Monotropic phase transition

Monoterpenoid ketones, 24 536-541 Monoterpenoids, 24 468, 470, 472, 484-541 Monothiocarboxylic acids, 23 739 Monotropic phase transitions, 15 101 Monounsaturated fatty acids, 10 830 Monounsaturated olefins, hydrogenation of, 26 879-880 Monovinylacetylene, 1 230 Monsanto acetic acid process, 19 646 Monsanto adiponitrile process, 17 236 Monsanto aluminum chloride-based Alkylation process, 23 333 Monsanto Prism separator, 16 21 Monsanto process (Lummus-UOP Classic process), 16 74 23 339, 341 Monsanto-Washington University collaboration, 24 390, 400-401 Montanic acid... 

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. 

The properties of some monomers are listed in Table I. Only the diphenyl derivatives exhibit enantiotropic liquid crystalline properties whereas the phenylesters melt to isotropic fluids. A monotropic liquid crystalline phase is obtainable if long spacers and para substituents are used. Because of the high tendency to crystallize the monotropic phase transitions of the monomers were not determined. 

Tg = glass transition temperature, M = different columnar phases, in part their structures are as yet unknown, = monotropic phase transition "decomposition, % parent radial pentayne (R = H) with this particular R is non-thermomesomorphic, see its melting point in [17,44,56,60], two crystalline modifications, Et = ethyl group, "this transition temperature is reversible and was obtained on cooling the isotropic melt, chiral molecule carrying five (= R) (S)-2-methylbutyloxy, respectively one (= R ) or five (= R) (S)-3,7-dimethyloctyloxy substituents, diirai nematic discotic (N d) phase, data given in opposite order assigning inverse j ase sequences. 

If a modification is unstable at every temperature and every pressure, then its conversion into another modification is irreversible such phase transitions are called monotropic. Enantiotropic phase transitions are reversible they proceed under equilibrium conditions (AG = 0). The following considerations are valid for enantiotropic phase transitions that are induced by a variation of temperature or pressure. 

On heating from a crystalline phase, DOBAMBC melts to form a SmC phase, which exists as the thermodynamic minimum structure between 76 and 95°C. At 95°C a thermotropic transition to the SmA phase occurs. Finally, the system clears to the isotropic liquid phase at 117°C. On cooling, the SmC phase supercools into the temperature range where the crystalline solid is more stable (a common occurrence). In fact, at 63°C a new smectic phase (the SmF) appears. This phase is metastable with respect to the crystalline solid such phases are termed monotropic, while thermodynamically stable phases are termed enantiotropic. The kinetic stability of monotropic LC phases is dependent upon purity of the sample and other conditions such as the cooling rate. However, the appearance of monotropic phases is typically reproducible and is often reported in the phase sequence on cooling. It is assumed that phases appearing on heating a sample are enantiotropic. 

Depending on temperature, transitions between distinct types of LC phases can occur.3 All transitions between various liquid crystal phases with 0D, ID, or 2D periodicity (nematic, smectic, and columnar phases) and between these liquid crystal phases and the isotropic liquid state are reversible with nearly no hysteresis. However, due to the kinetic nature of crystallization, strong hysteresis can occur for the transition to solid crystalline phases (overcooling), which allows liquid crystal phases to be observed below the melting point, and these phases are termed monotropic (monotropic phases are shown in parenthesis). Some overcooling could also be found for mesophases with 3D order, namely cubic phases. The order-disorder transition from the liquid crystalline phases to the isotropic liquid state (assigned as clearing temperature) is used as a measure of the stability of the LC phase considered.4... 

Bashir et al. (1998), in a study of phase transitions in a monotropic liquid-crystal polyester, found that PC and HF instruments performed differently in cooling mode. Better resolution of thermal events (i.e., better separation of peaks on the thermogram) was obtained with PC instruments. These authors pointed out that the performance of any DSC instrument might not be the same during heating as during cooling. 

Fig. 2.8 Diagrams illustrating (a) enantiotropic and (b) monotropic phase relationships for two organic compounds, cw-decahydronaphthalene and n-propylbenzene, respectively. Note that the y scale is actually given in units of entropy calculated from the energy terms. 7), n/i represents the transition point between phases I and II, being above the melting point in (a) and (by extrapolation) below the melting point in (b). (From Westrum and McCullough 1963, with permission.)...

A few series of azo and azoxy group containing liquid crystalline copolyesters were prepared by limura and coworkers and their phase transition temperatures were examined. No unusual phenomena were found, although monotropic mesophases were observed for the following copolyesters, depending on the combinations and fractions of alkylene groups ... 

The study by Percec, Tomazos and Willingham (15) looked at the influence of polymer backbone flexibility on the phase transition temperatures of side chain liquid crystalline polymethacrylate, polyacrylate, polymethylsiloxane and polyphosphazene containing a stilbene side chain. Upon cooling from the isotropic state, golymer IV displays a monotropic nematic mesophase between 106 and 64 C. In this study, the polymers with the more rigid backbones displayed enantiotropic liquid crystalline behavior, whereas the polymers with the flexible backbones, including the siloxane and the polyphosphazene, displayed monotropic nematic mesophases. The examples in this study demonstrated how kinetically controlled side chain crystallization influences the thermodynamically controlled mesomorphic phase through the flexibility of the polymer backbone. 

Figure 10. Transformation of a virtual or monotropic mesophase of the monomeric structural unit (M ) into an enantlotroplc mesophase by Increasing the degree of polymerization. The upper part (a) describes the influence of molecular weight on the dependence between the free energies of the crystalline (G ), liquid crystalline (G ) and isotropic (G.) phases and transition temperatures. The translation of this dependence into the dependence phase transition temperature-molecular weight is shown in the lower part (b).

The steeper slope of the T c, , ., -M dependence versus that of the Tk-i(lc) dePendence has ev K ore important implications on the moledular weight-phase transition temperature dependence for the situation when the monomer structural unit displays only a monotropic or a virtual mesophase (Figure 10). 

Where ( ) denotes a monotropic transition [ ] denotes a virtual transition (observed on fast supercooling) denotes a phase transition where the enthalpy was too small to be measured. 

On cooling the Sml phase for 10,090,10 a second modulated hexatic phase is observed, the Sm2 phase [155], This is a monotropic phase and its optical texture is indistinguishable from the preceding Sml phase. In addition, the phase transition could not be observed using differential scanning calorimetry. 

Table 4. Phase transition temperatures (°C) of selected alkane-1,2-diols, 7a-h [78, 79]. rci=clearing temperatures of these compounds saturated with water (50 wt%), = monotropic mesophases.

When fluorine groups were introduced in the 2- and 3-positions of the terminal aromatic ring of the biphenyl unit ((48) M = Cu R = OC8Hn w=l) no columnar phase was observed, but a monotropic nematic phase instead, probably due to the reduction of the molecular symmetry When the lateral chain was elongated, the columnar phase disappeared at the expense of an enantiotropic nematic phase ((48) M = Cu R = OC H2 +i, n = 6, 8, 10-12, 14 and m = 2 n=l2, m = 3 and n = 4, 8, m = 4), which became monotropic ((48) M = Cu, R = OC H2 +i, n=l2,m = 4 n = 8, m = 8 and n=12, w = 12,). Furthermore, re-entrant (Section 7.9.3.2.5) nematic phases, between two crystalline phases, were also observed for the compounds in the series m = 2. Both the temperatures of the crystal-to-mesophase (163-219 °C) and mesophase-to-isotropic liquid (175-224 °C) decreased very rapidly with increasing m and n. The stability of the nematic mesophase was also found to depend on the bulkiness of the lateral chain. For instance, using a branched side group such as HMeEt did not suppress the monotropic phase but reduced the transition temperatures considerably. The cross-over between the Coh and the nematic phase would then correspond to a decrease of the lateral interactions between molecules due to the steric hindrance of the chains. 

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