Thermotropic polymer molecular structures

The article covers synthesis, structure and properties of thermotropic liquid-crystalline (LC) polymers with mesogenic side groups. Approaches towards the synthesis of such systems and the conditions for their realization in the LC state are presented, as well as the data revealing the relationship between the molecular structure of an LC polymer and the type of mesophase formed. Specific features of thermotropic LC polymers and copolymers of nematic, smectic and cholesteric types are considered. 

The design and synthesis of new liquid crystalline polymeric materials endowed with intrinsc chirality deserve attention, as chirality can offer probes of the supermolecular structure and a tool for modulating specific responses of the polymers (1). The chemical transformation of preformed thermotropic polymers can add novel opportunities for the realization of various molecular architectures conventionally unfeasible and best suited for mesophase modification. 

Thermotropic polyesters derived from unsubstituted aromatic diols and diacids usually have melting points which approach or exceed the thermal decomposition point. Thus it is reasonable to expect that some modification in molecular structure would be required to render them melt-processable, even though some adverse effects on liquid crystallinity and mechanical properties of the polymers would result. 

Secondly, thermotropic liquid crystalline polymers are characterized in a temperature range that is often too high to keep the molecular structure unchanged. The post-polymerization and decomposition are two possible results of a prolonged study of a polymeric liquid crystalline phase. There have been reports on liquid crystalline phases observable only... 

One major conclusion that can be drawn from the presented results is related to the effect of the molecular structure of the rigid, anisotropic core of the polymers under investigation on their thermotropic behaviour. Thus, if one takes both the mesophasic-to-isotropic liquid transition temperature and the temperature range of persistence of such mesophase as a qualitative index of the effectiveness of the central group X in imparting liquid crystal properties to the polymer system, the following order can be established ... 

Although instances of lyotropic PLCs predate studies of thermotropic PLCs, as they involved solutions of comparatively esoteric species — virus particles and helical polypeptides — studies of these liquid crystals were isolated to a few laboratories. Nevertheless, observations on these lyotropic PLCs did stimulate the first convincing theoretical rationalizations of spontaneously ordered fluid phases (see below). Much of the early experimental work was devoted to characterizing the texture of polypeptide solutions. (23) The chiral polypeptides (helical rods) generate a cholesteric structure in the solution the cholesteric pitch is strongly dependent on polymer concentration, dielectric properties of the solvent, and polymer molecular weight. Variable pitch (<1 - 100 pm) may be stabilized and locked into the solid state by (for example) evaporating the solvent in the presence of a nonvolatile plasticizer.(24)... 

Random copolymerization is an effective way to disturb the regular molecular structure of the polymer chain. The lack of periodicity along the chain inhibits crystallization and, thus, reduces the crystal size and perfection and depresses Tm. If the chosen comonomer is essentially linear, losses of chain mesogenicity can be minimized. This approach has been followed in designing thermotropic copolyesters, such as Vectra A by Celanese (now Ticona), which continues to enjoy commercial success. Vectra A is the copolyester of p-acetoxybenzoic acid (ABA) and 2,6-acetoxynaphthoic acid (ANA) with a mole ratio of 73/27. The melting point of this LCP is around 280°C, which is much lower than the melting points of the either poly (ABA) or poly (ANA) [6]. 

Donald et al. [2] reported banded structures formed by several thermotropic polymers oriented by shear at temperatures above their softening points. Similar structures were also noted in fibers drawn from polymers with rigid backbones above the softening points. Viney et al. [3] point out that the banded structures observed in shear are due to the variation in the direction of the long molecular axis with respect to the direction of shear. Evidence obtained by both optical microscopy and electron diffraction measurements supports this view. Donald and Windle [4] studied the banded structure by electron microscopy and commented that The near sinusoidal variation in the direction of the principal axis of the refractive index ellipsoid is indeed reflecting the variations in the molecular orientation. Their transmission electron microscopy indicates that the transition from... 

The results above show that the Frank moduli are determined mainly by the structure of mesogenic units which are similar for conventional nematics and thermotropic polymers (the situation changes considerably for the lyotropic solutions of long rod-like polymeric molecules, see the next section). On the other hand, the dynamics of reorientation are strongly influenced by the backbone. Field response and relaxation times depend dramatically on the molecular mass of a polymer though, in the first approximation, obey the same equations (4.30, 4.31). Figure 4.42 shows field-response times as a function of temperature for a comb-like acryl polymer H... 

Thus, the factors critical in the molecular design of a commercially viable thermotropic polymer for fiber/resin applications include (1) processability — ideally the polymer should process in the range of 250-350°C (2) melt anisotropy — a careful balance of molecular symmetry is required (3) end-use properties -- considerable structure-property experimentation needed to optimize and, of course, (4) minimum monomer cost. 

A lot of molecular structures give rise to thermotropic liquid crystallinity, but only the aromatic ester type polymers and copolymers are successfully prepared as structural materials. Aromatic polyesters can be classified into three types based on their molecular composition and thermomechanical property [74]. 

We are all familiar with gases, liquids and crystals. However, in the nineteenth century a new state of matter was discovered called the liquid crystal state. It can be considered as the fourth state of matter (although plasmas are also candidates for this accolade). The essential features and properties of liquid crystal phases and their relation to molecular structure are discussed in this chapter. Specifically, the focus is on thermotropic liquid crystals (defined in the next section). These are exploited in liquid crystal displays (LCDs) in digital watches and other electronic equipment. Such applications are outlined later in this chapter. Surfactants and lipids form various types of liquid crystal phase but this was discussed separately in Chapter 4. Finally, this chapter focuses on low molecular weight liquid crystals, liquid crystalline polymers being touched upon in Section 2.10. 

In the experimental study of the optical properties of molecules by the method of flow birefringence (FBF), the question of the molecular dispersion of the solution is always important, particularly in the analysis of the properties of mesogenic macromolecules. The complex structure of thermotropic polymers and their tendency to form supermolecular structures require special monitoring of possible association or decomposition of the macromolecules in solution. The linear dependence of An in a wide range of rate gradients (Figs. 3.7 and 3.8) and... 

Fibers with very high strength and modulus can be fabricated from polymers that have a molecular structure in which the chains are packed in small cross-sectional areas with strong bonds and low elongation. However, some of these aromatic polymers have melting points that are higher than their decomposition temperatures due to the rigidity of their molecules. It is therefore impossible to process them in thermotropic liquid crystal form. 

Fig. 1. The molecular structure of the thermotropic liquid crystal polymers, poly a,co-[4,4 -(2,2 -dimethylazoxyphenyl)]alkandioates and their nematic-isotropic transition temperatures NI as a function of the number of methylene groups n in the flexible spacer. 

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