Thermotropic liquid crystal polymers determination

The main part of the side group in these macromolecules consists of the alkoxy-benzoic acid moiety. This acid may form thermotropic liquid crystals. The investigation of the hydrodynamic properties of PPhEAA molecules in dilute solutions has revealed that the equilibrium rigidity of their main chains is relatively low (Table 12). Hence, since for all flexible-chain polymers, the shear optical coefficient An/Ar in PPhEAA solutions is independent of molecular weight the segmental anisotropy - tt2 and the anisotropy of the monomer unit Aa may be determined by use ofEq. (67). 

Originally. TOA was devised and used for analyses of chain mobility in polymers (7) and polymer blends (8.91 by monitoring birefringence disappearance during programmed heating in scratches scribed on film surfaces. Lenz (10.Ill has reported the use of TOA in the study of liquid crystal polymers, and recently we have used the technique in determining melt- and isotropization temperatures for thermotropic fully aromatic liquid crystal polymers (4.51. 

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. 

Thermotropic liquid crystalline polymers (TLCPs) show its liquid crystallinity in melt phase (Brehmer and de Jeu 2012 Shibaev et al. 1984 Popa-Nita et al. 2009). Thermotropic phases are those that occur in a certain temperature range. If the temperature is raised too high, thermal motion will destroy the ordering of the LC phase, pushing the material into a cmiventional isotropic liquid phase. At too low a temperature, most LC phases will form a conventional anisotropic crystal. Many thermotropic LCs shows a variety of phases as temperature is altered (National Materials Advisory Board 1990 Popa-Nita et al. 2009 Davidson 1999). The melt temperature and the thermal history stored within the polymer system plays a vital role in determining the liquid crystallinity of TLCPs. 

The optical properties are directly related to the polymer and LC structure formation, consequently it is of paramount importance to determine the effect of side chain size and number of substituents, degree of esterification and degree of polymerization in pitch values and the optical properties of the mesophases of thermotropic liquid crystals. 

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... 

The isotropic-to-nematic transition is determined by the condition [1 — (2/3)TBBWBB/k T] = 0 whereas the spinodal line is obtained when the denominator of XAA is equal to zero. These conditions are evaluated in the thermodynamic limit (Q = 0) in Fig. 7 for a Maier-Saupe interaction parameter Web/I bT = 0.4xAb and for NA = 200, N = 800, vA = vB = 1. When the volume fraction of component A(a) is low, the isotropic-to-nematic phase transition is reached first whereas at high < >A the spinodal line is reached first. In the second case, the macromolecules do not have a chance to orient themselves before the spinodal line is reached. This RPA approach is a generalization of the Doi et al. [36-38] results (that were developed for lyotropic polymer liquid crystals) to describe thermotropic polymer mixtures. Both approaches cannot, however,... 

The differences between standard thermotropic LCs and macromolecular condis crystals are summarized in Fig. 8. The first three and the last two points make it easy to experimentally identify low molecular mass LCs. For macromolecules, however, the viscosity may be suflBciently large to lose the obvious liquid character the birefringence does not always show the well-known LC texture (55) the small ASj of LCs may be confused with partial crystallinity of the condis crystals and in polymers, some larger main-chain rigid groups are not always easily identifiable as mesogens. This leaves points four and eight for differentiation between the two mesophases. Points five and six are more difficult to establish, and solid state NMR and detailed X-ray structure-determinations may be necessary for full characterization. Furthermore, borderline structures may be possible between thermotropic LCs, amphiphilic LCs, and condis crystals. A few examples and the resolution of their structures are discussed next, to illustrate the resolution of some of these problems. 

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