Liquid Crystalline Polymers Architecture

Figure 1. Example Liquid Crystalline Polymer Architectures.

Fig. 3 Schematic representations of some possible supramolecular liquid crystalline polymer architectures, a-e Different classes of main-chain structures, f-i Different side chain polymers, j-m Different types of networks...

FIGURE 5.7 Schematic Representation of typical, (partially) electroluminescent LC polymer architectures. (a) Rodlike structure, (b) Hairy-rod structure, (c) Combined main-chain-side-chain system, (d) Semiflexible segmented structure, (e) Semiflexible segmented structure with disklike mesogen. (After Weder, C. and Smith, P., Main-chain liquid-crystalline polymers for optical and electronic devices, in Encyclopedia of Materials Science and Technology, Buschow, K.H., Cahn, R.W., Flemings, M.C., Ilschner, B., Kramer, E.J., and Mahajan, S., Eds., Elsevier Science, New York, 2001.)... 

Figure 2 Some molecular architectures of liquid crystalline polymers incorporating rodlike mesogens and flexible spacers. ...

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. 

Liquid crystalline polymers (LCPs) have gained attraction as materials with interesting optical, mechanical and rheological properties [3-7]. This review summarizes research on thermotropic liquid crystalhne polymers synthesized by metathesis routes, as this chemistry has proven to be a versatile way to build up well-defined polymer architectures [8]. Recent results promise to ejq)and the possible uses of these methods. 

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.)... 

Fig 1 Common architectures for thermotropic liquid crystalline polymers... 

Liquid Crystalline Polymers are an important class of polymeric materials because they may exhibit optical properties similar to low-molar-mass liquid crystals and high mechanical properties of polymers. These polymers are broadly classified based on their molecular architecture, i.e. attachment of the mesogen to the polymeric backbone, as main-chain liquid crystal polymers (i) or side-chain liquid crystal polymers (2). In main-chain liquid crystal polymers, mesogens are incorporated into the backbone. The mesogens may be of different shapes and sizes, and are usually rodlike or disklike. Such polymers have not been used for optoelectronic applications because it is very difficult to reorient these materials by electric field. Instead, these materials find applications that use their exceptional mechanical properties. Even side-chain liquid crystal polymers, whose mesogen is attached to the polymer backbone through a flexible spacer switch too slowly for... 

Also for polymer solutions, the properties depend on interactions between solvent and dissolved components and vary strongly with concentration and temperature. Therefore, beside the molecular analysis, there is the important question for the understanding of the relationship between structure and properties, whether and how PLCs in a solution can be distinguished from those containing non-liquid crystalline polymers with a similar molecular architecture. Another interesting question is, to what extent the conformation of the macromolecules in solution is influenced by interactions between mesogenic groups. As a consequence of that, the hydro- and thermodynamic properties of the solution should also be affected. 

Broadband dielectric spectroscopy (BDS) is a versatile experimental tool to study the dynamics of polymeric systems. In its modem form it covers the extraordinary frequency range from 10 Hz to 10 Hz with the option to extend both limits to lower and higher values, respectively. This enables one to analyse the molecular d3mamics on a large time scale especially if the temperature of the sample is varied as well. In the present review article examples will be discussed for polymers of widely varying molecular architectures (linear and cyclic chains, star-branched systems, and liquid crystalline polymers). 

Mesogenic groups can be incorporated into polymeric systems [7]. This results in materials of novel features like main chain systems of extraordinary impact strength, side-chain systems with mesogens which can be switched in their orientation by external electric fields or—if chiral groups are attached to the mesogenic units—ferroelectric liquid crystalline polymers and elastomers. The dynamics of such systems depends in detail on its molecular architecture, i.e. especially the main chain polymer and its stiffness, the spacer molecules... 

For copolymer fibers, the tensile strength and modulus were found to be much higher than those of the PI fibers. Furthermore, Figure 8 shows that tensile strength and modulus of PBTA/PI molecular composite fibers increase with an increase of the PBTA content in block copolymers. It is evident that introdudng a liquid-crystalline polymer in molecular architecture makes considerable reinforcing effects in molecular composites. 

Main ehain liquid crystalline polymers can be constructed from rod-like (calamitic) and disc-like (discotic) units by a condensation process (see Chapter 8). Seen here in racemic form, polymer 11 is a poly ether with repeating mesogenic-like core units separated by flexible alternating hydrocarbon spacers. This typical architecture ensmes a sufficiently low melting point for liquid crystalline phases to be exhibited. Polymer 11 has a molecular weight of 17,000 and a polydispersity of 1.7. 

Figure 12. The molecular architecture of liquid crystalline polymers containing crown ether ligands.

The possibilities are not restricted to flexible polymers. One of the blocks can be rigid rodlike, in which case a rod-coil block copolymer [54-57] is formed if the architecture is of the diblock type (see Section 2.3.1 for another example [15]). Other interesting cases comprise diblock copolymers where one of the blocks is a side-chain liquid-crystalline polymer [4, 58-62]. Finally, we mention the important class of hairy rods obtained for a comb copolymer architecture consisting of a rigid backbone and flexible side chains [4, 58-61, 63-66], to be discussed in more depth later in this review. 

---