Molecular MCLCP

ROMP Ring-opening metathesis polymerization ADMET Acyclic diene metathesis polymerization ALTMET Alternating diene metathesis polycondensation MCLCP Main chain liquid crystalline polymer SCLCP Side chain liquid crystalline polymer mru molecular repeating unit... 

Liquid crystallinity can be attained in polymers of various polymer architectures, allowing the chemist to combine properties of macromolecules with the anisotropic properties of LC-phases. Mesogenic imits can be introduced into a polymer chain in different ways, as outhned in Fig. 1. For thermotropic LC systems, the LC-active units can be connected directly to each other in a condensation-type polymer to form the main chain ( main chain liquid crystalline polymers , MCLCPs) or they can be attached to the main chain as side chains ( side chain liquid crystalline polymers , SCLCPs). Calamitic (rod-Uke) as well as discotic mesogens have successfully been incorporated into polymers. Lyotropic LC-systems can also be formed by macromolecides. Amphiphihc block copolymers show this behavior when they have well-defined block structures with narrow molecular weight distributions. 

The transition temperature and transition heat for polyester MCLCP vary with the thermal treatment conditions. By changing the sample mass, heating rate and thermal treatment conditions, satisfactory results from DSC measurement for the rigid-rod MCLCP are usually obtainable. In some cases, self-polycondensation of polyesters proceeds continuously on thermal treatment by DSC. Thus, the relative molecular mass increases and a closer packed structure is obtained, resulting in more evident transition peaks. As the transition peaks on the DSC curve for the rigid-rod MCLCP are relatively small, careful inspection is necessary for their identification. When the sample is annealed at a temperature slightly below the isotropic transition temperature, more distinct peaks are frequently obtained. 

From the point of view of molecular architecture, as schematically shown in Figure 9.3, two types of LCP have been developed (1) main-chain LCPs (MCLCPs), having the monomeric liquid crystals (i.e., mesogenic group) in the main chain of flexible links, and (2) side-chain LCPs (SCLCPs), having the monomeric liquid crystals attached, as a pendent side chain, to the main chain. 

LCPs combine the mechanical properties of polymers with the order of LCs. The low molar mass mesogenic units, which form a LC phase, consist of a rigid core. If this core is extended, so-called main chain liquid crystalline polymers (MCLCPs) are obtained. A second method to obtain LCPs is to connect low molecular weight LC (LMWLC) units via flexible spacers. This approach allows the preparation of side chain liquid crystalline polymers (SCLCPs) (Box 1). 

The appearance of liquid-crystalline order in neat liquid-crystalline polymers should have an influence on the chain dynamics. However, since molecular architectures of mcLCPs and scLCPs significantly differ from each other, the extent of this influence is different for each type of PLC. 

Dielectric measurements of mcLCPs are very sparse and were essentially performed as auxiliary to mechanical studies, reflecting principally interest in the mechanical properties of new materials rather than in peculiarities of the molecular dynamics caused by the liquid crystallinity. 

Low-molecular-weight liquid-crystalline compounds have been known for about 100 years. However, main chain liquid crystal polymers (MCLCPs) have attained prominence only in the last 15 years. In 1956 Flory predicted lyotropic behaviour and this theoretical prediction was well demonstrated in the synthetic polymer area with the discovery by Kwolek of the aramids, e.g. poly(/ -phenyleneterephthamide). This led to interest in thermotropic main chain LCPs and, although aromatic polyesters which are thermotropic were described during this period in patents issued to and Carborundum Co., their liquid-crystalline... 

The molecular morphology is the underlying key to the behaviour of MCLCPs and dictates to a large extent the macro- and micromor-phological behaviour of MCLCPs. 

The ordered regions in MCLCPs appear to play a similar role to crystals in conventional polymers, i.e. they tie the molecules together and impart molecular rigidity. However, the level of crystallinity in MCLCPs is low, often under 20% for an unannealed sample, and the crystallites in MCLCPs account for only a minor proportion of the total material. The... 

The rheology of MCLCPs is complex. In general, the shear viscosity of MCLCPs is much lower than that of conventional polymers at a comparable molecular weight, and the transition from the isotropic state to the liquid-crystalline state is generally accompanied by a significant decrease in melt viscosity. At the onset of nematic behaviour, the melt viscosity of the MCLCPs is three decades less in order of magnitude than that of a similar but non-mesogenic polymer. 

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