, b, c show the temperature dependences of viscosity for the solutions under study. The above dependences are described by curves with well-pronounced sharp maxima. This behavior is typical of the solutions with LC transitions (Kulichikhin Golova, 1985, Vshivkov Rusinova, 2008, Gray, 1962). According to Gray (1962), this profile of the temperature dependences of viscosity corresponds to the (isotropic liquid)-(nematic liquid crystal) phase transition. Therefore, upon cooling of HPC, CEC and PBG solutions under deformation conditions, no cholesteric crystals are formed in other words, under dynamic conditions, a liquid crystal changes its type from cholesteric to nematic. The results obtained are in good agreement with the data of other authors (Volkova et al., 1986), who showed that the shear deformation of CEC solutions (c= 30%) in trifluoroacetic acid and a 2 1...

In this context, literature [90] states that at room temperature, acetoxypropyl cellulose exhibits both chiral nematic phases—the lyotropic and the termotropic one. When subjected to specific conditions of shear flow, the cellulose derivative cholesteric liquid crystal suffers transformations, such as cholesteric helix and cholesteric-to-nematic transition. The films prepared from anisotropic solutions of termotropic acetoxypropyl cellulose in an isotropic solvent exhibit anisotropic mechanical properties, generated by the molecular orientation of the solution under shear stress. Thus, liquid crystalline solutions give rise to films with anisotropic mechanical properties the films are brittle when stretched parallel to the shear direction and ductile when stretched perpendicular to it. 

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