Thermotropic side-chain LCPs

The fairly good quality of the fits validates both Leadbetter s assumptions and the Maier-Saupe distribution function. However, the values of S obtained and even the quality of the fits obviously depend on the odd or even number of (CH2) groups in the flexible spacer. This odd-even effect is widespread and well known in the field of main-chain LCPs and will be discussed later in this article. The nematic order parameter of main-chain LCPs may reach values as high as 0.85 which demonstrates the very high orientation of the nematic phase of these polymers. Such a large orientation is undoubtedly responsible for the good mechanical properties of this type of materials. The treatment described above therefore provides a very easy way of characterizing the orientational order of a nematic phase. It has also been tested for thermotropic side-chain LCPs and found to be satisfactory as well [15]. Unfortunately, it has not been used yet in the case of lyotropic LCPs except for some aqueous suspensions of mineral ribbons (Sect. 5) which are not quite typical of this family of materials. 

Both thermotropic and lyotropic liquid crystal polymers exhibit characteristic features with regard to their microstructureJ Anisometrical monomers such as rods or discs are connected to chains in an appropiate manner. These anisometrical monomers are considered to be the mesogens and may be part of main chain LCP, side chain LCP, or of both types together (Fig. 6). Between the mesogens are located flexible spacers of non-mesogenic character. Sufficient flexibility is a prerequisite for liquid crystal formation, with an increase in either temperature or solvent concentration. 

Most LCPs of commercial interest are thermotropic main-chain nematic materials. The other main structural type of interest are the side chain LCPs which are generally more complex to synthesise and have not yet found significant commercial applications. The author s Mesogenic Index model, which is useful for predicting whether a given polymer has a mesophase, has been applied only to main-chain LCPs to date. 

ADMET has been used to make copolymers of main-chain and side-chain LCPs [87]. An all-hydrocarbon main-chain thermotropic LCP polymer has also been prepared by ADMET [134]. 

A common feature of different types of liquid crystal polymers (LCPs), e.g., thermotropic side-chain or main-chain (either stiff or with flexible spacers) polymers, is their slowed-down dynamics compared to low molecular weight liquid crystals (LCs). Often polymers can be quenched to a glassy state in which the liquid-crystalline order is preserved but motions are completely frozen out. Such liquid-crystalline glasses provide a unique opportunity to determine, in principle, the full orientational distribution function, whereas only its second moment is available from motionally averaged NMR spectra. Thus LCP studies have made fundamental contributions to LC science. 

A more detailed description of side-chain LCPs can be found elsewhere [6]. The rest of this chapter will deal with main-chain thermotropic LCPs. 

Effect of Bulkiness of Pendent Side Groups on the Rheo-Optical Behavior of Thermotropic Main-Chain LCPs... 

Traditionally, two major classes of thermotropic liquid crystalline polymers have been identified the so called main chain (longitudinal) and side chain (comb) types (MCLCPs and SCLCPs, respectively) (Fig. 2.5). More recently other variants have appeared these are combined LCPs which are hybrid between MCLCPs and SCLCPs, and the rigid rod types described by Watanabe et A great wealth of literature already exists in the form of unified texts and reviews which detail both the major classes of LCPs. Bibliographic data have been compiled" and reviews more or less specific to main chain or comb " polymer systems have appeared. We shall be concerned here only with main chain and side chain LCPs. 

Figure 1.69 General structure of main-chain (M.C.) and side-chain (S.C.) LCPs. Adapted from T. S. Chung, The recent developments of thermotropic liquid crystalline polymers. Polymer Engineering and Science, 26(13), p. 903. Copyright 1986, Society of Plastics Engineers.

In the subsequent chapters dynamic magnetic resonance experiments of thermotropic side and main chain polymers are presented. Computer simulations provide the orientational distributions and conformations of the polymer chains and the correlation times of the various motions. The results, referring to all polymer phases, are related to the exceptional material properties of LCPs. The discussion clearly demonstrates the power of dynamic magnetic resonance in characterizing such complex chemical systems. 

The preparation methods and the novel permselective characteristics of (polymer/LC) composite films have been extensively studied (1-3). Also, various types of (polymer/LC) composite systems have been reported as large area and flexible light-intensity controllable films (4-11). Since thermotropic liquid crystalline polymers (LCPs) with mesogenic side chain groups exhibit both inherent mesomorphic properties of LC and excellent mechanical characteristics of polymeric materials, LCPs... 

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