The nature of liquid crystallinity

The previous section gave us a certain background, since we now know something about classes of polymer-based materials other than PLCs. Moreover, it was noted that rigidity is related to the anisotropy of shapes and properties. We are now ready to tackle the nature of liquid crystallinity. Common to all liquid crystals is the fact that the molecules are oriented approximately parallel to a preferred axis in space called director. The degree of alignment is defined by the so-called order parameter or anisotropy factor s  

In a PLC we can expect somewhat lower anisotropy factors, but we should realize that the rigid sequences impart orientation to flexible sequences between. Again Escher has provided something that can be used to illustrate a scientific point. Look at Fig. 1.3 and assume that the black lizards represent rigid PLC sequences and white animals the flexible sequences. It is easy to see that the flexible sequences necessarily become oriented to a certain extent whether they like it or not . 

Liquid crystallinity can appear for more than one reason. Materials in which liquid crystalline properties are induced by the presence of a solvent are called lyotropic. If liquid crystallinity appears in definite temperature intervals, we have thermotropic liquid crystals. Hsiao, Shaw and Samulski found that liquid crystalline properties can be also brought about by elevation of pressure I have called such LCs barot-ropic. Their existence is not surprising, since pressure and temperature changes produce similar (although not identical) effects in terms of affecting free volume. 

Delineations between the above three classes of liquid crystals are not necessarily very sharp, and in general liquid crystalline phases of mixed  

With the importance of anisotropy well established, Krigbaum, Brelsford and Cifferi studied the temperature variation of the axial ratio Iqjd, where q is the persistence length and d the chain diameter (the persistence length is the average sum of the projections of all bonds on bond i in an indefinitely long chain). They found a large variation the axial ratio falls with increasing temperature faster than was expected. Thus, a critical value is reached and a liquid crystal becomes an isotropic liquid. A different theoretical approach is that of Picken, which Northolt and Sikkema describe in Chapter 6 of this book (Section 6.2.1). 

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The structure of the mesogenic moiety appears to exert a strong influence on the nature of liquid crystalline order developed in a polymer. 

To acquire a certain perspective, I shall provide first a brief history of MLCs and PLCs. Then I shall discuss mesophases liquid crystals constitute only one of three kinds of mesophases. Further, I shall compare heterogeneous (that is, ordinary) composites, molecular composites and PLCs. Then we shall go to the heart of this chapter the nature of liquid crystallinity and its manifestations. On this basis we shall be able to survey existing and potential structures of PLCs, using a classification developed earlier. We shall see connections between a place in this classification and properties. Thus, this chapter provides an overview of the field. Chapters 1-3 form the first part of the book, including structures, characterization and dynamics. Subsequent chapters deal with specific properties, synthesis procedures, morphologies, processing and applications. 

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