Phase transitions isotropic-liquid crystalline

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... 

Figure 16 shows a schematic diagram of phase transformations for rigid-chain polymers separated from isotropic solutions by introducing a nonsolvent into the system (this is a usual method of obtaining fibres and films) (cf. >). The initial isotropic solution with the polymer concentration V2 and the value of the Huggins-Flory parameter is in the monophase region. The critical concentration of the transition into liquid crystalline state for this system is v. When a nonsolvent is introduced, i.e. when x is increased up to the value >0.5 (x ), two routes of the phase transition... 

Although the so-called a-phase of the fatty alcohols—a thermotropic type smectic B liquid crystal with hexagonal arrangement of molecules within the double layers—is initially formed from the melt during the manufacturing process, it normally transforms into a crystalline modification as it cools. However, the crystallization of the gel matrix can be avoided if the ot-phase can be kept stable as it cools to room temperature. This can be achieved by combining appropriate surfactants such as myristyl or lauryl alcohol and cholesterol, a mixture of which forms a lamellar liquid crystal at room temperature. Due to depression of the melting point, the phase transition temperature of crystalline to liquid crystalline as well as liquid crystalline to isotropic decreases. Therefore, a liquid crystalline microstructure is obtained at room temperature. 

Fig. 3 (a) Phase diagrams for a ternary solution of PPLA in TFA/CHCI3 (3/97 w/w). Solid lines indicate phase boundaries between liquid crystalline (LC) and isotropic (/) states and dashed lines those between right- and left-handed a-helical states B biphasic region, (b) PPLA conformation-induced the transition of LC handedness. Reprinted with permission from [77]. Copyright 1997 Wiley-VCH... 

In the case of thermotropic liquid crystals, a liquid-crystalline phase is obtained by heating a solid mesomorphic compound. At the melting point (Tm), the thermal motion of the molecules has increased to such an extent that the material passes from the crystalline phase to the liquid-crystalline phase. A mesomorphic compound that exists in the glass state will enter the liquid-crystalline phase at the glass-transition temperature (Tg). On further heating, the orientational order of the molecules is lost as well. The LC transforms into an isotropic, clear liquid at the clearing point (or isotropization point T. Many materials are liquid-crystalline at room temperature. Several types of liquid-crystalline phases can occur between the solid state and the isotropic liquid state. Sometimes decomposition of the material occurs before the... 

FIGURE 28 Influence of the lanthanide ion on the transition temperatures of the complexes [R (LH)3(N03)3]. LH is the Schiff s base ligand L4H with C H2 +1=C8Hi7 and C 2m+1 =CigH37. Cr, crystalline phase SmA, smectic A phase I, isotropic liquid. (Drawn using the data in Table 1 of Binnemans et aL, 1999c). 

FIGURE 45 5,5 -Substituted 2,6-6w(l-ethyl-benzimidazol-2-yl)pyridines and their transition temperatures (in °C). Abbreviations Cr = crystalline phase, SmC = smectic C phase, SmA = smectic A phase, N = nematic phase. Col/, = hexagonal columnar phase, I = isotropic liquid. 

We find an increase in the transition temperature linear in Ve, and a continuous second order transition above a certain critical external field. The external field can cause ordering in the isotropic phase (fig. 5), As an application, the magnetic birefringence in a isotropic liquid crystalline polymer melt has been calculated and good agreement with measurements of Haret is found. 

FIG U R E 16 Transition isotropic liquid phase (neither positional nor orientation order) nematic phase (orientation order but still no positional order) ordered crystalline phase. 

Techniques.—A novel method for the determination of the number average molar mass (M ) is reported by Kronberg and Patterson, based on the observation that polystyrene and poly(ethylene oxide) are soluble in the nematic and isotropic phases of the liquid crystalline iV-(/>-ethoxybe zylidene)/ n-butylanaline. Presence of a polymer depresses the first-order nematic-Tsotropic melting transition, by decreasing the nematic order, and as liquid crystals tend to exhibit large values of the cryoscopic constant, molar masses of up to 10 may be studied with some accuracy. 

In conclusion, the number and kinds of thermal phase transitions of singlechain surfactant molecules vary from a simple phase transition, the solid crystalline to isotropic liquid, to a complex polymorphism and mesomorphism. More than one thermotropic state may exist in the same system, each being the stable phase within a particular range of temperature (and pressure). Phase transitions are usually reversible through all the intermediate forms to a structure that is thermodynamically stable at room temperature, or they are partially reversible through one or more, but not all, of these transitions, and the room temperature product is an undercooled form of a phase that is stable at some intermediate temperature [26]. 

The thermodynamic parameters, measured at various temperatures for the isotropic, nematic, or smectic phase of a liquid crystalline stationary phase, are related to changes in the ability of a solute to interact with the solvent. Several theoretical treatments, including the refined infinite dilution solution model [453, 480, 481] and the lattice model [455,456], have been postulated for the general interpretation of the thermodynamic parameters in terms of the retention and selectivity parameters in gas chromatographic separations. However, attempts to correlate the thermodynamic quantities with specific solute-solvent interactions remain qualitative. Nevertheless, a few general observations can be noted. Upon cooling from an isotropic phase into a liquid crystalline phase, the values of decrease and those of 7 increase. Plots of Vg or 7 versus temperature show discontinuities at phase transition temperatures. The activity coefficient of any solute in an anisotropic environment (Ya) is found to be more positive (and typically, 7r>l) than in its isotropic phase (7i°°). This trend in is a reflection of vari-... 

Different liquid crystal phase transitions will be more or less difficult to detect using DSC. If the transition is first order, meaning that the order parameter is discontinuous across the phase boundary, a significant latent heat will be measurable, and usually a clear peak can be observed. An example of a first-order phase transition in liquid crystals would be the crystalline-to-smectic or -nematic phase. The nematic-to-isotropic phase is also first order. Some liquid crystal phase transitions are much more... 

Figure 11 Inverted phase transition for (hydroxypropyl)ceUulose in HjO. An isotropic solution with polymer volume fraction > 0.05 produces a biphasic (isotropic + liquid crystalline) mixture at T > 42°. The condition T = Q 0.5) occurs at T = 41 °C. (From Ref. 63. Copyright 1980 Am. Chem. Soc.)...

Aggregation in colloidal systems can be introduced by various mechanism. Attractive interactions between the colloids is the most prominent example. Another possibility is to confine the colloids in one phase of a phase separating mixture, e.g., in the isotropic phase of a liquid crystalline fluid that is undergoing the isotropic-to-nematic transition [52, 53]. This unusual soft solid consists of a foam hke structure, where the bubbles are filled with liquid crystal in the nematic phase and the colloids are confined in the walls separating the bubbles [54]. 

In a liquid crystal the molecules have an orientational order but do not have translational order a liquid crystalline state is therefore intermediate between a liquid and a solid. The simplest liquid crystalline phase is the nematic phase and most liquid crystalline materials have a nematic-isotropic phase transition (see Figure 8). Because of the applications of liquid crystals in, for example, displays, understanding the properties of these materials is of considerable practical interest. For more details on the physics and applications see Ref, 33,... 

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