Polyimides thermotropic liquid-crystalline

Recently much effort has been made to induce liquid crystallinity in polyimides. Kricheldorf et al., Chen et al., and Sato et al. have suggested that an aromatic im-ide ring may be an excellent mesogen through their studies on thermotropic liquid-crystalline polyimide-esters [41-54] and polyimide-carbonates [55, 56]. Some thermotropic properties of aromatic polyether-imides have also been reported by Asanuma et al. [57]. 

More recently, we have found that the terphenyltetracarboxylic acid-based polyimides P-XTP showed thermotropic liquid-crystalline phase [21,22]. As far as we know, this is the first example of the simple polyimides showing thermotropic liquid crystallinity. 

Figure 4.21 Temperature dependence of the fluorescence peak wavelength during heating of thermotropic liquid-crystalline polyimide. Excitation at 320 nm. (Reprinted from Polymer, Volume 40, H.W. Huang, T.I. Kaneko, K. Horie and J. Watanabe, Fluorescence study on intermolecular complex formation between mesogenic terphenyldiimide moieties of a thermotropic liquid-crystalline polyimide, 3826, copyright 1999, with permission from Elsevier Science)...

The large body of thermotropic liquid crystalline polymers is polyesters [14], some of which have been successfully commercialized. And very recently, quite a lot of thermotropic liquid crystalline polyimides have been reported [15]. In this chapter, the crystallization behavior and phase transition kinetics of thermotropic polyesters and polyimides will be discussed. 

Thechemical structures [43] ofthermotropicpolyimideandpolyamideLCP s are described in Chapter 3, along with their crystallization kinetic studies. Kircheldorf et al. [44] carried out XEiD and PLM characterization studies of these TLCPs and found the polyimide to be a smectic liquid crystalline. Pramoda et al. [45] studied the thermal degradation kinetics of TLC-PI (thermotropic liquid crystalline polyimide) and TLC-PA (thermotropic liquid crystalline polyamide). The apparent activation energy in the case of TLC-PI is found to be higher than the TLC-PA for degradation. 

K. G. Blizard and R. R. Haghighat, Processing and properties of polyimide melt blends containing a thermotropic liquid crystalline polymer, Polym. Engr. and Sci., vol. 33, No. 2, 1-9. 

Shoji, Y. Ishige, R. Higashihara, T. Watanabe, J. Ueda, M., Thermotropic Liquid Crystalline Polyimides with Siloxane Linkages Synthesis, Characterization, and Liquid Crystalline Behavior. Macromolecules 2010, 43,805-819. 

It is significant that the reinforcement degree corresponds to a class of polymer forming a nanocomposites matrix. The largest values of / are obtained for polymers whose chains are able to stretch on the silicate platelet surface (rigid-chain polyimide, crystallising polypropylene and thermotropic liquid crystalline polyester), intermediate values for polymers whose chains are able to stretch only partly (polycarbonate, poly (butylenes terephthalate) and amorphous polyamide-6) and the smallest values for nanocomposites on the basis of epoxy polymer, the capability of chains stretching of which decreases sharply because of the availability of transverse covalent bonds network [30]. 

Keywords Liquid-crystalline polyimides, Poly(ester-imide)s, Thermotropic, Nematic, Smectic, Cholesteric, Mesogen, Layer structure, Aliphatic spacer, Trimellitimide... 

By far the largest group of liquid crystalline polymers containing imide groups are copoly(esterimide)s (PEIs). These have been sub-divided into flexible copolymers containing aliphatic spacers and wholly aromatic copolymers. There are several reports of copoly(amide-imide)s and copoly(ether-imide)s (both thermotropic and lyotropic) and these are treated separately. There are only a few examples of wholly aromatic imide containing LCPs, as polyimides frequently melt well above their decomposition temperature. Successful attempts have been made to incorporate flexible spacers, but in most instances the spacers are based on ether units, and the polymer is more strictly classified as a poly(etherimide). 

While no variation of the diamine used was carried out in this program, the properties of the polyimide can undoubtedly be affected by use of various diamines. Flexibility could be Introduced by longer chain length diamines, and anisotropy may be possible through the use of thermotropic or lyotropic liquid crystalline diamines. 

Evans et al. systematically studied various polyimides derived from PMDA [56]. None of those polyimides was thermotropic, despite the fact that PMDA was expected to be a mesogenic unit because of its flat, rigid, and linear strue-ture. The authors concluded, we are unaware of any low molecular weight liquid crystalline compounds which contain the imide functional group. This combined with the results of the studies reported here suggests that it may be... 

---