Polymers conjugation defects

The linear and nonlinear optical properties of one-dimensional conjugated polymers contain a wealth of information closely related to the structure and dynamics of the ir-electron distribution and to their interaction with the lattice distorsions. The existing values of the nonlinear susceptibilities indicate that these materials are strong candidates for nonlinear optical devices in different applications. However their time response may be limited by the diffusion time of intrinsic conjugation defects and the electron-phonon coupling. Since these defects arise from competition of resonant chemical structures the possible remedy is to control this competition without affecting the delocalization. The understanding of the polymerisation process is consequently essential. 

The reactions and their mechanisms have been discussed and reviewed in detail. For the application in the field of electronics, it is desirable that the polymer formed is perfectly r-conjugated. Defects are responsible for a reduced performance of light-emitting devices. 

Upon doping, a charge-transfer bond is formed between the polymer chain and the p or n dopant, and the electronic structure of the chain is strongly perturbed, with the generation of conjugational defects. The problem, whether doped sites are randomly distributed within the polymer chain, or whether they combine into T-D clusters", is yet unsolved experimentally. 

D. J. Klein and A. T. Balaban, Clarology for conjugated carbon nanostructures Molecules, polymers, graphene, defected graphene, fractal henzenoids, fullerenes, nanotuhes, nanocones, nanotori, etc.. Open Org. Chem. J. (Suppl. 1-M3) (2011) 27-61. 

To explain the unusually high conductivity of these polyblends of polyaniline Min et al. [112,113] have suggested secondary doping, which has been defined as an inert substance (vapour or liquid) which promotes the conductivity of an already doped form of the conducting polymers. These may also induce molecular conformational changes to reduce conjugation defects resulting in increased intramolecular conductivity and may also enhance crystallinity. It has been observed that in the absence of w-cresol the conductivity of camphor sulphonic acid-doped polyaniline film is... 

Parameters such as, synthesis medium (HCl, camphor sulfonic add (CSA) or dodecylbenzene sulfonic acid (DBSA)), molar ratio aniline/oxidant, method for oxidant addition, temperature, polymerization duration, dedoping conditions will have effects on polymer properties in terms of yield, chain length, effective conjugation, defects rate, and electrical properties. Temperature was kept to 0 C, -30 C or -40 C and reactions were stopped after various durations in order to control the molecular weight of samples. Polymerization durations were varied from 1 hour to 5 days and depended on the reaction temperature. The inherent viscosities of the polyanilines (Pani) were determined at 25 °C in 0.1%w solutions in concentrated sulfuric acid (95 %), using an Ubbelohde viscometer. For instance, high viscosity samples (1.4 dl/g) were obtained after 3 days at -40 °C vsMe low viscosity samples (0.6 dl/g) were obtained after 1 hour at 0 A relationship was found between polymerization duration and inherent viscosity for polymers synthesized at low temperature (- 40 °C). Inherent viscosity increases from 0.6 to 1.4 when duration of polymerization increases from one to five days. In the case of synthesis at 0°C no correlation was obtained between duration of polymerization and inherent viscosity. A careful control of other parameters (synthesis medium, molar ratio aniline/oxidant, method for oxidant addition) have permitted to get samples with inlierent viscosities ranging from 0.55 to 2.1 dl/g in a reproducible way. 

We can expect that in these highly conducting polymers electrons are much less localized and that they will reflect certain features of inorganic metals. But the chain-like structure of the conjugated polymers will give rise to a very anisotropic behaviour and some aspects of one-dimensionality will show up, such as the metal-insulator transition (Peierls-transition / /). The most exciting speculation is probably that on the existence of mobile conjugation defects, which can be described as non-linear excitations and are often referred to as solitons /5/ (Fig. U). These defects share many properties with non-linear excitations in other fields of physics and offer an interdisciplinary connection all the way from chemistry to elementary particle physics and field theory /6/. 

For charge transport along chains with conjugated double bonds the mechanism of soliton motion has been proposed (this would be a theory for R-j in the terminology of the previous section). If the conjugation defect of Fig. h is called soliton , this term is used to stress two features of this defect it does not disperse while it moves (just as solitary water waves do not disperse. This is where the name comes from) and it has certain symmetry properties. If most polymer chains in today s polyacetylene films are very short, solitons will not be able to move very far since they are confined to the polyene chains. Therefore the non-dispersivity will be hard to test. In addition solitons will not be important for electrical conductivity under these circumstances. 

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