Liquid crystalline state history

The effect of the chemical ermstitution of the crosslinker oti the local topology of the network is the second new feature to be considered. If the crosslinker molecule is flexible it can behave like an isotropic solvent. In that case, essentially only the phase transition and phase transformatiOTi temperatures of the LC phase are affected [90]. If, however, the chemical constitution resembles that of a mesogen of the constituent polymer backbone, the history of the crosslinking process becomes important. Under these conditions the crossUnker adopts the state of order in which the final crosslink process of the network occurs and thus determines the local topology of the crosslink [120,121]. The mechanical properties and the reorientational behavior are considerably modified for networks with the same chemical constitution but crosslinked either in the isotropic or in the liquid crystalline state [122-124]. Other important aspects of the local topology at the crosslink concern the phase transformation behavior [125] as well as the positional ordering in smectic systems [126]. 

Thermotropic liquid crystals hold a dominant position in the field of the LCD however, researchers have also to pay attention to another type of liquid crystals, lyotropic liquid crystals, fi om the aspect of the life science field. Essential properties of cell membranes originate from their liquid crystalline behavior. The point of view of biophysics exists in the liquid crystal discovery time inferred from the monograph of Otto Lehmaim titled The liquid crystal and life flieory . In the experimental research of material science, the development of science cannot be expected without collaboration with a physicist, a physical chemist, and a synthetic chemist, as showing the history of research not only as that of liquid crystals but also of macromolecules and colloid science, among others. Because a considerable portion of a living organism (cell membrane, skin structure, etc.) is composed of liquid crystalline states, participation of researchers from many different fields is necessary for the bio-matter liquid crystal. I would hope to see the development of medical science, pharmacy, and foods by the full utilization of the potential of liquid crystal materials. 

Godinho, M.H. van der Klink, J.J. Martins, A.F. 90. Shear-history dependent equilibrium states of liquid-crystalline hydroxypropylcellulose solutions... 

In order to characterize polymeric fluids and to test rheological equations of state it is customary to use simple, well defined flows. The two main flows are simple shear and simple elongational. These are shown schematically in Figure 1. In shear flow, material planes (see Figure 1) move relative to each other without being stretched, whereas in extensional flow the material elements are stretched. These two different flow histories generate different responses in not only flexible chain polymers but in liquid crystalline polymers. When these flows are carried... 

In the field of conventional engineering thermoplastics we have a detailed understanding of the isotropic state, we appreciate the stress history deployed in a moulding process, we can measure relaxation phenomena and so predict residual orientation, and so we can deduce the property spectrum of a final product. If the time-scale between recognition of liquid crystalline phenomena in melts and its commercial exploitation appears protracted we need only note the observation of Professor J. L. White summing up at a recent conference in Kyoto, that, for liquid crystal polymers, stress history, optical anisotropy and texture are independent variables. In fact, to make the connection from basic material property to performance in the final product, industrial technologists have had to learn a new science. 

A crystalline or semicrystalline state in polymers can be induced by thermal changes from a melt or from a glass, by strain, by oiganic vapors, or by liquid solvents (40). Polymer crystallization can also be induced by compressed (or supercritical) gases, such as C02 (41). The plasticization of a polymer by C02 can increase the polymer segmental motions so that crystallization is kinetically possible. Because the amount of gas (or fluid) sorbed into the polymer is a direct function of the pressure, the rate and extent of crystallization may be controlled by controlling the supercritical fluid pressure. As a result of this ability to induce crystallization, a history effect may be introduced into polymers. This can be an important consideration for polymer processing and gas permeation membranes. 

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