Mesomorphs

Balmbra R R, Clunie J S and Godman J F 1969 Cubic mesomorphic phases Nature 222 1159-60... 

Liquid crystals represent a state of matter with physical properties normally associated with both soHds and Hquids. Liquid crystals are fluid in that the molecules are free to diffuse about, endowing the substance with the flow properties of a fluid. As the molecules diffuse, however, a small degree of long-range orientational and sometimes positional order is maintained, causing the substance to be anisotropic as is typical of soflds. Therefore, Hquid crystals are anisotropic fluids and thus a fourth phase of matter. There are many Hquid crystal phases, each exhibiting different forms of orientational and positional order, but in most cases these phases are thermodynamically stable for temperature ranges between the soHd and isotropic Hquid phases. Liquid crystallinity is also referred to as mesomorphism. 

X-ray, uv, optical, in, and magnetic resonance techniques are used to measure the order parameter in Hquid crystals. Values of S for a typical Hquid crystal are shown in Figure 3. The compound, -methoxyben2yHdene-/) - -butylaniHne (MBBA) is mesomorphic around room temperature. The order parameter ranges from 0.7 to 0.3 and discontinuously falls to 2ero at T, which is sometimes called the clearing temperature (1). 

A Hquid crystal compound in more cases than not takes on more than one type of mesomorphic stmcture as the conditions of temperature or solvent are changed. In thermotropic Hquid crystals, transitions between various phases occur at definite temperatures and are usually accompanied by a latent heat. 

The importance of unsaturation is illustrated by the fact that 2,4-nonadienoic acid [21643-39-0] forms a Hquid crystal phase, whereas the aHphatic carboxyHc acids do not. The two double bonds enhance the polarizabiHty of the molecule and bring iatermolecular attractions to a level that is suitable for mesophase formation. The overall linearity of the molecule must not be sacrificed ia poteatial Hquid crystal candidates. For example, whereas /n j - -aIkoxyciaaamic acids (5) are mesomorphic, the cis isomers (6) are not, a reflection of the greater anisotropy of the trans isomer. 

Bulky, even if highly polari2able, functional groups or atoms that are attached anywhere but on the end of a rod-shaped molecule usually are less favorable for Hquid crystal formation. Enhanced intermolecular attractions are more than countered as the molecule deviates from the required linearity. For example, the inclusion of the bromine atom at position three of 4-decyloxy-3-bromoben2oic acid [5519-23-3] (9) prevents mesomorphic behavior. In other cases the Hquid crystal phases do not disappear, but their ranges are narrower. 

Organized Langmuir films and poly layers of mesomorph cyclolinear polysilox-anes 98MI35. 

Similar lamellar structures are formed for l-alkyl-3-methylimida2olium cations with [PdCl4] when n > 12. As with the pyridinium systems, mesomorphic liquid crystal structures based on the smectic A structure are formed [24]. 

These differences on the stress-strain behavior of P7MB and PDTMB show the marked influence of the mesomorphic state on the mechanical properties of a polymer. When increasing the drawing temperatures and simultaneously decreasing the strain rate, PDTMB exhibits a behavior nearly elastomeric with relatively low modulus and high draw ratios. On the contrary, P7MB displays the mechanical behavior typical of a semicrystalline polymer. 

Characteristic of the microstructure of PET fibers in their final production form is the occurrence of three types of polymer phases crystalline, mesomorphous, and amorphous. The first phase is the result of crystalline aggregation of PET molecules, the second phase—of mesomorphous or, in other words, paracrys-talline aggregation, the third phase—of amorphous aggregation. The mesomorphous and amorphous phases together form a noncrystalline part of the fiber. 

The mesomorphous phase, also called an intermediate phase or a mesophase, is formed by molecules occurring in surface layers of the crystallites. It can be assumed that the mesophase is made up largely by regularly adjacent reentry folds. However, it cannot be excluded that the mesophase is also composed of some irregular chain folds, which are characterized by a long length and run near the crystal face in the direction perpendicular to the microfibril axis. 

The quantitative proportion of the mesomorphous phase in PET fibers is strongly diversified and depends on the conditions of fiber manufacture. According to Lindner fl7], it is estimated to be in the range 0.21-0.36. For poorly stretched fibers (below 300% of draw ratio), the mesophase constitutes almost exclusively an ordered part of the polymer in the fiber. With an increase in the draw ratio of the fiber, the proportion of the mesophase in the whole fiber mass increases, mainly at the cost of the amorphous phase. It is assumed that the mesophase constitutes about 0.5 of the amorphous phase mass for poorly stretched fibers and about 1.2 of this mass for highly stretched fibers. 

Caffrey, M. Structural, Mesomorphic and Time-Resolved Studies of Biological Liquid Crystals and Lipid Membranes Using Synchrotron X-Radiation. 151, 75-109 (1989). 

All solid forms lacking order to a substantial degree and ordered liquids (smectic, nematic) are called at times mesomorphic forms. 

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... 

In this subsection some examples are presented of polymorphic polymers, for which one or more forms are solid mesomorphic, in the sense described in the Sect. 2.1. Let us recall that in this review we will not deal with liquid mesomorphic forms. 

A conformationally disordered mesomorphic form is present, for instance, in the high-temperature phase I of PTFE. In this form, a long-range 3-D order is present only in the periodic pseudohexagonal placement of the chain axes [49]. In fact intramolecular helix reversals would produce the conformational disorder [50-52] and a complete intermolecular rotational disorder would be also present [49, 52,53]. 

Differently from the cases of ETFE, PTFE, 1,4-trans-polybutadiene, and PE, in the mesomorphic form of i-PP it is believed that there is a nearly complete conformational order, in the chains packed with parallel axes, (intrachain long range 3-D order) and only short-range lateral order in the positioning of the chain axes [60, 61]. 

It is, possibly, also worthy to cite the relevance of some mesomorphic forms in the optimization of the drawing conditions to get highly oriented manufacts although this is, possibly, mainly related to the peculiar morphological organization of these solid mesomorphic forms. 

In particular, it has been shown that the most important factor determining the drawing conditions of fibers and films of i-PP is the structure (a or mesomorphic) which characterize the yarn or the film obtained by extrusion. The drawing from mesomorphic samples requires, indeed, a lower tension and, generally, higher draw ratios are obtained [125,126],... 

Analogously, for polyethyleneterephtalate, drawing procedures in multiple steps, with preliminary drawings at room temperature generating a mesomorphic form [127], have been suggested by several authors [128-130], in order to get high modulus and high tenacity fibers. 

The synthesis of well defined block copolymers exhibiting controlled molecular weight, low compositional heterogeneity and narrow molecular weight distribution is a major success of anionic polymerization techniques 6,7,14-111,112,113). Blocks of unlike chemical nature have a general tendency to undergo microphase separation, thereby producing mesomorphic phases. Block copolymers therefore exhibit unique properties, that prompted numerous studies and applications (e.g. thermoplastic elastomers). 

The solubility of chitin is remarkably poorer than that of cellulose, because of the high crystallinity of chitin, supported by hydrogen bonds mainly through the acetamido group. Dimethylacetamide containing 5-9% liCl (DMAc/IiCl), and N-methyl-2-pyrrohdinone/LiCl are systems where chitin can be dissolved up to 5%. The main chain of chitin is rigid at room temperature, so that mesomorphic properties may be expected at a sufficiently high concentration [67,68]. 

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