Structural effects of amines on the photooxidative degradation of polybenzoxazines

The photooxidative behavior of a series of polybenzoxazines based on various amines with the same phenolic unit (bisphenol-A), upon exposure to ultraviolet (UV) radiation (λ>290 nm), is examined. BA-m polymer (containing methylamine) is shown to have the highest degree of substituted benzoquinone formation followed by those polymers derived from ethylamine, propylamine, cyclohexylamine, aniline, and meta-toluidene, the latter five of which had the same rate and degree of photooxidation. Fourier transform infrared spectroscopy studies suggest that the intramolecular hydrogen bonding interaction between the hydroxyl group of the phenolic unit and the nitrogen of the amine can play a key role in ionizing the oxygen of the phenolic hydroxyl group when conditions are advantageous, thus leading to high relative degrees of photooxidative degradation. In addition, model compound studies demonstrate that various degrees of non-intramolecularly bound hydrogen density around the phenolic hydroxyl group may impact upon the efficiency of the intramolecular interactions. Other model compound studies suggest that the size of the amine functionality is another critical factor influencing the degrees of photooxidation. A mechanism of UV-induced molecular modification involving contributions from the amine functionality is proposed.