Mesophase Order

The various types of mesophases have usually been treated separately, and rarely did researchers review more than one mesophase at a time. One of the few descriptions of liquid and plastic crystals was given by Smith7). We will expand on this attempt and try to combine the description of all three major types of mesophase order, namely ... 

Mesophases of supermolecular structure do not need a rigid mesogen in the constituent molecules. For many of these materials the cause of the liquid crystalline structure is an amphiphilic structure of the molecules. Different parts of the molecules are incompatible relative to each other and are kept in proximity only because of being linked by covalent chemical bonds. Some typical examples are certain block copolymers50 , soap micelles 51 and lipids52. The overall morphology of these substances is distinctly that of a mesophase, the constituent molecules may have, however, only little or no orientational order. The mesophase order is that of a molecular superstructure. 

Several liquid-crystalline mixtures have been employed In this work. They and several of their physical characteristics are collected In Table I. As expected, the bulky BN molecules disturb mesophase order causing transition temperatures to be lowered. Monotropic c+K transitions are approximate since they depend upon the rate of cooling and other factors. 

As is well-known, density functional theories are very successful in describing ffuid-solid transitions in general [336, 337], and hence this approach is most promising for the related transition of mesophase ordering using the information on the disordered melt. Unlike the Landau-type expansion, Eq. (184X high-order nonlinear terms are included selfconsistently. 

We reconsidered the folded-chain fringed-micelle model, proposed nearly forty years ago, and found it to be appropriate to explain mesophase ordering and crystallisation in the polymer melt and amorphous state. Putting together the evidence provided by Strobl for crystallisation as a multi-stage process [13], the folded-chain fringed-micellar grain model [202-206], the smectic phase of iPP [6,152,153], density fluctuation before crystalliza-... 

The three-phase structures 12, 13, and 14 are of particular interest because of the even more severe violation of the phase rule. At constant pressure, in equilibrium, only one-phase areas should be stable. Drawn fibers of poly(ethylene terephthalate) (PET) are discussed as three-phase sttucture with Figs. 5.68-72 and 5.113-115. As in polyethylene, the drawing to fibers or films orients the amorphous nanophase and achieves an arrested mesophase order, proven with Fig. 5.72. Since the drawing of PET fibers is much less efficient in extending the molecules than gel-spinning, there remains a sizeable portion of the amorphous phase, as shown in Fig. 5.71. The mobile mesophase of PET has not been found as a stable phase, as in polyethylene. Copolymers of PET with stiffer repeating units, such as oxybenzoate, however, have stable mesophases (see Chap. 7). 

The discovery of mesophase order in densely crosslinked networks has led to some innovative concepts in designing and processing thermoset materials, for example, these networks could be oriented under external fields during curing reaction and result in highly anisotropic materials. These LC thermosets exhibit unique properties that show promise for applications in the aircraft, aerospace, and electronic packaging fields as well as for surface coatings with superior impact resistant properties. 

Figure 30 shows the possible mesophase orderings that one might expect [99]. Early work [102,103] has focused on the possibility of microphase separation in the form of Janus cylinders [104], i.e., the cylinder splits into two halves, with a planar A-B interface (containing the cylinder axis, taken to be the z-axis henceforth). 

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