Melts isotropic

No coherent threadline could be maintained and the extmdate flew off the windup as short, brittle, crystalline lengths. Not until many years later did other workers show that this polymer on cooling exhibits a mesophase transition directly from the isotropic melt to a smectic A phase. Good sources of information on Hquid crystals and Hquid crystal polymers are available (212—216). 

Figure 1 DSC curves of P7MB representing a cooling cycle (lower) starting from the isotropic melt, and the subsequent heating cycle (upper). Scanning rate 10°C/min.

The variation of the transition temperatures of these polybibenzoates with the number of methylene units in the spacer is shown in the lower part of Fig. 5. Melting temperatures, Tm, (crystal-isotropic melt transition) are obtained [9] for m > 7 and m = 3 (monotropic behavior), while for the other members, Tm really represents the... 

Figure 4 DSC melting endotherms of P7MB after isothermal crystallization at 135°C, starting from the isotropic melt [10]. The curves correspond to 0, 3,6, and 35 min of crystallization time, from bottom to top. Scanning rate 5°C/min.

A further confirmation that mirrorlcss lasing is restricted to single domains comes from an experiment in which an Oocl-OPV5 film has been crystallized from the isotropic melt phase (above 204 "C). Melt crystallization resulted in the formation of large domains with dimensions up to several millimeters (see Fig. 16-29 C). Tlie normalized emission spectra for different excitation energies are shown in Figure 16-47. The excitation spot diameter was 1 mm in these ex-... 

Fig. 16. Gibbs energy-temperature diagram if FCC and ECC are present in the system. Ai-isotropic (undeformed) melt, A2-deformed melt (nematic phase) points 1 and 4 - melting temperatures of FCC and ECC under unconstrained conditions (transition into isotropic melt) points V and 2 -melting temperatures of FCC and ECC under isometric conditions (transition into nematic phase), point 3 - melting temperature of nematic phase (transition into isotropic melt but not completely randomized)...

If this sample contains also folded-chain crystals (reasons for their appearance during orientational crystallization were stated before), under isometric conditions they undergo melting at a higher temperature (at point 1 with respect to the oriented melt with transition to line A2) than under the conditions of free heating (point 1 with transition in the isotropic melt to line At). 

Melting of ECC involving transition into the isotropic melt was shown by Flory to be a first-order process. It can be seen in Fig. 18 b that there occurs a transition from a complete order to a fully random chain arrangement in the isotropic melt (Fig. 16, point 4). 

First of all the term stress-induced crystallization includes crystallization occuring at any extensions or deformations both large and small (in the latter case, ECC are not formed and an ordinary oriented sample is obtained). In contrast, orientational crystallization is a crystallization that occurs at melt extensions corresponding to fi > when chains are considerably extended prior to crystallization and the formation of an intermediate oriented phase is followed by crystallization from the preoriented state. Hence, orientational crystallization proceeds in two steps the first step is the transition of the isotropic melt into the nematic phase (first-order transition of the order-disorder type) and the second involves crystallization with the formation of ECC from the nematic phase (second- or higher-order transition not related to the change in the symmetry elements of the system). 

Figure 36 is a three dimensional representation of the order parameter P at 350 K after 19.2 ns of simulation, where about 25% of the system has transformed into the crystalline state. The black regions near both side surfaces correspond to the crystalline domains with higher P values, while the white regions are in a completely isotropic state of P = 0. Detailed inspection of these data has shown that no appreciable order is present in the melt. A simple interface model between the crystal and the isotropic melt seems to be more plausible at least in this case of short chain Cioo-... 

This work was extended to include acetylides (alkynyls) of the type (RNG)Au-G=C-C6H4-/)-C H2 +1, with n — 6-12 (even numbers), and R as above. The compounds were prepared from (tht)AuCl and the alkynes /)-HC=C-C6H4-C H2 +1 in aqueous acetone with soda as a weak base. The resulting insoluble, polymeric alkynylgold(i) complexes were dissolved in an organic solvent by adding equivalent quantities of the isocyanides. All products show distinct transitions from a crystalline to a smectic A phase and finally to the isotropic melts.206... 

Figure 8.31 Photomicrograph illustrating chiral coiling tubes of smectic LC obtained as B7 phase of MHOBOW grows from isotropic melt.

Thermotropic LCs can be further divided into (a) enantiotropic materials, in which the LC phases are formed on both heating and cooling cycles, and (b) mesotropic materials, in which the LCs are stable only on supercooling from the isotropic melt. Mesotropic LCs have been further divided into three groupings as follows ... 

Liquid crystal forming materials have thus five possible pure phases (fully ordered -crystal, LC-glass, amorphous glass, liquid crystal, and isotropic melt). In addition, there may be four two-phase materials (fully ordered crystal and LC-glass, fully ordered crystal and amorphous glass, fully ordered crystal and isotropic melt, and fully ordered crystal and liquid crystal). The correlations between these phases are shown in Fig. 3. 

As a result of the strong drive to equilibrium, it is usually difficult to quench the isotropic melt to an amorphous glass when liquid crystal formation is possible. Extraordinary quenching techniques may be needed. Once produced, the amorphous state loses its metastability on heating above the glass transition Tga. The melt is quite unstable, so that it may not be possible to keep the melt from changing to the mesophase 21. ... 

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