Crystal nematic transition

Turning to the low temperature transition of the homopolymer of PHBA at 350 °C, it is generally accepted that the phase below this temperature is orthorhombic and converts to an approximate pseudohexagonal phase with a packing closely related to the orthorhombic phase (see Fig. 6) [27-29]. The fact that a number of the diffraction maxima retain the sharp definition at room temperature pattern combined with the streaking of the 006 line suggests both vertical and horizontal displacements of the chains [29]. As mentioned earlier, Yoon et al. has opted to describe the new phase as a smectic E whereas we prefer to interpret this new phase as a one dimensional plastic crystal where rotational freedom is permitted around the chain axis. This particular question is really a matter of semantics since both interpretations are correct. Perhaps the more important issue is which of these terminologies provides a more descriptive picture as to the nature of the molecular motions of the polymer above the 350 °C transition. As will be seen shortly in the case of the aromatic copolyesters, similar motions can be identified well below the crystal-nematic transition. 

On the other hand, as opposed to the randomizing reactions which occur in the nematic melt if one anneals these copolyesters near their crystal nematic transition a completely different process appears to be operative. Thus several workers [11, 14], have reported that heating the HBA/HNA system near its melting point results in a dramatic increase in Tcn by approximately 50 °C. As... 

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.

Recently very interesting results have been reported on the behaviour of polymer/liquid crystal membrane consisting of bisphe-nol A polycarbonate (PC) and of N-ethoxybenzylidene A -n-butyl aniline (EBBA). This substance shows a crystal-nematic transition at 304° K and nematic-to-isotropic phase transition at 355° K. 

The discussion of mixtures is worthy of further comment. It has been known for some time that mixtures of linear, nonmesomorphic molecules with nematic compounds are characterized by a sharp decrease in both the crystal-nematic and nematic-isotropic liquid transitions with increasing concentration of nonmesomorphic component. However, mixtures of two or more nematic compounds which possess subtle differences in molecular structure do not exhibit sharp decreases in the nematic-isotropic liquid transition temperatures with molar composition although eutectic points for the crystal-nematic transitions may be obtained. Thus, the nematic-isotropic liquid transition temperatures form a smooth curve over the entire range of molar composition. Similar smooth curve surfaces are observed in ternary mixtures. This remarkable stability of the nematic mesophase is undoubtedly due to the fact that all the molecules are oriented in the same direction resulting in the formation of a pseudo-lattice type structure. On the other hand, the molecules of a nonmesomorphic guest are randomly oriented in a nematic host and their presence results in the disruption of nematic order. 

In addition, nine mixtures of compounds from the series had crystal >nematic transition temperatures below 30 C. 

In SMPs with a liquid crystalline transition the specific heat capacity increases significantly up to the transition point due to long range fluctuations of the order parameter near the transition [49]. At the transition temperature, a first-order phase transition occurs. The recorded DSC peak of the liquid crystalline transition will be the mixture of these two contributions. A schematic example for a liquid crystalline polymer is shown in Fig. 2 (diagram e), showing transitions in the form of sharp endothermic peaks, Tc-n for the crystal-nematic transition and for the nematic-isotropic transition. 

To study the addressed phenomena, a commercial sample of p-cyanophenyl p-n-hexylbenzoate (hereafter abbreviated as 6CPB) (Figure 2-6) was obtained from Roche (Basel, Switzerland), and used without further purification [53, 54]. The transition temperatures of this LC are also presented in Figure 2-6. It should be mentioned, that the nematic phase will be retained during cooling down to 7 °C below the crystal-nematic transition point. To demonstrate the reduction of... 

Sasabe and Ooizumi measured the pressure dependence of the static permittivity of MBBA in order to determine the crystal-nematic transition line. They used a guarded electrode system that was pressurized hydrostatically. 

This Datareview has concentrated on the nematic-isotropic transition except for the case of the truxenes where the discotic nematic to a columnar phase was considered. No smectic or crystal phase data have been considered here even though the literature reviewed often contained pressure-temperature data involving these phases. Data for the crystal-nematic transition are almost always included in the studies, but only sometimes even when the compound exhibits a smectic phase are such data included. No surprises exist in the body of pressure-temperature data for the nematic-isotropic transition. This transition for any nematogen could be described by the polynomial fitting equation with surprisingly small uncertainties in the average fitting parameters for example... 

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