Mesophase materials

A study of the effect of the mesophase layer on the thermomechanical behaviour and the transfer mechanism of loads between phases of composites will be presented in this study. Suitable theoretical models shall be presented, where the mesophase is taken into consideration as an additional intermediate phase. To a first approximation the mesophase material is considered as a homogeneous isotropic one, while, in further approximations, more sophisticated models have been developed, in which the mesophase material is considered as an inhomogeneous material with progressively varying properties between inclusions and matrix. Thus, improvements of the basic Hashin-Rosen models have been incorporated, making the new models more flexible and suitable to describe the real behaviour of composites. 

Liquid Crystalline Polymers. One class of polymers that requires some special attention from a structural standpoint is liquid crystalline polymers, or LCPs. Liquid crystalline polymers are nonisotropic materials that are composed of long molecules parallel to each other in large clusters and that have properties intermediate between those of crystalline solids and liquids. Because they are neither completely liquids nor solids, LCPs are called mesophase (intermediate phase) materials. These mesophase materials have liquid-like properties, so that they can flow but under certain conditions, they also have long-range order and crystal structures. Because they are liquid-like, LCPs have a translational degree of freedom that most solid crystals we have described so far do not have. That is, crystals have three-dimensional order, whereas LCPs have only one- or two-dimensional order. Nevertheless, they are called crystals, and we shall treat them as such in this section. 

Mesophase materials are possible for all three classes of molecules. Biological mesophases were already discovered in the middle of the 19th Century. Reinitzer later described the special two-stage melting of cholesteryl benzoate. These materials were then named liquid crystals by Lehmann in 1904 5). Small molecule mesophase materials will be referred to from time to time in this review as reference materials. 

Mesophase materials of linear, flexible macromolecules have gained attention only more recently, when it was found that parallel molecular orientation is easily achieved in some of these mesophases. This orientation can lead to high modulus and tensile strength 6). Presently there exists a certain amount of confusion in the literature about the description, properties, and nomenclature of these macromolecular mesophases and their place in the arrangement of all matter. Even for the better understood small-molecule mesophases there are some problems in the description of glasses and in the separation of orientational and conformational disorder. Also, the distinction between mesophases based on molecular structure and on super-molecular structure is not always made. We will try in this review to clarify some of these points. 

The need ultimately to include conformational disorder in the system of mesophase materials has already been pointed out by Smith (Ref.7), p. 193) and becomes obvious when reading discussions of the behavior of typical condis crystals (see, for example, Ref.108))... 

The third group of mesophase materials represents the conformationally disordered crystals, called condis crystals. The physical properties of condis crystals, which largely maintain positional and orientational order, change in much too subtle a way from the fully ordered crystals so that a common property could be attached to their name. 

Calculated from the equation unit cell parameter = mesopore diameter + wall thickness. Mesoporous mesophase material of the MCM-41 type. 

The materials to be studied promise to be ever more challenging, ranging from soots and combustion exhausts (76-78), to mesophase materials from petroleum (79), through the graphite-diamond interface (80-81), antiaromatic compounds (82), carbon clusters (83, 84), and interstellar materials... 

The internal molecular arrangement of a solid in general can be projected as a continuum between well-ordered crystalline state and completely disordered amorphous state (Fig. 1.2). A crystalline material is depicted as having three-dimensional long-range symmetry operators over a domain of at least 1000 individual molecules. A mesophase material (liquid crystals, plastic crystals) is depicted as having intermediate symmetry operators, and an amorphous state has no symmetry operators (Klug and Alexander 1974). 

Pulsed laser deposition involves striking a target made of pre-prepared surfactant-templated inorganic material with an intense laser beam, causing the ejection of atoms, ions and fragments containing template that deposit onto a substrate. Continuous well-adhered films of nanometer scale particles of mesophase material tens of nanometers to... 

Shizuka et al " and Wildeman et al. have reported that the Wurtz-type reaction may also be used to prepare alternating copolymers as in the coupling of diorganolithium compounds (6) with dihalosilanes (7) (Scheme 2). The product (8) of the reaction in this case forms a liquid crystalline, mesophase material that exhibits o-jt conjugation. 

---