Thermotropic nematic polymers

The distribution of chain sequence extension, calculated by using RIS models, is compared with isotropic-nematic transition characteristics for a number of thermotropic polymers comprising rigid groups connected by polymethylene spacers. The distribution depends strongly not only on the odd-even character of the number of methylene units of the spacers, but also on the specific groups (or atoms connected at the ends of polymethylene spacers. 

The order and mobility of a labeled flexible alkyl spacer in the linear thermotropic polymeric nematic liquid crystal poly(2,2 -dimethyl-4,4 -dioxyazoxybenzenedodecanedioyl-dj0) (poly[oxy(3-methyl-1,4-phenylene)azoxy 2-methyl-1,4-phenylene)oxy(1,12-dioxo-1,12-dodecanediyl-d2oll) is explored with deuterium NMR. The quadrupol splittings of the spacer methylene segments in the nematic melt of the polymer are reported as a function of the temperature and are contrasted with observations on model compounds solubilized in a nematic solvent. 

The development is reviewed of liquid-crystalline polymers whose mesophase formation derives from the nature of the chemical units in the main chain. The emphasis lies primarily on highly aromatic condensation polymers and their applications. The general properties of nematic phases formed by such polymers are surveyed and some chemical structures capable of producing nematic phases are classified in relation to their ability to form lyotropic and thermotropic systems. The synthesis, properties, physical structure and applications of two of the most important lyotropic systems and of a range of potentially important thermotropic polymers are discussed with particular reference to the production and use of fibres, films and anisotropic mouldings. 

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,... 

It is now generally accepted that folding is universal for spontaneous, free crystallisation of flexible polymer chains. It was first of all found in crystallisation from very dilute solutions, but it is beyond doubt now, that also spherulites, the normal mode of crystallisation from the melt, are aggregates of platelike crystallites with folded chains, pervaded with amorphous material. "Extended chain crystallisation" only occurs under very special conditions in the case of flexible chains for rigid polymer chains it is the natural mode ("rigid rod-crystallisation" from the melt in case of thermotropic polymers, and from solution in case of the lyotropic liquid-crystalline polymers both of them show nematic ordering in the liquid state). 

Yoon, D. Y, Bruckner, S., Volksen, W, Scott, J. C., and Griffin, A. C., Configurational characteristics and nematic order of semiflexible thermotropic polymers, Faraday Discuss. Chem. Soc., 79, 41-53 (1985). 

S.H. Chen, H. Shi, J.C. Mastrangelo, J.J. Ou, Thermotropic chiral nematic side-chain polymers and cyclic oligomers. Prog. Polym. Sci. 21, 1211-1233 (1996)... 

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... 

Figure 2 (4,5) summarizes some key points on what was the first reported and wel 1-characterized thermotropic polymer. The Eastman workers melt copolymerized p-acetoxybenzoic acid with poly(ethylene terephthalate) (PET) to form a series of thermotropic aliphatic-aromatic polyesters (X7G polymers). Some of these compositions showed the characteristic melt optical anisotropy typical of nematic small molecules, that is, the melt between crossed polaroids on a hot stage microscope transmits light. This nematic order was persistent. Of course, shear birefringence is also shown by isotropic polymer melts but light transmission decay is quite rapid. 

Fig. 5.72 A polarized light micrograph of a sectioned nematic naphthalene thermotropic polymer (NTP) reveals a schlieren texture.

Figure 6.39 shows one of the important signatures of nematics, namely their low viscosity. Thermotropic liquid-crystalline polymers such as Vectra are of very low viscosity and complicated, tortuous moulds are readily filled. A generalized shear rate-viscosity curve for liquid-crystalline polymers (nematics) is shown in Fig. 6.40. Shear thinning occurs in both regions I and III. Some nematics only show parts of this curve.

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