Structure MCLCP

All Liquid Crystal Polymers are characterised by the fact that they contain stiff meso-genic groups, often inserted in flexible chain systems (so called "spacers") and connected to them by linking functional groups the mesogenic unit is inserted either in the main chain or in the side chains or (in exceptional cases) in both. We shall discuss MCLCPs and SCLCPs. A schematic representation of common structures of LCPs is displayed in Fig. 6.14 (Jansen, 1996). An example of a SCLCP with disc-like mesogens is displayed in Fig. 6.15 (Franse et al., 2002, 2004). 

Estimate the characteristic temperatures of the MCLCP with the following structural 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. 

Preparation of Main Chain Liquid Crystalline Polymers (MCLCPs) Table 1. Representation of some typical structures of polyesters. 

The effect of structural disclinations upon texture and the origins of these in local deformations of or discontinuities in the arrangement of the molecules have been studied only in the case of nematic MCLCPs. The primacy aim of this chapter is therefore to present to the reader defects and textures in nematic MCLCPs. Some of consequences of constructing a nematic phase from long... 

In addition to this SCLCP host, MCLCPs have also been used. Li et al. [ 168] dissolved molecules of an oligomeric fragment of po-ly(p-phenylenebenzobisthiazole) 10, which was terminated by opposing nitro and di-methylamino groups, in the polymer itself. The resultant material has a value of 6x10 esu. Stupp et al. [169, 170] carried out a careful analysis of the system of Disperse Red 1 (11) dissolved in a chemically aperiodic nematic polymer containing the following three structural units (12). 

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. 

There are different polymer structures that exhibit LC phases. The extension of the mesogen s rigid aromatic core leads to main chain polper systems. Other approaches are based on the spacer concept. Here rigid cores that are common in LMWLCs are decoupled by flexible alkyl chains. The mesogenic units can be placed either in the main or in the side chain. Also combined MCLCP/SCLCPs have been prepared (figure 3). 

In the search for MCLCPs one may be inclined to select the monomers that give rise to the most rigid, extended structures. However, the requirement of a completely rigid structure must be mitigated by the requirement of polymers with accessible melting temperatures. The series of p-phenylene oligomers illustrates this point. 

As already mentioned, mesogens embrace a diversity of structure, but the great majority of the mesogenic groups in MCLCPs are aromatic in character and of general structure... 

Because of the structural similarity between semi-rigid thermotropic polymers and LCPs, i.e. the presence of rigid moieties separated by flexible spacers, all polymers of this kind are commonly referred to as the main chain liquid crystal polymers (mcLCP). 

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