Mesomorphic state, liquid crystal polymers

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Liquid crystal polymers (LCPs) were introduced over the last three decades. In the liquid state, either as a solution (lyotropic) or a melt (thermotropic), they lie between the boundaries of solid crystals and isotropic liquids. This polymeric state is also referred to as a mesomorphic structure, or a mesophase, a combined term adopted from the Greek language (mesos = intermediate morphe = form). This state does not meet all the criteria of a true solid or a true liquid, but it has characteristics similar to both a solid and a liquid. For instance, the anisotropic optical properties of LC polymeric fluids are like those of crystalline solids, but their molecules are free to move as in liquids. 

The state of mesomorphism is a spontaneously ordered liquid-fluid crystalline state. Liquid crystals were discovered as early as 1888. They are materials that exhibit order in one or two dimensions but not in all three. By comparison, the amorphous materials lack any order, while the crystalline ones exhibit order in three dimensions. All liquid crystalline polymers exhibit some degree of fluidity. They were investigated extensively in the 1900s and became commercially important in 1960s. 

The first liquid crystal polymers (LCPs), in which structural moieties known to lead to mesomorphic behavior in small-molecule liquid crystals have been incorporated into the main chain, were reported in early 1980s. The presence of the mesogenic units in the polymer chain enhances the tendency for the material to form the liquid crystalline state, and these polymers also show better thermal stability. Both the nematic and smectic phases can be observed in these materials, (see Fig. 4.12). 

Dynamics of the mesogenic unit and of the backbone in the mesomorphic and isotropic states. Above the static glass transition two new intramolecular motions become active in side-chain liquid crystal polymers. Firstly, as in flexible-chain polymers, the micro-Brownian segmental motions of the backbone chain are successively liberated. The increased mobility of the backbone chain enables in turn large-scale reorientations of the long axis of the pendant mesogenic units. As a result, new DR relaxation processes appear. ... 

The contemporary period is characterized by a rapid accumulation of information about thermotropic polymeric liquid crystals. It is yet too diversified and, as a rule, gives only a qualitative description of the observed phenomena. The next step in LC polymer investigations should be aimed at a quantitative description of their behaviour. This approach should necessarily lead to progress in the comprehension of the quite peculiar mesomorphic state of matter as well as in the introduction of LC polymers into technological practice. 

The mesogenic structure of a benzoic acid dimer has been introduced as a noncovalent cross-linker for polysiloxanes [79]. Polymer 57 exhibits a smectic C phase due to the dynamics of H-bonding. In contrast, mesomorphic order is locked in the solid state of poly[(4-acryloyl)benzoic acid] by polymerization in its mesophase [128]. No liquid-crystalline state is observed for this material because of the lack of flexibility of the structures. Main-chain-type polymeric liquid-crystal associates are formed from carboxyl-bifunctionalized aromatic compounds [129]. 

Indeed, the polymers discussed here are not fluid materials but are hard and brittle, and are not liquid-crystals in the usual sense of this term. In most cases heating does not change the level of organization up to temperatures where the polymer starts to decompose. Other comparisons and distinctions will be drawn between these polymers and low molecular weight mesomorphic states below. Some comments will also be made on the organization of the polymers as compared to semi-crystalline polymers with small extent of crystallinity. 

In between the amorphous phase and the crystalline phase, there sometimes occurs an important intermediate phase for polymers carrying anisotropic groups (called mesogen groups) either on the chain backbone or on the chain branches, which is referred to mesophase. The mesogen groups become orientational-ordered under suitable thermodynamic conditions. This ordered state could be the well-known liquid crystal (LC) state, as a typical mesomorphic state between the amorphous state and the crystalline-ordered state for LC polymers. 

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