Optically anisotropic polymers

Fig. 31. Illustration of procedure due to Utiyama124 for determining M of optically anisotropic polymer. Example relates to three concentrations cj, c2, C3 over a range of angles 0...

For an optically anisotropic polymer the conventional light scattering analysis, in which the initial slope of a c/i vv vs. plot at infinite dilution... 

E. Hanelt, F. Sandmeyer, and N. Haberle, Liquid-crystalline nematic organosiloxanes which can be crosslinked to form optically anisotropic polymer layers, US Patent 6 300 454, assigned to Wacker-Chemie GmbH (Munich, DE), October 9, 2001. 

The polyamides are soluble in high strength sulfuric acid or in mixtures of hexamethylphosphoramide, /V, /V- dim ethyl acetam i de and LiCl. In the latter, compHcated relationships exist between solvent composition and the temperature at which the Hquid crystal phase forms. The polyamide solutions show an abmpt decrease in viscosity which is characteristic of mesophase formation when a critical volume fraction of polymer ( ) is exceeded. The viscosity may decrease, however, in the Hquid crystal phase if the molecular ordering allows the rod-shaped entities to gHde past one another more easily despite the higher concentration. The Hquid crystal phase is optically anisotropic and the texture is nematic. The nematic texture can be transformed to a chiral nematic texture by adding chiral species as a dopant or incorporating a chiral unit in the main chain as a copolymer (30). 

Anisotropy in polymer solutions is rather rare, but, where it occurs, the effect on the derived molecular weight can be large enough to warrant appropriate corrections. The procedures have been developed by Utiyama124) and by Utiyama and Kurata125). Without a polariser or analyser the normal reciprocal scattering function Kc/Rtf can be measured. It is denoted by Z(0) the form of which contains an anisotropy parameter 5 which is a function of the number of optically anisotropic elements and the principal polarisabilities in the chain ... 

If one follows the solution viscosity in concentrated sulfuric acid with increasing polymer concentration, then one observes first a rise, afterwards, however, an abrupt decrease (about 5 to 15%, depending on the type of polymers and the experimental conditions). This transition is identical with the transformation of an optical isotropic to an optical anisotropic liquid crystalline solution with nematic behavior. Such solutions in the state of rest are weakly clouded and become opalescent when they are stirred they show birefringence, i.e., they depolarize linear polarized light. The two phases, formed at the critical concentration, can be separated by centrifugation to an isotropic and an anisotropic phase. A high amount of anisotropic phase is desirable for the fiber properties. This can be obtained by variation of the molecular weight, the solvent, the temperature, and the polymer concentration. 

For instance, for a nematic polymer with positive anisotropy of dielectric constant (Ae > 0) orientation of mesogenic groups along the applied field takes place (homeo-tropic orientation). The fact of orientation is illustrated in Fig. 25, which shows that under crossed polarizers the optical transmittance I of a film of nematic polymer with optically anisotropic texture (taken for 100%) falls practically to zero when a low-frequency field is switched on. 

The fragments of macromolecules with ordered cholesterol group sequences, that are formed in bad solvents, may serve as nuclei of supermolecular order in films, obtained from these solvents. Structural and optical studies have shown that PChMA-11 films produced by solvent evaporation display different properties those obtained from chloroform and toluene solutions (small relaxation times, see Table 17) are optically isotropic, and those obtained from heptane solutions (large relaxation times, see Table 17) are optically anisotropic, what reflects the differences in conformational state of polymeric chains in these films. Contrary to the optically isotropic films, a high degree of side branch ordering characterizes optically anisotropic films, which is confirmed by X-ray studies. The observed difference of LC polymer structure in the bulk is thus the consequence of their different conformational state in solution this reveals some possibilities for the control of LC polymer structure at the initial steps of mesophase nucleation in solutions. 

Abstract Macromolecular coils are deformed in flow, while optically anisotropic parts (and segments) of the macromolecules are oriented by flow, so that polymers and their solutions become optically anisotropic. This is true for a macromolecule whether it is in a viscous liquid or is surrounded by other chains. The optical anisotropy of a system appears to be directly connected with the mean orientation of segments and, thus, it provides the most direct observation of the relaxation of the segments, both in dilute and in concentrated solutions of polymers. The results of the theory for dilute solutions provide an instrument for the investigation of the structure and properties of a macromolecule. In application to very concentrated solutions, the optical anisotropy provides the important means for the investigation of slow relaxation processes. The evidence can be decisive for understanding the mechanism of the relaxation. 

The last polymer mentioned in Table 10.9 is poly(p-phenylene terephthalamide). This polymer is spun from its solution in pure sulphuric acid (100%), a dope that exhibits mesomorphic (=liquid crystalline) behaviour it is optically anisotropic and is nematic in character. A number of other polymers, containing rigid elements in the chain have melts of polymer liquid crystals, with a high birefringence and a non-linear optical behaviour in electric fields. 

Liquid crystals, due to the presence of the ordered domains, are optically anisotropic, i.e. birefringent. This can be easily verified by observing the liquid crystal, e.g. aramid and sulfuric acid solution, at rest between crossed polarizers. The parallel arrays of polymer chains in liquid crystalline state become even more ordered when these solutions are subjected to shear as, for example, in extruding through a spinneret hole. It is this inherent property of liquid crystal... 

Separation applications are also worthy of investigation if some entirely novel principle is involved. For example, Dunkin et al. have recently demonstrated [29] the ability to form an anisotropic imprinted polymer. This was achieved by selective blocking or filling of cavities employing polarised photo-chemsitry and a photochemically active template. An anisotropic polymer of this type might offer an opportunity to use polarised spectroscopy in an optical sensor device. 

An intriguing report by Steinke et al. [ 17] describes the creation of molecularly imprinted anisotropic polymer monoliths . Optically transparent blocks of MIP using either methacrylic acid (MAA) or 2-(acrylamido)-2-methylpropanesulphonic acid (AMPSA) as functional monomers and TRIM (trimethylolpropane trimethacrylate 2-ethyl-2-(hydroxymethyl)-l,3-propanediol trimethacrylate) as the cross-linker were synthesised using the photoactive template Michler s ketone... 

Crazing in anisotropic polymers has seldom been studied by means of optical interferometry. A block copolymer (Isoprene-Styrene) with a highly oriented fibrillar structure, and a PMMA oriented by means of drawing above Tg, have been studied so far. These results are reported in and The same subject has often been studied, however, using other experimental techniques... 

Kurihara, S., Matsumoto, K., and Nonaka, T. Optical shutter driven photochemieally from anisotropic polymer network containing liquid crystalline and azobenzene molecules. J. App/. P s. Lea. 73,160 (1998). 

The time of polymer solutions free relaxation (xdetermined experimentally, is the parameter, sensitive to macromolecules sizes and conformation change. It characterizes the rate, with which the optically anisotropic molecules primary orientation disappears, having established under... 

Smectic and nematic meso phases can be easily interconverted in the case of low-molar-mass compounds, and this gives melts of these compounds the characteristics of liquids. On the other hand, these melts are optically anisotropic because of their one-dimensional order, and so, have characteristic colors. Consequently, they are also known as liquid crystals. Solutions of rod-shaped macromolecules exhibit similar ordered behavior they are called tactoidal solutions. One-dimensional order can be induced in polymer melts by lowering the temperature below the melt or glass transition temperature, whereby the one-dimensional order is frozen in. The characteristic X-ray diagrams discussed above are then obtained. 

Amorphous, nonoriented polymers are not optically birefringent, because their optically anisotropic monomeric units are randomly ordered with respect to one another. A birefringence first occurs when the chains are oriented or placed under strain. Generally, the following relation holds ... 

Because of their structural features stiff polymers are often optically anisotropic, and hence their light scattering data have to be analyzed with care. 

This arises when there is a physical ordering of optically anisotropic elements e.g. chemical bonds) along some preferential direction— see Fig. 3. This can occur in polymers by aligning amorphous or crystalline chains as by an extension or drawing deformation. Orientation birefringence is the quantity that is most generally measured (or desired)... 

Spherulitic structure is common to many crystalline polymers. These spherulites are optically anisotropic aggregates of crystals arising from growth from a heterogeneous nucleus at their centre. In most cases, they grow until they fill the volume of the polymer and meet at planar surfaces so as to form polygonal structures (Fig. 37). The sizes of these spherulites... 

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