Mesomorphic modifications

Polymers showing crystalline mesomorphic modifications, characterized by chains in disordered conformations, are exceptions to the equivalence principle. Examples of these cases are shown in Section 2.6. 

Crystalline forms presenting large amounts of disorder of the kind (ii) or (iii) are generally called mesomorphic modifications (Section 3.6), in analogy with the ordered liquids (smectic and nematic). In these cases the lack of periodicities in one or two dimensions (e.g., along the chain axes or along the directions normal to the chain axes) prevents the definition of a unit cell. Typical features in the X-ray diffraction patterns of mesomorphic forms are diffuse halos on the equator or on the layer lines depending on the kind of disorder present. 

Disordered structures belonging to the class (i) are interesting because, in some cases, they may be characterized by disorder which does not induce changes of the lattice dimensions and of the crystallinity, and a unit cell may still be defined. These particular disordered forms are generally not considered as mesomorphic modifications. A general concept is that in these cases the order-disorder phenomena can be described with reference to two ideal structures, limit-ordered and limit-disordered models, that is, ideal fully ordered or fully disordered models. 

The majority of noncrystallizable atactic POCS-4 and -5 exhibit only one columnar mesomorphic modification (mesophase I). The X-ray pattern of all POCS compounds in mesophase I show a single intense reflection, whose angular position (20 ) depends on the type of polymer (Figure 1). 

It is important to note that, for important sub-cases of case /), which will be discussed in more detail in Sect. 2.4, there is a low extent of disorder entropy effects, if any, are small and changes of the lattice dimensions are absent or small. These particular disordered forms are not considered as mesomorphic. In such cases, the limiting models which are fully ordered or fully disordered may be designated respectively as ordered or disordered crystalline modifications, if their consideration is useful for the structural description of a polymeric material. Note... 

Another approach to get new liquid crystals is the lateral fluorination of the stilbazole ligands,337 which is a common and highly effective tool to exert control over mesomorphism, crystal phase stability, and physical properties. Other modifications include the use of more alkoxy substituents and other alkyl sulfate anions.338-344 Ionic silver amino complexes also display liquid crystalline behavior at rather low temperatures they are of the form [Ag(NH2 -CJl +OJX (X = N03, n = 6,8,10,12,14 X = BF4, = 8,10,12,14).345... 

It is intriguing that even some flexible siloxane polymers form mesomorphic (liquid-crystalline) phases.34 139-166 Some illustrative data are given in Table 4.2. Both poly(diethylsiloxane) and pol y(di-n-propylsiloxane) show two crystalline modifications as well as a mesomorphic phase. (The other major class of semi-inorganic polymers, the polyphosphazenes, are also relatively flexible, and show similarly interesting behavior.)10167... 

The chief reason for the interest in graft copolymers originates from the incompatibility between polymer chains of different chemical nature. Intramolecular phase separation results, because grafts and backbone repell each other, and these compounds exhibit a marked tendency to form mesomorphic phases like block copolymers and soaps do. When these species are mixed with a solvent that exhibits a preferential affinity for one of the components (grafts or backbone) the incompatibility may be enhanced. This intramolecular phase separation has led to a number of applications. If small amounts of a graft copolymer are included into a homopolymer of the same nature as the grafts (or the backbone), surface modifications can result as described below. 

Lamellae obtained by direct crystallization from the melt into stable crystalline modifications at temperatures of 60-80 °C have thicknesses which differ only marginally from those of the fully mesomorphic samples. Such a finding, if confirmed, is highly relevant because it suggests that the lamellar organization depends only to a very limited extent on the particular crystal form while, to a first approximation, the pre-existing arrangement in the undercooled melt appears to play a key role. 

The first synthesized mesomorphic fulleropyrrolidine, G2, (Fig. 73A) [309] was chosen as a representative reference for a systematic variation of the R group in order to evaluate the effects on the mesomorphic properties. When R = H (Fig. 73B) [308], the compound exhibited a broad temperature range SmA phase (SmA 161 I). Modification of the R group by oligophenyleneviny-lene conjugates (OPV) led also to similar results (Fig. 73C G 50 SmA 171 1, D G 50 SmA 169 1) and indicated that the overall behavior is dominated by... 

Similar structural modifications using chiral aliphatic chains (citronellyloxy and its hydrogenated analog) were performed, and these yielded complexes with a chiral nematic phase, though when both terminal chains were chiral, mesomorphism was suppressed. The phases typically occurred between 120 and 160 C. Siting the bulky rhenium fragment in one extremity of the molecule resulted in the destabilization of the mesomorphism, but not its suppression. 

The general, 8-diketonate template has an overall lath-like structure, and subtle structural modifications can lead to crossover between smectic and columnar mesomorphism, or to structural change between discotic to calamitic shape, simply by changing the nature of the substituent and R (Figure 33). This is an important aspect of this type of complex, which would not be appreciated fully and understood if discussed separately. For instance, symmetrical systems (R = R, D2h) can have either disk-like or X-shaped structures, whereas the unsymmetrical compounds (R R ) can possess either zigzag- or cross-shaped structures (Figure 33). For the latter, both cis (C2v) and trans (C2h) isomers are also possible, and it is reasonable to think that mixtures are always present and that transicis interconversion occurs at elevated temperatures. This study is clearly a textbook case in addition to being a perfect illustration of induction, modification, or suppression of mesomorphism on complexation. 

The bi- and trinuclear bis- and tris-cw-enaminoketonato complexes investigated by Pyzuk et al. also represent an interesting approach in designing new molecular species forming liquid crystals, with a perfect control of the location of active sites within the mesophase, thus allowing for specific interactions to take place. The choice of enaminoketone as template ligands was dictated by their relative ease of synthesis, formation of chemically stable mesomorphic complexes vide supra), and facile shape modification. 

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