Liquid structure melting transition

As the temperature is decreased, free-volume is lost. If the molecular shape or cross-linking prevent crystallisation, then the liquid structure is retained, and free-volume is not all lost immediately (Fig. 22.8c). As with the melt, flow can still occur, though naturally it is more difficult, so the viscosity increases. As the polymer is cooled further, more free volume is lost. There comes a point at which the volume, though sufficient to contain the molecules, is too small to allow them to move and rearrange. All the free volume is gone, and the curve of specific volume flattens out (Fig. 22.8c). This is the glass transition temperature, T . Below this temperature the polymer is a glass. 

Unlike low molar mass liquid crystals, these materials do not undergo a nematic-isotropic transition. Instead, they adopt liquid crystal behaviour throughout the region of the phase diagram for which they are in the melt. Above a particular temperature, rather than adopting an isotropic liquid structure, they decompose. 

Figure 4.16. Schematic F- phase diagram of decanethiol on Au(lll). The different regions and phases are denoted as S (stripes), IS (intermediate structures), C [c(4 X 2)] and L (liquid). The broken lines indicate phase boundaries of the IS, which are not yet fully established. The solid curve between C and L (melting transition) exhibits a sharp rise near full coverage. Adapted from Schreiber et al, 1998.

The parameters of the JT distortions were calculated by the X -method for a series of crystals in good agreement with experimental melting temperatures [14]. The details of the theory and specific calculations seemingly require additional refinements, but the main idea of the JT origin of the liquid-crystal phase transition seems to be quite reasonable. This work thus makes an important next step toward a better understanding of the relation between the macroscopic property of SB and the microscopic electronic structure, the JT effect parameters. 

With the melting explained as a JT-induced SB in addition to the SB in atom-molecule, molecule-molecule transitions and structural phase transitions in crystals, only the transition gas-liquid remains not considered from the JT vibronic coupling point of view. In Ref. [14] this possibility is mentioned, but not realized. Let us discuss this problem in some more detail (see also Ref. [4]). 

The liquid crystal melt, which comes into being at the glass-rubber transition or at the crystal-melt transition, may have several phase states (Mesophases) one or more smectic melt phases, a nematic phase and sometimes a chiral or cholesteric phase the final phase will be the isotropic liquid phase, if no previous decomposition takes place. All mesophase transitions are thermodynamically real first order effects, in contradistinction to the glass-rubber transition. A schematic representation of some characteristic liquid crystal phase structures is shown in Fig. 6.13, where also so-called columnar phases formed from disclike molecules is given. 

A number of molten salt systems [e.g., the simple ionic system Ca(N03)2-KN0j], have the property of being able to be supercooled, i.e., to remain liquid at temperatures below the melting point down to a final temperature. This is called the glass transition temperature, and at this temperature the salts form what is called a glass. This glass is only apparently solid. It is a highly disordered substance in which a liquid structure... 

It is clear that to differentiate between the two proposed mechanisms, in-depth studies of the structure present in the liquid crystalline melt must be performed. DSC studies in which the melt structure has been quenched might be helpful. The only difficulty is that the transitions observed in the DSC are often very difficult to detect. 

We note here that systematic studies of the melting transition of dry or nearly dry phospholipids bilayers (e.g., vesicles) have been scarce. While there is an abundant experimental and theoretical literature concerning the structure and properties of bilayers in water, less is known about their behavior when water is removed. We have therefore initiated a systematic experimental study of the gel-liquid crystal transition of pure DPPC and DPPC-cholesterol vesicles freeze-dried with and without disaccharides and oxyanion-disaccharide complexes. Some of our results to date are shown in Figure 9.3. 

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