Polymeric Liquid Crystalline Materials

Polymeric materials have advantages because of their stability and structureforming properties. Electron- and ion-active organic polymeric materials have attracted attention for new devices. In Chapter 5, Kato and co-workers focus on polymeric liquid crystalline materials that are used for the development of functional materials transporting ions and electrons. The nanostructures such as smectic and columnar phases exhibited by side-chain, main-chain, dendritic, and network polymers may exhibit one- and two-dimensional transportation properties. 

General Factors Influencing Polymeric Liquid Crystalline Materials... 

The potential of vibrational spectroscopy for polymer characterization can be considerably enhanced if it is applied as time-resolved technique to polymers trader external perturbations or combined with other analytical techniques, thereby providing different types of physical data that have been simultaneously acquired on the same sample. In this respect, specifically the combination of vibrational spectroscopy with thermal or mechanical measurements has contributed toward a better understanding of the stmctural changes as a consequence of the external perturbation. Furthermore, FT-IR spectroscopic studies of segmental mobility of polymeric, liquid-crystalline materials as a function of external electric or electromagnetic perturbation have proved of scientific and industrial interest and will be discussed in some detail. 

More recently, Raman spectroscopy has been used to investigate the vibrational spectroscopy of polymer Hquid crystals (46) (see Liquid crystalline materials), the kinetics of polymerization (47) (see Kinetic measurements), synthetic polymers and mbbers (48), and stress and strain in fibers and composites (49) (see Composite materials). The relationship between Raman spectra and the stmcture of conjugated and conducting polymers has been reviewed (50,51). In addition, a general review of ft-Raman studies of polymers has been pubUshed (52). 

Even within a particular class of polymers made by step-growth polymerization, monomer composition can be varied to produce a wide range of polymer properties. For example, polyesters and polyamides can be low-Tg, amorphous materials or high-Tg, liquid crystalline materials depending on the monomer composition. 

Kato, T. Mizoshita, N. Kanie, K. Hydrogen-bonded liquid crystalline materials Supramolecular polymeric assembly and the induction of dynamic function. Macromol. Rapid Commun. 2001, 22, 797-814. 

Polyoxybenzoate is a stiff chain, lyotropic liquid crystalline material, as was discussed on the basis of its copolymers with ethylene terephthalate (see Sect. 5.1.4). The crystal structure of the homopolymer polyoxybenzoate was shown by Lieser 157) to have a high temperature phase III, described as liquid crystalline. X-ray and electron diffraction data on single crystals suggested that reversible conformational disorder is introduced, i.e. a condis crystal exists. Phase III, which is stable above about 560 K, has hexagonal symmetry and shows an 11 % lower density than the low temperature phases I and II. It is also possible to find sometimes the rotational disorder at low temperature in crystals grown during polymerization (CD-glass). 

The study of liquid crystalline materials by X-ray diffraction has until recently been confined to low molecular weight compounds because of the lack of availability of suitable polymeric mesogens. Based on the observations by X-ray diffraction made with small molecules as reviewed by Azaroflf a system for identifying the type of mesophase has been developed by DeVries and this system is now being applied to polymeric liquid crystals. 

A comprehensive review about the engineering and preparation of side-chain liquid-crystalline polymers by living polymerization methods including ROMP has been given recently. Consequently, only the more recent contributions shall be covered briefly. One convenient approach for the preparation of side-chain liquid-crystalline materials entails the ROMP of norbornenes carrying mesogenic side groups in the... 

However, the preparation of latex particles may be perceived as having reached a level at which the potential for a fundamental breakthrough in the final materials per se is rather limited. Pioneering efforts may instead be expected in the development of polymeric microcompartmentalized materials. This development, in a limited form, may be exemplified by the work of Gan and colleagues [28], who polymerized organic monomers solubilized in bicontinuous microemulsions and obtained microporous organic polymers. This area is, of course, of future interest, but the problem of lack of correlation between the microemulsion colloidal structure and the microstructure of the final material may result in a focus on the polymerization of liquid crystalline material where even complex systems [29,30] have been shown to retain their microstructure after polymerization. This area of polymerization has been further developed and systematized by Antonietti [31,32], Antonietti et al. [33], and Fendler [34]. 

A simple application of the self-assembly of complementary bifunctional small molecules to form extended-chain structures, or longitudinal PLCs, is given in references 6-8. Griffin and coworkers report that hydrogen bond-driven association between bispyridyl-terminated and bisbenzoic acid-terminated species can lead to liquid crystalline materials with polymeric characteristics. Typical compounds employed are shown in Figure 3.2. None of the starting components are liquid crystalline. 

Ferroelectric materials are a subclass of pyro- and piezoelectric materials (Fig. 1) (see Piezoelectric Polymers). They are very rarely foimd in crystalline organic or polymeric materials because ferroelectric hysteresis requires enough molecular mobility to reorient molecular dipoles in space. So semicrystalline poly(vinylidene fluoride) (PVDF) is nearly the only known compoimd (1). On the contrary, ferroelectric behavior is very often observed in chiral liquid crystalline materials, both low molar mass and poljuneric. For an overview of ferroelectric liquid crystals, see Reference 2. Tilted smectic liquid crystals that are made from chiral molecules lack the symmetry plane perpendicular to the smectic layer structure (Fig. 2). Therefore, they develop a spontaneous electric polarization, which is oriented perpendicular to the layer normal and perpendicular to the tilt direction. Because of the liquid-like structure inside the smectic layers, the direction of the tilt and thns the polar axis can be easily switched in external electric fields (see Figs. 2 and 3). 

Abstract The aim of the present review is to illustrate to the reader the state of the art on the construction of supramolecular azobenzene-containing materials formed by halogen bonding. These materials include several examples of polymeric, liquid crystalline or crystalline species whose performances are either superior to the corresponding performances of their hydrogen-bonded analogues or simply distinctive of the halogen-bonded species. 

---