Characterization of Liquid Crystalline Polymers

A complete characterization of liquid crystalline polymers should include at least two aspects the characterization of the molecular structure and that of the condensed state structure. Since the first characterization is nothing more than what is practiced for non-liquid-crystalline polymers, we will restrict the discussion to only a short introduction of methods mostly used in the characterization of the presence and the main types of polymeric liquid crystal phases. The methods include the mostly used polarizing optical microscopy (POM, Section 4.1), differential scanning calorimetry (DSC, Section 4.2) and X-ray diffraction (Section 4.3). The less frequently used methods such as miscibility studies, infrared spectroscopy and NMR spectroscopy will also be discussed briefly (Section 4.4). 

Secondly, thermotropic liquid crystalline polymers are characterized in a temperature range that is often too high to keep the molecular structure unchanged. The post-polymerization and decomposition are two possible results of a prolonged study of a polymeric liquid crystalline phase. There have been reports on liquid crystalline phases observable only 

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Characterization of Liquid Crystalline Polymers for Electro-optic Applications... 

Finkelmann, H., Koldehoff, J., Ringsdorf, H. 1979. Synthesis and Characterization of Liquid-Crystalline Polymers with Cholesteric Phase. Angew. Chem. Int. Ed. Engl. 17, 935... 

Many liquid-crystalline polymers are semi-crystalline and have only one mesophase that is nematic. The DSC thermograms of such polymers are very similar in appearance with Figure 4.22. Many other polymers are noncrystalline but have one and only one mesophase. The DSC will then show a glass transition and a transition from the mesophase to the isotropic liquid phase. Yet many other polymers may have more than one liquid crystalline phase so that their DSC curves should also include the transitions from one mesophase to another. DSC is obviously a very useful and very convenient technique for the characterization of liquid crystalline polymers. Nevertheless, the following points should be emphasized in order to interpret the DSC results with less ambiguity. 

H.Finkelmann, J.Koldehoff, and H.Ringsdorf, Synthesis and characterization of liquid crystalline polymers with cholesteric phase, Angew.Chem.Int.Ed.Engl. 17 935 (1978). 

Ringsdorf and his group have reported the synthesis and thermal characterization of liquid crystalline polymers with methacrylate and acrylate backbones in which the mesogenic groups are in pendant side chains.2,9,11 Some of their results are shown in Figure 6. 

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