Thermotropic polymer

In 1975, the synthesis of the first main-chain thermotropic polymers, three polyesters of 4,4 -dihydroxy-a,a -dimethylbenzalazine with 6, 8, and 10 methylene groups in the aHphatic chain, was reported (2). Shortly thereafter, at the Tennessee Eastman Co. thermotropic polyesters were synthesized by the acidolysis of poly(ethylene terephthalate) by/ -acetoxybenzoic acid (3). Copolymer compositions that contained 40—70 mol % of the oxybenzoyl unit formed anisotropic, turbid melts which were easily oriented. 

Thermotropic polymers, 13 371, 372 Thermotropic polymer liquid crystals, 15 107... 

Kricheldorf [17] studied liquid-crystalline cholesteric copoly(ester-imide)s based on 1 or 2. The comonomers to obtain these chiral thermotropic polymers were N-(4-carboxyphenyl)trimellitimide, 4-aminobenzoic trimellitimide, 4-aminocinnamic acid trimellitimide, adipic acid, 1,6-hexanediol, and 1,6-bis(4-carboxyphenoxyl) hexane. Apparently the poly (ester imide) chains are so stiff that the twisting power of the sugar diol has little effect. 

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. 

Both polymer melts and polymer solutions sometimes form phases with orientational and positional ordci. Thermotropic polymer liquid crystals possess at least one liquid crystal phase between the glass-transition temperature and the transition temperature to the isotropic liquid. Lyotropic polymer liquid crystals possess at least one liquid cry stal phase for certain ranges of concentration and temperature. 

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 possibility of obtaining thermotropic polymers was considered theoretically by Papkov and his collaborators in 1973 I9) and by Ciferri in 197 5 20) and is implicit in Flory s treatment2) of the rigid-rod situation. 

Thermotropic polymers require no solvent for formation of a liquid-crystalline phase, which occurs instead within a defined temperature range. The chemical units useful for thermotropic polymer formation are generally those already exemplified in Figs. 1 and 2, but these units in homopolymer form give rise to crystalline polymers with melting points above their decomposition temperatures. The problem of polymer design is to reduce the melting temperature in order to obtain a liquid-crystalline phase at a temperature below that of decomposition. Whereas the lyotropic systems are... 

The rheological behaviour of thermotropic polymers is complex and not yet well understood. It is undoubtedly complicated in some cases by smectic phase formation and by variation in crystallinity arising from differences in thermal history. Such variations in crystallinity may be associated either with the rates of the physical processes of formation or destruction of crystallites, or with chemical redistribution of repeating units to produce non-random sequences. Since both shear history and thermal history affect the measured values of viscosity, and frequently neither is adequately defined, comparison of results between workers and between polymers is at present hazardous. 

Most high polymers exhibit die swell on extrusion. Thermotropic polymers do not, and indeed values of the ratio of extrudate diameter to capillary diameter as low as 0.9 have been observed 104). Since die swell is associated with viscoelasticity in the melt, it is interesting to note that even in the isotropic phase little or no elastic behaviour is observed in a thermotropic polymer 105),... 

Most of the patents that claim thermotropic polymers also claim fibres made from them, although full spinning conditions and physical data for the products are not always provided. Tensile data for the samples of highest tenacity after annealing from some representative patents are fisted in Table 3. 

Despite their attractive properties, no clear application for thermotropic polymers of the rigid main-chain types has yet emerged. Fibre manufacture is an obvious... 

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). 

Figure 2 shows an example of a phase diagram of a mixture consisting of a thermotropic polymer—ethyl cellulose (EC)--dissolved in an ordinary organic solvent—acetic acid. 

Fig. 1. Thermotropic Polymers with Either Main Chain or Side Chain Mesogenic Units...

The term mesogenic group , for the purposes of this review, refers to the part of the polymer chain that is composed of the rigid, linear segments and the atoms or functional groups which link them together in a linear array. It is this part of the polymer chain that ultimately determines whether or not the polymer will be liquid crystalline, within what range the transition temperatures will occur for a thermotropic polymer, and what type of mesophase can be formed. 

Most of the subject polymers that have been studied to date have contained flexible spacers composed of polymethylene chains. The monomers or precursors for these spacers are readily available as either diols or diacids or as dihalides, and this availability has led to the preparation of several homologous series of thermotropic polymers with different mesogenic units. Indeed, the first reported main chain, thermotropic LC polymer contained a polymethylene flexible spacer... 

The mesophase exhibiting a homeotropic texture can still shew stir opalescence, which is a method of identification particularly suited for thermotropic polymers. This somewhat crude method of characterizing a liquid crystalline material is performed by shearing a thin film of the mesophase and looking for momentary appearances of turbidity in the otherwise transparent melt. No microscope is need to observe stir opalescence, but simple shearing of the homeotropic melt between crossed-polars can also reveal the mesophase. 

Fig. 10a and b. X-ray Diffraction Patterns of Thermotropic Polymers Polymer 15 Obtained at Room Temperature, a and in the Liquid Crystal State, b... 

Among the many different classes of thermotropic polymers, only a limited number of polyesters based on aromatic ester type mesogenic units have been studied by rheological methods, beginning with the publication by Jackson and Kuhfuss of their work on the p-oxybenzoate modified polyethylene terephthalate, PET, copolymers. They prepared a series of copolyesters of p-hydroxybenzoic acid, HBA, and PET and measured the apparent melt viscosity of the copolymers as a function of their composition by use of a capillary rheometer. On inclusion of low levels of HBA into PET, the melt viscosity increased because of partial replacement of the more... 

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