Chromophore impurities

Y Impurity chromophore in 1 excited singlet state, which lies at lower energy than M ... 

If any ground-state interactions occur among the chromophores of the polymer, the UV absorbance will be altered. However, such alterations are more likely to be caused by impurity chromophores on the polymer chain. Since it is extremely difficult to obtain high polymer which is free from defects and impurities, another standard for the polymer UV absorbance is desired. 

These molecules have no absorption in the near UV, so they cannot act as internal filters. There is no evidence that they could act as quenchers of the excited impurity chromophores in common polymers, and it has been mentioned already that such quenching action would be, in any case, unlikely to be important in relatively rigid systems such as polymers. This remark does not apply to free radical scavengers, because a free radical has an unlimited lifetime since it can disappear only through a chemical reaction with another open-shell molecule. 

None of these products contribute towards the initiation of photooxidation of the polymer other than having some u.v. antioxidant action. On long-wavelength irradiation (365 nm) photoinitiation occurs via impurity chromophore whereas with light of wavelengths shorter than 300 nm direct excitation of the diphenylcarbonate unit occurs. 

The fluorescence and phosphorescence excitation and emission spectra of commercial polypropylene and poly(4-methylpent-l-ene) are examined using a fully compensated spectrofluorimeter. The excitation spectra of the polymers are compared with the absorption spectra of model chromo-phores of those believed to be present in the polymers. The fluorescence emission is associated primarily with the presence of enone and the phosphorescence is associated with dienone impurity chromophoric units. Bromination of cold hexane extracts of the polymers reduces significantly the intensity of the fluorescence confirming the presence of ethylenic unsaturation. The behavior of the luminescent enone and dienone groups during irradiation under sunlight-simulated conditions is examined also. Possible mechanisms for the participation of these chromophoric units in the photooxidation of the polymers are discussed. 

Table 7.1 Impurity chromophores commonly contained in commercial polyalkenes or poly(vinyl chloride)s.

Most of the radicals generated by photoreactions of impurity chromophores can abstract hydrogen atoms from the surrounding polymer. This applies especially to hydroxyl and chlorine radicals. 

Scheme 7.4 Generation of free radicals by photoreactions of impurity chromophores and ensuing hydrogen abstraction from the polymer.

It is generally accepted that the elimination of HCl occurs by way of a free radical chain reaction. As shown in the lower part of Scheme 7.16, chlorine atoms function as propagating species. Likely initiation mechanisms involving some of the impurity chromophores listed in Table 7.1 are presented in the upper part of Scheme 7.16. 

The solar light-induced dehydrochlorination of PVC plasticized with phtha-lates has been reported to be sensitized by the plasticizer [38, 39]. In marked contrast, more recent work has revealed a weak protective effect of phthalates with respect to C-Cl bond cleavage and polyene formation. Phthalates are likely to quench electronically excited states of impurity chromophores [40]. 

The behavior depicted in Fig. 7.2 is observed with many polymers upon exposure to sunlight, including with commercial polyalkenes such as polyethylene and polypropylene. In the latter cases, impurity chromophores act as initiators of the autoxidation process (see Scheme 7.4 in Section 7.1.3). Important elementary reactions determining the autoxidation process are described in the following. Free radicals Rx formed during the initiation phase abstract hydrogen atoms from macromolecules PH, thus forming macroradicals P [Eq. (7-11)]. 

Polymers with Impurity Chromophores. Although all the polymers with intrinsic chromophores are inherently sensitive to degradation, at least some are regarded as relatively stable materials, able to resist solar exposure for long... 

Impurity chromophores which do not form part of the molecular structure of the polymer. These impurities can be low molecular external compounds and/or internal bonds to polymer macromolecules (in-chain and/or end-chain) ... 

All the TLS existing in a polymer with impurity centres can be divided into two groups extrinsic TLS and intrinsic TLS. The intrinsic TLS are spread throughout the whole bulk of a solvent. The distance between intrinsic TLS and an impurity chromophore, a rule, is large. Chromophores do not affect these TLS and they determine bulk properties of a polymer at low temperature. These very TLS are responsible for the low temperature anomalies in heat capacity. 

Commercial PVC products commonly contain plasticizers (up to 40%) such as phthalates or mellitates. In the past, phthalates were found to exert a weak protective effect with respect to C—Cl bond cleavage and polyene formation by quenching the electronically excited states of impurity chromophores [101]. 

Polymers with Impurity Chromophores. Although all the polymers with intrinsic chromophores are inherently sensitive to degradation, at least some are regarded as relatively stable materials, able to resist solar exposure for long periods. In contrast, polymers such as the polyolefins and the hydrocarbon rubbers have remarkably poor lifetimes when exposed to solar light. Unstabilized PP film, for example, has a lifetime of only a few weeks in outdoor exposure. 

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