Discotic thermotropic polymers

NMR Studies of Orientational and Conformational Order in Discotic Thermotropic Polymers... 

There are several different phases in thermotropic liquid crystals. The structural nature of mesophases is influenced by the molecular shape and therefore depends on whether the liquid crystal is formed by rod-like or disc-like molecules. Thermotropics of rod-like molecules may be divided into two main categories nematic and smectic phases. There exist many types of smectic phases, labeled as 5, 5b, S /. When an ordered solid of a liquid crystal melts (see Fig. 1.1), it may melt into a nematic phase or a smectic A phase. Upon further heating, it eventually turns into an isotropic liquid. First, classical thermotropic liquid crystals are described, and then a group of more exotic liquid crystals like discotic thermotropics, lyotropics, and liquid crystalline polymers. 

The second group comprises thermotropic polymers. The phase transitions of thermotropic liquid crystals are achieved when a determined temperature range is reached. The fundamental unit that induces structural order in this kind of polymer presents with high rigidity and anisotropic shape [26,27]. Two major subclasses can be distinguished according to this shape discotic (disc-Uke molecules) and... 

It was first reported in the early 1970s that these melt processible polymers could best be described as thermotropic systems which usually display an nematic texture in the melt phase [5]. Subsequently, a number of additional phases have been reported ranging from discotic structures to highly ordered smectic E G systems with three dimensional order. In the last several years an IUPAC sponsored study on nomenclature on thermotropic LPCs has been underway. A more complete set of definitions will be available shortly as a result of Recommendation No. 199 IUPAC [6]. 

Liquid crystallinity can be attained in polymers of various polymer architectures, allowing the chemist to combine properties of macromolecules with the anisotropic properties of LC-phases. Mesogenic imits can be introduced into a polymer chain in different ways, as outhned in Fig. 1. For thermotropic LC systems, the LC-active units can be connected directly to each other in a condensation-type polymer to form the main chain ( main chain liquid crystalline polymers , MCLCPs) or they can be attached to the main chain as side chains ( side chain liquid crystalline polymers , SCLCPs). Calamitic (rod-Uke) as well as discotic mesogens have successfully been incorporated into polymers. Lyotropic LC-systems can also be formed by macromolecides. Amphiphihc block copolymers show this behavior when they have well-defined block structures with narrow molecular weight distributions. 

Thermotropic liquid crystals can then be furflier subdivided into high molecular mass, main and side-chain polymers [10] and low molecular mass, the latter class of compounds being one of the areas of this review. The phases exhibited by the low molecular mass molecules are then properly described with reference to the symmetry and/or supramolecular geometry of the phases, which are briefly introduced here and are discussed in more detail further below. Thus, the most disordered mesophase is the nematic (N), which is found for calamitic molecules (N), discoidal molecules (Nq) and columnar aggregates (Nc), among others. The more ordered lamellar or smectic phases (S) [11, 12] are commonly shown by calamitic molecules, and there exists a variety of such phases distinguished by a subscripted letter (e. g. Sa, Sb)- Columnar phases (often, if incorrectly, referred to as discotic phases) may be formed from stacks of disc-like molecules, or from... 

Lyotropic liquid crystals occur abundantly in nature, being ubiquitous in living systems.Their structures are quite complex and are only just beginning to be elucidated. However, in this monograph we shall be confining our attention mainly to the physics of low molecular weight thermotropic liquid crystals and do not propose to discuss polymer and lyotropic systems in any further detail. In chapters 2-5, we deal with the nematic, cholesteric and smectic mesophases of rod-like molecules and in chapter 6 discotic systems. 

This book does not intend to cover the whole field of thermotropic liquid crystalline (TLC) materials as it is extremely difficult to cover within a single book. Instead it presents a collection of Chapters written by experts on various exciting topics in the field. Properties of recently developed TLCs (such as banana-type, thiophene-based, and columnar TLCs), phase biaxiaUty, and novel polymeric TLCs are discussed In detail. Solid-state NMR studies to obtain atomistic-level structural and geometrical information of TLCs are presented. Synthesis of liquid crystalUne conjugated polymers, fast switching of nematic materials by an electric field, and photoconducting discotic systems are also presented. 

As mentioned above, LLCs, which are columnar stacks of discotic molecules, can be photoaligned, thus forming a thin polarizing layer [61-63], However, the thermotropic discotic LC can also be ordered by a photoalignment method [64-66], A thin film of 25 nm thickness has been prepared on a fused silica surface from a photosensitive polymer with a side-chain p-cyanoazobenzene photosensitive group [64] (Figure 4,22),... 

---