Photo-degradation of poly(neopentyl isophthalate). Part II: Mechanism of cross-linking

The mechanism of cross-linking of poly(neopentyl isophthalate) (PNI) by photo-degradation in nitrogen atmosphere was investigated. The exposure of PNI to UV light resulted in gel (insoluble material) formation. The gel material was collected and the morphology of the gel material was characterized with SEM. The gel has the highest density near the coating surface. To obtain information on the cross-linking at a molecular level the gel was decomposed by methanolysis and the decomposition products were analysed with LC-MS. Besides the expected “grafting” types of cross-links (phenyl-to-phenyl coupling) also “chain coupling” types of cross-links were detected (recombination of neopentyl glycol based moieties). Analysis of samples that had been exposed to UV light in air indicates that the phenyl-to-phenyl coupling also occurs in the presence of oxygen.