The formation of chromophore groups

Various chromophore groups, colouring the polymer yellow to brown, are formed during the oxidative degradation of polymers. The production of such groups was identified in the case of polystyrene [2, 255], but their structure is not yet entirely elucidated. Achhammer et al. [2] suggested that the colour of oxidized polystyrene is due to quinomethane formed by the reaction of polystyryl radicals  

Wall et al. [648—650] reported that the chromophores are related to benzalacetophenone the precursor of which is the hydroperoxide  

Grassie and Weir [25] and Jellinek and Lipovac [313] have suggested that a conjugated structure in polystyrene is responsible for its colour namely, 

Polyacrylonitrile presents a particularly interesting case of the formation of many chromophore groups during the thermal oxidation [Refs. 99—101, 153, 234, 343, 475, 582]. The heating of polyacrylonitrile in air does not result in its degradation but in the formation of the so-called ladder structure involving nitrone groups [101, 234]  

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The formation of chromophoric groups such as carbonyls (C=0) and unsaturated carbon bonds (C=C and C=C) in the backbone of a polymer results in colour formation in white and transparent plastics and discolouration of coloured materials. Chromophoric groups can be formed when polymers undergo oxidation (see section 6.2.3) or hydrolysis (see section 6.2.4). 

Chung [34] concluded that the semiconducting properties of a metal species influence discoloration. In contrast to metals belonging to the insulator group, metals belonging to the semiconductor group promote yellowing, perhaps due to catalysis of the polymerization of vinyl esters. The formation of chromophores is enhanced when the metal has a variable valency with a reduction potential near to zero. 

The lignin model compounds selected for this study possess two functional groups, a phenolic hydroxyl group, and an aliphatic hydroxyl group located on the side chain para to the phenolic hydroxyl group. Experiments were performed to determine the effect of blocking each of these two sites relative to the formation of chromophoric structures. 

The first law of photochemistry [the Grotthus-Drapper principle (30)] states that for a photochemical reaction to occur, some component of the system must first absorb light. The second law of photochemistry [the Stark-Einstein principle (3J)j states that a molecule can only absorb one quantum of radiation. The absorbed energy causes the dissociation of bonds in the molecules of the wood constituents. This homolytic process produces free radicals as the primary photochemical products. This event, with or without the participation of oxygen and water, can lead to depolymerization and to formation of chromophoric groups such as carbonyls, carboxyls, qui-nones, peroxides, hydroperoxides, and conjugated double bonds. 

Surprisingly, some polymers which have no chromophores in their structure such as PVC, polyethylene, poly (methyl methacrylate), nylon and polystyrene are degraded by exposure to radiation with wavelengths around 200nm, undergoing discolouration and weakening. This may be attributed to the formation of chromophoric carbonyl groups, by oxidation. Impurities or additives in plastics may also contribute chromophores. 

Scheme 11.5 Formation of block and graft copolymers following the photodissociation of chromophoric groups. For the sake of simplicity, chain-termination reactions are not included.

For the analysis of aliphatic anionic surfactants by HPLC, other detection systems than UV or fluorescence detection have to be used because of the lack of chromophoric groups. Refractive index detection and conductivity detection provide a solution for this t5rpe of anionic smfactants but their detection limits are rather high and gradient elution is not usually possible. Another possibihty is the application of indirect photometric detection, which is based on the formation of ion pairs between UV-active cationic compoimds, such as N-methylpyridinium chloride, used as mobile phase additives and the anionic surfactants followed by UV detection [60]. Gradient elution with indirect photometric detection is possible in principle but the detection limits increase considerably [61]. 

This effect is especially well noticeable when comparing samples from a mixture of VAVAC and PHB (20%) and individual VAVAC (Fig. 1, curves 3 and 1). Apparently, this is due to a higher rate of formation of chromophore groups in the presence of PHB. A higher value of a stationary optical density of the sample of the polymers mixture is also due to this reason. 

Self-assembly of functionalized carboxylate-core dendrons around Er +, Tb +, or Eu + ions leads to the formation of dendrimers [19]. Experiments carried out in toluene solution showed that UV excitation of the chromophoric groups contained in the branches caused the sensitized emission of the lanthanide ion, presumably by an energy transfer Forster mechanism. The much lower sensitization effect found for Eu + compared with Tb + was ascribed to a weaker spectral overlap, but it could be related to the fact that Eu + can quench the donor excited state by electron transfer [20]. 

Another common hydrogen transfer reaction of carbonyl triplet is the photoenolization of the c-methylbenzoyl chromophore, illustrated in reaction 3 for the syn conformer of c-methylaceto-phenone (j+). Reaction 3 can act as a very efficient energy sink, and a number of properties of this group led us to believe that this process could be used to reduce photodegradation i.e. the excellent absorption characteristics of the chromophore, the short triplet lifetime and the fact that the disappearance of the carbonyl triplet does not take place at the expense of the formation of another excited state. 

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