Liquid-crystalline phases differential scanning calorimetry

Stilbenoid dendrimers are able to undergo aggregation. Depending upon the generation number, some of the pure substances form liquid-crystalline phases (Dha discotic hexagonal disordered phase Dra discotic rectangular disordered phase Dob discotic distorted phase). Differential scanning calorimetry (DSC) revealed phase transitions between 99°C and 0°C. 

Differential scanning calorimetry (DSC) is a common technique for the classification of individual phase transitions in liquid-crystalline materials and has been applied for the phase characterization of alkyl-modified chromatographic surfaces. Hansen and Callis [187] applied DSC to investigate phase changes in Cig and C22... 

Phase transition is an important property of membranes. Below the phase transition temperature, lipids are tilted and highly ordered. They are in their solid or "gel" state. Increasing the temperature leads to a pre-transition, characterized by periodic undulations and straightening of the hydrocarbon chain. Further increase of the temperature causes the main phase transition. Above the main phase transition temperature, lipids are fluid or "liquid crystalline." Figure 3 shows the phase diagram for the interaction of water with a lipid as well as its inferred arrangements in a model membrane (5). Phase transitions in membranes and membrane models have been extensively studied by spectroscopic techniques and by differential scanning calorimetry. 

FIGURE 13.20. Differential scanning calorimetry trace. The heating curve for 1,1,2-trichloro-2,3,3-trifluorocyclobutane. The larger peak is due to the transition from the anisotropic crystalline phase to the plastic crystalline phase the smaller peak is due to the transition from the plastic crystalline pheise to the liquid phase. (Courtesy V. B. Pett and David L. Powell, The College of Wooster, Ohio.)... 

Membrane fluidity is determined by following anisotropic rotation of fluorescent or spin probes. Liquid-crystalline (or fluid) to gel thermotropic phase transition of lipids (Figure 1) (cf. Section 3.1.1 l)in liposomes or intact biomembranes can be followed by Fourier transform infrared (FTIR) spectroscopy or differential scanning calorimetry (DSC). 

Characterization. The liquid crystalline properties of the side-chain monomers (III) and polymers (I) have been studied by Differential Scanning Calorimetry (DSC), Polarized Optical Microscopy (POM) and X-ray diffraction. The thermal transition data and phase types for all monomers (III) and polymers (I) are summarized in Table HI. A representative DSC scan for the monomer (El) and polymer (p with a four-carbon tail (n=4) and six-carbon flexible spacer (m=6) are shown in Figures 1 and 2 respectively. The first peak at -24°C shown in Figure 1 is the crystal to smectic... 

The monomer exhibits complex phase behaviour, in particular, a monotropic liquid crystalline phase, which is only apparent on cooling, such materials can only be properly characterized by using differential scanning calorimetry (DSC) in conjuction with optical microscopy and also by X-ray scattering (see Chapter 1). 

A central part of the application-oriented evaluation of liquid crystals are so-called virtual clearing temperatures, electrooptic properties, and viscosities. These data are obtained by extrapolation from a standardized nematic host mixture. 7 Af, An, and jy are determined by linear extrapolation from a 10% iv/iv solution in the commercially available Merck mixture ZLI-4792 (Tfji = 92.8°C, Af = 5.27, An = 0.0964). For the pure substances the mesophases are identified by optical microscopy and the phase transition temperatures by differential scanning calorimetry (DSC). The transition temperatures in the tables are cited in °C, numbers in parentheses denote monotropic phase transitions which occur only on cooling the sample C = crystalline, S = smectic A, Sg = smectic B, S = smectic G, S> = unidentified smectic phase, N = nematic, I = isotropic. 

A complete characterization of liquid crystalline polymers should include at least two aspects the characterization of the molecular structure and that of the condensed state structure. Since the first characterization is nothing more than what is practiced for non-liquid-crystalline polymers, we will restrict the discussion to only a short introduction of methods mostly used in the characterization of the presence and the main types of polymeric liquid crystal phases. The methods include the mostly used polarizing optical microscopy (POM, Section 4.1), differential scanning calorimetry (DSC, Section 4.2) and X-ray diffraction (Section 4.3). The less frequently used methods such as miscibility studies, infrared spectroscopy and NMR spectroscopy will also be discussed briefly (Section 4.4). 

Fig. 7. The transition from the L/S gel phase, through the P/8 phase, to the La liquid-crystalline phase of dimyristoylphosphatidylcholine, as detected by differential scanning calorimetry and visualized by freeze-fracturing.

While the first paper on liquid crystalline elastomers [30] already reports the detection of a cholesteric-isotropic transition using differential calorimetry and polarizing microscopy, comparatively little work has been done to characterize thy physical properties in the vicinity of this phase transition (compare, however, also the discussion of electromechanical effects in the next section) [9, 30, 31]. Combined liquid crystalline elastomers have been synthesized and various of these materials show a cholesteric-isotropic transition using X-ray scattering, polarizing microscopy and differential scanning calorimetry [31]. Dynamic mechanical investigations have been carried... 

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