Coloration of polystyrene during UV irradiation

During UV (254 nm) irradiation in the presence of oxygen, the absorption spectrum of polystyrene is changed (Fig. 3.54) and the polymer sample becomes slightly yellow. 

Yellow colouring of polystyrene can result from the formation of many different chromophoric groups  

Polystyrene absorbing UV radiation can be excited to both singlet and triplet states. In the excited singlet state polystyrene easily forms an excimer, which may participate in the energy transfer process along the macromolecule. Transfer of energy has been shown to be operative from 

Formation of polyenes from unsaturated chain ends occurs through excitation by energy transfer from the phenyl groups in polystyrene. The excited unsaturated end-group then decomposes with evolution of hydrogen by a molecular mechanism to yield a conjugated diene  

Hydrogen evolution by such a mechanism has been observed during photolysis of alkyl substituted benzene [352]. Further energy transfer to the dienes results in the formation of trienes, and so on. Evidence for the formation of conjugated sequences of double bonds has been also obtained from fluorescence spectra. 

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