Main-chain LCPs

Materials with totally new property combinations may be achieved by blending two or more polymers together. Through blending of thermotropic main-chain LCPs with engineering thermoplastics, the highly ordered fibrous structure and good properties of LCPs can be transferred to the more flexible matrix polymer. LCPs are blended with thermoplastics mainly in order to reinforce the matrix polymer or to improve its dimensional stability, but LCP addition may modify several... 

LC polyesters belong to the class of thermotropic main-chain LCPs, which also comprises polymers such as polycarbonates, polyethers, polyphenylenes, polyester-imides, polymers containing azo- or azo V-oxide linking groups, some cellulose derivatives, and polypeptides such as po 1 y (y - be n zy 1 -1. - g 1 u tamate). Both from the academic and industrial points of view, polyesters are by far the most important representatives of this class of polymers. 

Flow behaviour of Lyotropic Main Chain LCPs... 

In main-chain LCPs, molecular flexibility can be distributed more-or-less uniformly along the chain, as is the case for PBLG, HPC, or Vectra A, or it can be concentrated in flexible spacers, as in OQO(phenylsulfonyl)lU (see Fig. 11-2). The former are called persistently flexible molecules, and are often modeled by the worm-like chain, with a uniform bending modulus, while for the latter, a reasonable model might be the freely jointed chain (see Fig. 11-3 and Section 2.2.3.2). For a recent discussion of the phase behavior and dynamics of worm-like chains, see Sato and Teramoto (1996). 

Both thermotropic and lyotropic liquid crystal polymers exhibit characteristic features with regard to their microstructureJ Anisometrical monomers such as rods or discs are connected to chains in an appropiate manner. These anisometrical monomers are considered to be the mesogens and may be part of main chain LCP, side chain LCP, or of both types together (Fig. 6). Between the mesogens are located flexible spacers of non-mesogenic character. Sufficient flexibility is a prerequisite for liquid crystal formation, with an increase in either temperature or solvent concentration. 

It is well known that below a critical molar mass, the properties of polymers are very sensitive to changes in molar mass. Very little, however, is known about the molar mass dependence of the properties of LCPs. This is partly due to the difficulties in controlling molar mass in the reactions usually used to synthesize LCPs and partly due to the lack of good molar mass data on most LCPs, which are often soluble only in foirly harsh solvents. The chapter by Kim and Blumstein describes the preparation of series of main-chain LCPs varying in molar mass and the effect of molar mass on the thermal properties. 

Utilization of the single hydrogen bond between pyridine and benzoic acids in SLCP s has been a source of inspiration for other groups in the development of main-chain supramolecular polymers based on diacids and dipyridines.53-56 Supramolecular rod-coil polymers have been developed by assembly of 4,4 -bipyridines and telechelic polypropylene oxide with benzoic acid end-groups, which show highly ordered liquid crystalline phases.57 The use of tartaric acid derivatives in combination with bipyridine units resulted in the formation of hydrogen-bonded, chiral main-chain LCP s, as has been shown by circular dichroism measurements, optical microscopy, and X-ray data.58,59... 

Next to side-chain LCPs and main-chain LCPs, supramolecular networks were obtained by complex-ation of bipyridines with polyacrylates containing pendant benzoic acid groups. In a related approach, Kato and Frechet have studied supramolecular networks based on low-molecular-weight components, in which a trifunctional benzoic acid derivative was combined with a difunctional pyridine derivative.62 The hierarchy of the LC-phase that was formed turned out to be dependent on the flexibility of the trifunctional compound used. 

Structure and Theoretical Aspects of Main-Chain LCPs.199... 

The author worked for many years at BP Research on the synthesis of LCPs and devised an empirical method called the Mesogenic Index, which employs functional group contributions on an additive score basis to predict whether a particular random copolymer is likely to exhibit a mesophase (subject to the polymer being soluble or fusible). This chapter explores the general features and theoretical aspects of the chemical structures of main chain LCPs and describes the Mesogenic Index and how it was successfully applied to polyesters, polyamides and polycarbonates. The final section describes the extension of the MI empirical method to the various types of LC polyimides reported in recent years. 

Most LCPs of commercial interest are thermotropic main-chain nematic materials. The other main structural type of interest are the side chain LCPs which are generally more complex to synthesise and have not yet found significant commercial applications. The author s Mesogenic Index model, which is useful for predicting whether a given polymer has a mesophase, has been applied only to main-chain LCPs to date. 

The fairly good quality of the fits validates both Leadbetter s assumptions and the Maier-Saupe distribution function. However, the values of S obtained and even the quality of the fits obviously depend on the odd or even number of (CH2) groups in the flexible spacer. This odd-even effect is widespread and well known in the field of main-chain LCPs and will be discussed later in this article. The nematic order parameter of main-chain LCPs may reach values as high as 0.85 which demonstrates the very high orientation of the nematic phase of these polymers. Such a large orientation is undoubtedly responsible for the good mechanical properties of this type of materials. The treatment described above therefore provides a very easy way of characterizing the orientational order of a nematic phase. It has also been tested for thermotropic side-chain LCPs and found to be satisfactory as well [15]. Unfortunately, it has not been used yet in the case of lyotropic LCPs except for some aqueous suspensions of mineral ribbons (Sect. 5) which are not quite typical of this family of materials. 

The conformation of the repeat unit of main-chain LCPs very strongly depends on the odd or even number of CH2 groups in the flexible spacer. This is called the odd-even effect and is illustrated in Fig. 5. 

Fig. 7. Conformation of a main chain LCP described as a cylinder. H is the magnetic field direction which aligns the nematic director...

The widths of the diffuse spots along the director and perpendicular to it are inversely proportional to the correlation lengths. For some reason, main-chain LCPs very often show SmC fluctuations in the nematic phase and when they crystallize, these diffuse spots usually condense into Bragg reflections. 

Fig. 8. a X-ray scattering pattern of a fiber of an organometallic main-chain LCP quenched from the nematic phase, b Schematic representation of a. (a) = wide angle diffuse ring (b) = diffuse spots due to the SmC fiuctuations (c) = equidistant diffuse streaks (d) = other weak diffuse spots e = faint lines due to a slight crystallization... 

Let us first consider the set of equidistant diffuse streaks (c) perpendicular to the director [la,b, 30]. These streaks are very similar to those observed on the X-ray scattering patterns of the nematic phases of main-chain LCPs discussed in Sect. 3.2 but they must be interpreted differently because the SmA phase shows (quasi) long-range positional order. These diffuse streaks also correspond to the intersection with the Ewald sphere of a set of equidistant diffuse planes. But here this set represents the Fourier transform of uncorrelated rows of side-chains displaced along the director from their equilibrium position inside the layers (Fig. 13). 

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