Rod structure

Several groups have studied introduction of phenylene ethynylene units into PPV backbones. The first material of this type, copolymer 163, was reported by Bunz and coworkers [193] (Chart 2.36). The material displayed blue luminescence in solution (A ax = 460 nm), but due to the polymer s rigid-rod structure, very strong aggregation in the solid state gave rise to... 

FIGURE 5.7 Schematic Representation of typical, (partially) electroluminescent LC polymer architectures. (a) Rodlike structure, (b) Hairy-rod structure, (c) Combined main-chain-side-chain system, (d) Semiflexible segmented structure, (e) Semiflexible segmented structure with disklike mesogen. (After Weder, C. and Smith, P., Main-chain liquid-crystalline polymers for optical and electronic devices, in Encyclopedia of Materials Science and Technology, Buschow, K.H., Cahn, R.W., Flemings, M.C., Ilschner, B., Kramer, E.J., and Mahajan, S., Eds., Elsevier Science, New York, 2001.)... 

Polymerization of terephthalic acid with 4,6-diamino-l,3-benzenediol via oxazole formation (Eq. 2-219) proceeds with a sharp and continuous decrease in reaction rate with increasing polymer molecular weight [Cotts and Berry, 1981]. Reaction becomes progressively more diffusion-controlled with increasing molecular size due to the increasing rigid-rod structure of the growing polymer. 

Figure 11.29 (a) Structure of Newkome s two-directional arboral molecule, (b) Negatively stained transmission electron micrograph of the arborol gel. (c) Schematic representation of the formation of a long fibrous rod structure via orthogonal stacking of the arboral molecules. Reprinted from Newkome et al. (1990). Copyright 1990 American Chemical Society. 

Figure 3.18. The process of changes in morphology due to interface instability. (a)-(e) Starting from a smooth interface, it is seen that the morphology changes to a cusp array, to a rod structure, to a dendritic structure.

The CTE of polyimides decreases with the introduction of a rigid-rod structure into polyimide molecules, and is also controlled by changing the composition of high- and low-thermal-expansion polyimides. Figure 15.7 shows... 

In many cases the disorder introduced into rigid-rod structures in order to depress the crystalline melting point to a temperature below the degradation temperature is such that the polymer can crystallise only to a small extent or not at all. In such cases, the polymer exhibits a frozen nematic structure at temperatures below the glass transition, with a typical micaceous sheen. 

Figure 8.6 Morphological variation of DNA. Transmission electron micrographs of T4 DNA are shown, except for the rod structure where A DNA is used. The scale bar is 100 nm. The giant toroid is formed under a high concentration of spermidine in the buffer solutions with rather high salt (Yoshikawa et al, 1999). The segregated structure is generated by PEG-A (polyethylene glycol with pendent amino groups Yoshikawa et al., 1997b). Further explanation on the experimental conditions is available in the text.

It is also known that a rod structure is generated in compact DNA under some conditions. However, it was not clear whether the rod-like structure is a thermodynamically stable state or a kinetically trapped metastable state (Chattoraj et al., 1978 Eickbush and Moudrianakis, 1978 Grosberg, 1979 Arscott et al., 1990 Plum etal., 1990 Perales et al., 1994). 

Morphological variabon of DNA. Transmission electron micrographs of T4 DNA are shown, 133 except for the rod structure where X DNA is used... 

One might think that quadrupole mass spectrometers would be less susceptible to this limitation because of their much larger entrance apertures. Unfortunately, this is only marginally true, and only for ions of low mass. This is because the electrical entrance aperture of the quadrupole Is usually considerably smaller than the physical entrance aperture. By "electrical entrance aperture" we mean that area of the physical entrance aperature through which ions must travel if they are to be transmitted through the quadrupole rod structure. The radius of this aperture, r, is small and mass-dependent as seen by the following equation... 

Wire, Bars, Rods, Structural Shapes, Rails, Pipe, Tubing, Blooms, and Billets... 

The exponent a in the intrinsic viscosity-molecular weight relationship ([rj] = K.M ) of a polymer is associated with the expansion of the polymer in solution, and hence with the conformation and stiffness of the polymer (Table 24). The a values of tobacco mosaic virus, Kevlar and helical poly(a-amino acids) are close to 2, which means that they take rigid-rod structures. The a values of vinyl polymers are usually 0.5-0.8, indicating randomly coiled structures. In contrast, the a values of substituted polyacetylenes are all about unity. This result indicates that these polymers are taking more expanded conformations than do vinyl polymers. This is atrributed to their polymer-chain stiffness stemming from both the alternating double bonds and the presence of bulky substituents. 

The liquid crystal state (LCS) shows order in one or two dimensions it lacks the three-dimensional long-range order of the crystalline state. LCS has characteristics intermediate between those of the crystalline and the disordered amorphous states. These phases are called liquid crystals because many of them can flow like ordinary liquids but they display-birefringence and other properties characteristic of crystalline soHds. In liquid crystal phases the molecules can move but the orientational order is conserved in at least ne direction. The LCS can be displayed by small molecules and by polymersj but in both cases a characteristic chemical structure is needed. The existence of the liquid crystal state is related to the molecular asymmetry and the presence of strong anisotropic intermolecular interactions (19-21). Thus, molecules with a rigid rod structure can form highly ordered... 

The experimental data for intermediate phases is less conclusive. Luzzati et al. proposed a planar rod structure for the R phase in lecithin-water mixtures... 

An intermediate phase of tetragonal syiiunetry - the T phase - has also been detected in a number of systems. A rod structure related to a square mesh surface was foimd to agree well with X-ray and NMR data on a perfluorinated surfactant-water mixture forming the T phase [22], [34]. These examples demonstrate that surfactant or lipid monolayers lining mesh surfaces as well and bilayers wrapped onto three-periodic minimal surfaces (IPMS) are indeed found in these self-assembled systems. 

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