Mesophase enantiotropic

Mesophase that is thermodynamically stable over a definite temperature or pressure range. Note The range of thermal stability of an enantiotropic mesophase is limited by the melting point and the clearing point of an LC compound or by any two successive mesophase transitions. 

Metastable mesophase that can be formed by supercooling an isotropic liquid or an enantiotropic mesophase at a given pressure to a temperature below the melting point of the crystal. 

Ferrocene derivatives 15 exhibited remarkable liquid crystal properties (Fig. 9-13). Indeed, they all gave rise to enantiotropic mesophases. Structures with n = 1 to 11 showed nematic phases. From n = 12 a smectic C phase formed. The latter was monotropic only for 15 (n = 12). The smectic C domain increased from n = 13 to n = 16, and, inversely, the nematic range narrowed. The last member of this series (n = 18) presented one smectic C phase between 159 °C and 179 °C. A nematic to smectic C transition and a focal-conic texture of a smectic C phase are presented in Figs. 9-14 and 9-15, respectively. 

All polymers Il-m of the second class investigated show one enantiotropic mesophase which was identified as nematic (Table II). There are no significant differences in the isotropization temperatures, but the isotropization entropies increase with increasing number m of methylene groups in the alkylene segment particularly for polymers containing the 1,3-phenylene residue (meta isomers, m). 

Zhou et al. (1986) studied the transformation of monotropic to enantiotropic mesophase of the main-chain type liquid crystalline polymer, (3.15), in terms of the molecular weight effect. 

The incorporation of lateral chains in the series of complexes 30 had dramatic effects on the mesomorphism [59]. Indeed, almost all the complexes displayed only monotropic mesophases, essentially nematic, with a few only showing Sc phases. The only enantiotropic mesophases were obtained for m=l for both Ni and Cu complexes (Crys 239 Sc 267 N 281 I and Crys 188 Sc 243 N 267 I, respectively). 

The widest columnar mesophase temperature ranges were obtained for the bis-[l,3-di-(substi-tuted-phenyl)-/3-diketonate] metal complexes bearing ten and twelve chains ((55) R = H or OC H2 +i). The ten-chain copper, palladium, and oxovanadium(IV) complexes ((55) M = Cu, Pd, VO R = H, = 6, 8, 10, 12, 14) were all mesomorphic and the enantiotropic mesophases were identified by optical texture and variable-temperature X-ray diffraction as columnar phases (Table 34). The copper and palladium complexes displayed a Coh phase for short chain length ( = 6, 8 for M = Cu = 6, 8, 10 for M = Pd), which transformed to a Coin phase as the chain length was increased. Surprisingly, no direct Cok-to-Colh phase transition was observed within the same compound, but weakly first-order Cok-to-Cok and Colh-to-Colh phase transitions were found for compounds with intermediate chain lengths. In contrast, the vanadyl complexes exhibited only one Coh mesophase. Infrared studies indicated that the VO complexes possessed a linear V=0—V=0 linear polymeric chain structure in the crystal phase, while no... 

Enantiotropic mesophase, 4—5 P-enaminoketonate complexes lanthanide crmiplexes, 64-65, 66t structure, 64/... 

Case 1. Both monomeric structural unit and polymer display an enantiotropic mesophase. Upon increasing the molar mass from dimer to trimer, etc., Si decreases and therefore G increases. Beyond a certain molar mass Gi remains for all practical considerations constant. However, the slope of the increase of Tic-i is steeper than that of Tc- c> The difference between these two slopes determines the relative thermodynamic stabilities of the mesomorphic versus the crystalline phases at different polymer molar masses. As the molar mass is increased, the liquid crystalline regime is increased for main chain and side chain liquid crystalline polymers. 

Case 2. The structural unit displays a virtual or a monotropic mesophase the polymer displays a monotropic or an enantiotropic mesophase. The slope of the Tic-c versus molar mass (M) is steeper than that for Tc i, and as a consequence there will exist a critical value of M below which a mesophase is not observed. This effect has been observed in main chain and side chain liquid crystalline polymers. 

Mesophases may be enantiotropic or monotropic. While enantiotropic mesophases are stable and exist above the melting temperature of the crystalline phase, monotropic mesophases are metastable and occur below the melting point of the crystalline lipid. Thus, they are formed upon cooling the isotropic melt. 

Let us first consider very briefly the influence of various parameters (i.e., nature of flexible spacer and its length, nature and flexibility of the polymer backbone and its degree of polymerization) on the phase behavior of a side chain liquid crystalline polymer. According to some thermodynamic schemes which were described elsewhere, the increase of the degree of polymerization decreases the entropy of the system and therefore, if the monomeric structural unit exhibits a virtual or monotropic mesophase, the resulting polymer should most probably exhibit a monotropic or enantiotropic mesophase. Alternatively, if the monomeric structural unit displays an enantiotropic mesophase, the polymer should display an enantiotropic mesophase which is broader. It is also possible that the structural unit of the polymer exhibits more than one virtual mesophase and therefore, at high molecular weights the polymer will increase the number of its mesophases. All these effects were observed with various polymer systems. ... 

Fig. 4.6 Schematic pictures of (a) enantiotropic mesophase and (b) monotropic mesophase...

An enantiotropic mesophase is observed both on cooling and heating. It is thermodynamically stable within a certain temperature region. Its free energy is at these temperatures lower than those of the isotropic and crystalline (or more ordered liquid-crystalline) phases (Fig. 6.31). 

These segmented polymers are both thermoplastic and liquid crystalline elastomers. When the molar mass of PCL is 4000 and the hard domain content is 40 wt%, an enantiotropic mesophase is formed. The tensile storage moduli values of these TPEs, both in the crystalline state below the melting temperature of the PCL block and in the rubbery state above its T, are much higher than those of the TPEs prepared from the systems PCL/HDI. 

The TPUs prepared from PCL/HDI/DHBR showed an enantiotropic mesophase in the hard domains only when the hard segment content was 40% or higher. The tensile moduli of the crystalline TPUs were higher than those of PCL/MDI/1,4-BD based TPUs at temperatures below and above the crystalline melting temperature of the PCL soft segments, [33]. The latter... 

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