Mesophase virtual

Polyethylene conforms to the situation of Figure la, l.e. under normal conditions it does not display a mesophase, only the familiar orthorhombic crystal form (o). Nevertheless, there exists a virtual mesophase which can be "uncovered" the ways in which this can be achieved is the subject of what follows. 

Figure 11. The narrowing of the temperature range of a virtual mesophase of the monomeric structural unit (M ) 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 (Glc) and Isotropic (G ) phases and transition temperatures. The translation of this dependence Into the dependence phase transition temperature-molecular weight is presented 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). 

Case 3. The Monomeric Structural Unit Displays a Virtual Mesophase the Polymer Displays a Virtual Mesophase. 

The third situation is illustrated in Figure 11 and also refers to a different case in which the monomeric unit displays only a virtual mesophase. Here as before, the slope of the T. (i.e., Tic k) M dependence is higher than that of the Tffl (i.e., T j -M dependence, the latter lies above the former throughout hence the two curves do not intercept each other. Therefore, the resulting polymer displays also only a virtual mesophase. This thermodynamic situation was recently applied to the synthesis of virtual liquid crystal polyethers containing both flexible mesogens and flexible... 

According to the previous discussion it is found that increasing the degree of polymerization decreases the entropy of the system and if the monomeric structural unit exhibits a virtual or monotropic or mesophase, the resulting polymer should most probably exhibit a monotropic or mesophase. Alternatively, if the monomeric structural unit displays a mesophase, the polymer should display a mesophase that 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 inerease the number of its mesophases. All these effects have been observed in various systems. 

Based on this discussion and on the thermodynamic discussion described previously,we can easily consider that the "polymer effect" can provide via its molecular weight and backbone flexibility the same effect. In an oversimplified way it can be considered that it provides an overall change in the entropy of the system. TTirough this change, it can transform, in a reversible way, a virtual mesophase into a monotropic and subsequently into an enantiotropic one. In addition, the kinetic factors provided by the glass transition and crystallization should always be considered. For example, the formation of a mesophase located in the close proximity of a glass transition temperature becomes kinetically controlled or even can be kineticallv prohibited. 

A virtual mesophase is potentially possible but thermodynamically less stable than the crystalline (or more ordered liquid-crystalline) phase at the same temperature (Fig. 6.31). The monotropic mesophase is a special case of the virtual mesophase. 

Figure 6.31 Plots of free energy (G) as a function of temperature (T) illustrating enantiotropic and virtual mesophases. The slopes of the lines are equal to the entropies of the different phases.

Figure 19. (a) Pressure-temperature diagram of benzene hexa-n-hexanoate. The mesophase-isotropic transition line extrapolated to atmosphere pressure yields a virtual transition temperature of 89 °C. (From Chandrasekhar et al. [63], reproduced by permission of Academic Press), (b) Miscibility diagram of benzene hexa-n-hexanoate and the heptanoate. The virtual mesophase-isotropic transition temperature for the hexanoate is 89 °C, in agreement with the value obtained from the pressure-temperature diagram (a). From Billard and Sadashiva [64], reproduced by permission of the Indian Academy of Sciences). 

Fig. 8 Free energy per chain bond of the liquid (L), of the bulk mesophase (M), and of the crystalline (C) polymer. Txy is the transition temperature from X to Y (X, Y = L, M, C). a In this case the mesophase is stable between 7cm and 7ml while Tcl is virtual. (From [11]) b The mesophase is virtual, and so are 7cm and 7ml melting of the crystal into the liquid is only observable transition occurring at Tcl...

The disk-shaped molecules are not rigorously oriented to parallel arrays the mesophase state constitutes only a preferred orientation, but with virtually all molecular layers lying within 25° of the director representing the average orientation (18). 

Other polymers with oligo ethylene oxide) spacers have been observed. The polymers of limura and coworkers, with mesogenic structures 10 and 12 of Table 1 and spacers of di-, tri- and tetraethylene oxide, had X-ray patterns in the melt that were virtually identical to those of the solid This result was also observed for Polymer 37 with a tetraethylene oxide spacer prepared by us Such patterns are consistent with a smectic E mesophase which maintains much of the solid state order in the liquid crystalline melt. 

Figure la. Schematic plot of free energies vs. temperature for a scheme that does not show a mesophase. G., G and G, are, respectively, the free energies of the crystalline, mesomorphic (virtual) and Isotropic liquid states. T.. = T Is the crystalline melting point. Here, as In subsequent Figures lb and lc, the heaviest lines correspond to the stablest state at a given temperature. 

Admittedly, the existence of the postulated transient mesophase would still require structural confirmation. Even so, the taking of a sharp drop In viscosity as Indicator of mesophase formation has well established precedents In the liquid crystal field. Such Is e.g. the well documented effect In Kevlar referred to above (12) which In many respects has similarities to the presently discussed PE, except that Kevlar Is "mesogenlc" and can exist as stable liquid crystal under ambient conditions, while the mesophase In the flexible PE Is "virtual". The latter "virtual" phase only becomes "real" transiently, which suffices to dramatically affect the entire flow behavior of the material, the effect through which It Is being detected. 

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. 

There is a second particularity to be noted many virtually water-insoluble polymeric amphiphiles can be swollen to yield polymeric lyotropic mesophases, even if the miscibility gap is broad (Fig. 37). Such behaviour seems to be widespread for vinylic polymerized surfactants with side-chain spacers [126, 231, 331], I.e., neither polysoap behaviour implies the capability to form lyotropic mesophases, nor the presence of lyotropic mesophases the classification as polysoap. 

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