Liquid crystalline polymers chemical structures

The development is reviewed of liquid-crystalline polymers whose mesophase formation derives from the nature of the chemical units in the main chain. The emphasis lies primarily on highly aromatic condensation polymers and their applications. The general properties of nematic phases formed by such polymers are surveyed and some chemical structures capable of producing nematic phases are classified in relation to their ability to form lyotropic and thermotropic systems. The synthesis, properties, physical structure and applications of two of the most important lyotropic systems and of a range of potentially important thermotropic polymers are discussed with particular reference to the production and use of fibres, films and anisotropic mouldings. 

Benoit et al. then reasoned that all polymers, regardless of chemical structure and chain architecture, should fit on the same plot of rj)M versus elution volume. And most of them do, as shown on the plot in Figure 12-42, which is called the universal calibration curve. (It is actually not quite universal, as data from things like liquid crystalline polymers that have extended chain rather than coil conformations in solution do not fall on this curve.)... 

The influence of chain packing (Le. free volume) on solubility, diffusivity and permeability in liquid crystalline polymers can be studied by comparing properties of LCPs in the disordered, isotropic state with those in the ordered, liquid crystalline state. HIQ-40 is a random, glassy, thermotropic, nematogenic terpolymer synthesized from 40 mole percent p-hydroxybenzoic acid and 30 mole percent each of isophthalic acid and hydroquinone. The chemical structures of the constituent monomers for fflQ-40are ... 

There are now numerous compositions of liquid crystalline polymers under consideration as fiber spinning and injection molding materials. However, the problems involved in processing these systems are similar. In particular, how can one process these polymers to yield desirable isotropic properties or at least have biaxial orientation how can one achieve the optimum properties from a given composition and how does the chemical composition and structure affect the properties In flexible chain systems one must quench in orientation in a time scale which is faster than the relaxation process of the molecules. Typically there is a distribution of relaxation times in which the longest relaxation time is a matter of a few seconds. This longest relaxation time also governs a number of other flow characteristics. 

It was also reported [225] that photo-reactivity of side-chain liquid crystalline polymers can align liquid crystals both in a parallel mode or perpendicularly, depending on the degree of the photoreaction of the polymers. Presumably, this particular polymer can multiphoto-ahgn the liquid crystal pattern without a change of the direction of the linearly polarized UV light. The chemical structure of such an aligning polymer is depicted as follows ... 

Figure 1. The chemical structures of liquid crystalline polymer and low molecular weight liquid crystals.

Phases in thermodynamic systems are then macroscopic homogeneous parts with distinct physical properties. For example, densities of extensive thermodynamical variables, such as particle number N of the fth species, enthalpy U, volume V, entropy S, and possible order parameters, such as the nematic order parameter for a liquid crystalline polymer etc, differ in such coexisting phases. In equilibrium, intensive thermodynamic variables, namely T,p, and the chemical potentials pi have to be the same in all phases. Coexisting phases are separated by well-defined interfaces (the width and internal structure of such interfaces play an important role in the kinetics of the phase transformation (1) and in other... 

There are several commercial thermotropic polyesters that exhibit outstanding high-temperature capabilities. These include (14,15) an increasing number of fibers and high temperature plastics. Similar to the lyotropic liquid crystalline polymers, the thermotropics exhibit unusually low viscosities because of orientation and lack of entanglement. Of course, the orientation serves to improve their mechanical properties. The chemical structure can be varied significantly. 

While heat resistance and chemical resistance are not closely linked, the presence of aromatic groups, special structures such as liquid crystalline polymers, dense cross-linking, or fillers are often beneficial. When examining chemical resistance, it must be in context to specific chemicals aqueous acids or bases, organic solvents, oxidizing agents, and so on. Flammability resistance is... 

The terms liquid crystal and mesophase are interchangeable. Meso, in Greek, means between , so a fluid can be called a mesophase if it has some properties that are characteristic of crystals. The ability of the fluid to form a liquid crystal is due to the molecules ability to align with each other and create local ordering. So, liquid crystalline polymers are those polymers that form liquid crystalline phases either in solution or in the melt. Molecules that form a mesophase are usually rod-like or disc-like. In the case of rodlike polymers, such as poly(p-phenyleneterephthalamide) (PPTA), the rigidity of the backbone is primarily responsible for the formation of a mesophase. The rigidity is, of course, dependent on a variety of factors such as the nature of the solvent used, the temperature of the solution, and the chemical structure of the molecule. 

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