Defects in a polymer

Fig. 10.14 Non-emissive defects in a polymer LED visual appearance (a), gas penetration at grain boundaries (b) and dust particles (c) in the cathode. Reproduced with permission of Wiley-VCH from Kim et al. (2002).

The presence of a small number of defects in a polymer chain can have a large influence on a polymer s properties. Two important aspects of polymer chemistry involve detection and structure identification of chain branching and junctions between segments in block copolymers, as low levels of these structures can have a large effect on a polymer s crystallinity. 

The following schematic representation in Fig. 19 shows the application of the SVET for two types of defects in a polymer coating on galvanized steel. 

In Fig. 19(a), a defect in a polymer coating on a metal substrate is shown. The anodic dissolution in the defect leads to a positive current peak. The intact polymer-coated area shows zero current. A second important situation, a cut edge of coil-coated galvanized steel, is shown in Fig. 19(b). The zinc dissolution leads to a positive peak while the area of oxygen reduction on the cathodically protected steel surface is characterized by a broad negative current peak. The activation, distribution, and passivation of these local anodes and cathodes can now be studied by the SVET as a function of coating compositions. 

The AFM probing of polymeric surfaces can, besides imaging the surface, also produce a number of anomalies. Surface contaminants, such as those caused by adsorbed polar molecules, were found to cause significant perturbations on the images produced. From the calculations, it appears that when the AFM tip encounters a polar defect, it is initially attracted (phenomenon) and becomes trapped for a short period of time. This type of stick-slip phenomenon leads to an enhanced frictional energy dissipation which, in turn, causes an increase in the surface temperature of both the AFM tip and the polymer surface. The increase in temperature can subsequently induce rotational defects in a polymer chain and ultimately cause deformations on a long time-scale. 

For sufficiently high concentrations, radical defects in a polymer are easily detectable, as demonstrated for electron-irradiated Teflon FEP (Fig. The... 

Crazing. Defects in a polymer which appear to be fine cracks and may extend in a network on or under the surface or through a layer of material. They are not cracks, but areas of lower-density, oriented polymer. This phenomenon usually occurs in the presence of an organic liquid or vapor, with or without the application of mechanical stress. 

FIGURE 6.12 Bottom A molecular map of a defect in a polymer film. This map is a plot of the ahsorhance at 1730 cm versus x-axis and y-axis position. Top A picture of the polymer defect from which the molecular map was obtained. 

Multi-walled CNTs (MWCNTs) are produced by arc discharge between graphite electrodes but other carbonaceous materials are always formed simultaneously. The main by-product, nanoparticles, can be removed utilizing the difference in oxidation reaction rates between CNTs and nanoparticles [9]. Then, it was reported that CNTs can be aligned by dispersion in a polymer resin matrix [10]. However, the parameters of CNTs are uncontrollable, such as the diameter, length, chirality and so on, at present. Furthermore, although the CNTs are observed like cylinders by transmission electron microscopy (TEM), some reports have pointed out the possibility of non-cylindrical structures and the existence of defects [11-14]. 

More recently, it was shown by List et al. [293-296] and later by Moses et al. [246] that the green emission of the PFs is due to fluoren-9-one defects in the polymer chain. This was confirmed by comparison of PL films annealed in an inert atmosphere and in air a progressive additional band in the green region was observed on annealing in air (Figure 2.12) [246],... 

The use of so-called toners to improve the color has been described above. It should be remarked that all measures to overcome defects of a polymer caused by disregarding the basic principles of chemistry are rarely of durable success, because any additive will complicate or affect the polymeric system in an undesired way. 

Occasional defects in the polymer chain remain isolated. When propylene reacts from the opposite prochiral face bringing about a syndiotactic error (Scheme 13.7, a), the original chirality center preceding the error continues to be formed. The same is true for a secondary defect or type 2-1 insertion (Scheme 13.7, b) which is a head-to-head addition and creates a CH2-CH2 sequence. If the chirality were controlled by the growing chain end, an error would be perpetuated, giving rise to a polymer block with altered chirality. Instead, isotacticity is maintained in both cases. 

Before describing 129Xe NMR experiments on polymers and polymer composites, it is worthwhile to show with a few selected examples, some of the general aspects of 129Xe NMR of polymers. The first question that needs to be addressed is where sorbed Xe atoms are located in a polymer material, or rather where Xe atoms are not located. With a diameter of 0.44 nm the Xe atom is clearly larger than the interchain distance for most crystalline polymers. In general it means, that if Xe atoms are found in crystalline domains, they must reside in areas which contain defects. In the examples studied, no evidence of Xe in crystalline domains of polymers was ever found. 

Selectivity in the transformation of hydrocarbon towards a specific hydroperoxide is usually low which is due to the parallel decomposition of the latter and to side reactions of peroxyl radicals with oxidation products. In the case of polymers this is moreover enhanced by the presence of reactive defect structures in a polymer, which are the sites of the oxygen attack in the first stages of the reaction. 

When interest was rekindled in the 1980s, the method commonly used to synthesise PAni was the oxidative coupling of aniline with ammonium persulphate in aqueous HC1. This produces partially protonated EM salt which can be deprotonated with ammonia to form EM base. The molecular mass can then be determined in dilute LiCl solution in N-methylpyrrolidone (NMP) typical values correspond to 160 of the repeat units shown in Fig. 9.6. The reaction shows the characteristics of a living polymerisation, since the molecular mass increases if further monomer and oxidant are added to the reaction mixture, when the chain length increases to a maximum of about 240 repeat units. NMR studies of the LM form show about 5% defects in the polymer backbones. When the reaction is performed at 248 K the polymer is produced in nearly 100% yield, with higher molecular mass, typically over 500 repeat units, and greatly reduced defect content (Adams et ai, 1996). Other oxidising... 

The synthetic organic chemist, on the other hand, who is used to deal with structurally defined, but small molecules should be aware that avoiding structural defects in conjugated polymers is by no means a trivial task. A side reaction in any of the polymer forming steps does not lead to a separable side product, but produces an inherent structural defect of the macromolecule. 

The sorption of carbon dioxide can result in swelling and/or dissolution of a polymer. The extent of either/or both swelling and dissolution depends on the solubility of carbon dioxide in the polymer and/or the solubility of the polymers in carbon dioxide. When the system pressure is reduced, absorbed carbon dioxide may nucleate into bubbles and cause the formation of foam, or small defects in the polymer structure that may significantly alter the mechanical properties of the material. In addition, carbon dioxide can extract the plasticizer, if present, in the polymer and thus cause embrittlement. In this chapter, we address such solubility issues. 

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