Anisotropic polymers

In packed beds of particles possessing small pores, dilute aqueous solutions of hydroly2ed polyacrylamide will sometimes exhibit dilatant behavior iastead of the usual shear thinning behavior seen ia simple shear or Couette flow. In elongational flow, such as flow through porous sandstone, flow resistance can iacrease with flow rate due to iacreases ia elongational viscosity and normal stress differences. The iacrease ia normal stress differences with shear rate is typical of isotropic polymer solutions. Normal stress differences of anisotropic polymers, such as xanthan ia water, are shear rate iadependent (25,26). 

Examples of polymers which form anisotropic polymer melts iaclude petroleum pitches, polyesters, polyethers, polyphosphaziaes, a-poly- -xyljlene, and polysdoxanes. Synthesis goals iaclude the iacorporation of a Hquid crystal-like entity iato the maia chaia of the polymer to iacrease the strength and thermal stabiHty of the materials that are formed from the Hquid crystal precursor, the locking ia of Hquid crystalline properties of the fluid iato the soHd phase, and the production of extended chain polymers that are soluble ia organic solvents rather than sulfuric acid. 

Examples. 2D SAXS/WAXS experiments on highly anisotropic polymer materials during melting and crystallization can be used to visualize and understand the evolution of nanostructure [56,57], Transformations of biopolymers in solution, e.g., virus crystallization can be studied in situ [58], It is possible to study solidification mechanisms of spider silk [59], or the self-assembly of micelles on a time-scale of milliseconds [60],... 

Orientation effects are strongly coupled to nonlinear behavior, discussed in Section V, and the stress-strain response discussed in Chapter 5, Orientation makes an initially isotropic polymer anisotropic so that five or nine modulus/compliance values arc required to describe the linear response instead of two, as discussed in Chapter 2. For an initially anisotropic polymer the various modulus/compliance components can be altered by the orientation. It may not be necessary to know all components for an... 

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...

Other difficulties are owing to the influence of the solvent. With stiff and bulky chains the so-called micro-form-effect becomes of importance, when the refractive index increment differs considerably from zero (7). In this case the random link approximately acts like a cylinder of length A and with a refractive index different from that of the solvent. Another effect occurs in good solvents which consist of anisotropic molecules. These molecules become oriented along the polymer chain, considerably contributing to its anisotropy [Frisman, Dadivanyan and Dyuzhev (752)]. In this way, the determination of the eigen anisotropy of weekly anisotropic polymer chains becomes rather doubtful. 

G. W. Calundann and M. Jaffe, "Anisotropic Polymers—Their Synthesis and Properties," Proceedings of the Robert A Welch Conference on Chemical Research XXVI, Nov. 1982, Houston, Tex., p. 247. 

The only flat-panel technology with the potential to pose a realistic challenge to LCDs in the medium term is OLED technology. The first factories for OLEDs using either small molecules or polymers have started production, if in relatively low volumes, see Table 1.1. Higher production volumes can be confidently expected as the market acceptability and awareness of the capability of OLEDs increases. A combination of the modulation of plane polarised light provided by an OLED back-light by an LCD to create a hybrid OLED-LCD may become a major commercial product in the near future. Oriented main-chain polymers or anisotropic polymer networks in the nematic liquid crystal-... 

The order must then be frozen in before crystallisation occurs, since this would result in the formation of grain boundaries and a reduction in transport or emission efficiency. Device breakdown is also a possibility. The most efficient way to fix the liquid crystalline order is the formation of anisotropic networks by the polymerisation of reactive mesogens in the liquid crystalline state.Anisotropic polymer networks formed from the thermal or photoinitiated polymerisation of polymerisable, so-called photoreactive, liquid crystalline monomers have been used in a wide variety of electrooptic applications, see Chapter This is a more attractive approach than cross-linking... 

Anisotropic polymer filaments could be produced by in-situ photopolymerization of oriented acrylate monomers. Ordering of the monomers was achieved by an elongational flow prior to the polymerization process. The produced polymers showed a high elastic modulus and a low thermal expansion coefficient in the direction of the orientation. 

Figure 4.28. Formation of an anisotropic polymer in the interlayer spaces of lipid bUayers...

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). 

High-resolution proton DQ MAS NMR is used as a new technique that is capable of revealing complex motional processes in entangled polymer melts. Theoretical analysis shows the connection of quantities relating anisotropic polymer dynamics to data obtained from our DQ-MAS NMR experiment. With this technique, dynamic chain ordering as well as scaling laws consistent with the reptation model was previously observed for polybutadiene (PB). 

Thermotropic LCPs have low melt viscosities because of the ready alignment of the molecules under flow and they are thus suitable for use in moulding, where they can fill complex moulds well. One problem, however, is that if the polymer is introduced into the mould through more than one gate the separate flows of anisotropic polymer might not interpenetrate when they meet, which can give rise to weak weld lines . This difficulty can be overcome satisfactorily by proper design of the moulds. 

The anisotropy of the stretched polyacetylene permits the construction of a polarizer. When polyacetylene (PA) strips are laid across each other, polarized light is extinguished in the region of overlap ABCD of the anisotropic polymer (Fig. 2). 

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