Structural effects of phenols on the photooxidative degradation of polybenzoxazines

The photooxidative behavior of a series of polybenzoxazines based on various phenols, upon exposure to ultraviolet (UV) radiation (>290 nm), is examined. Substituted benzoquinone has been demonstrated in a previous study to be generated in cured 6,6′-bis(2,3-dihydro-3-methyl-4H-1,3-benzoxazinyl)isopropylidene (BA-m) resin when exposed to UV radiation. In the present work, a comparison study is conducted on polybenzoxazines, all of which contained the same amine (methylamine), but with phenolic units that had different substituents on the para-positions of the phenyl rings. BA-m is shown to have the highest degree of substituted benzoquinone formation followed by those polymers derived from hydroquinone, 4,4′-(hexafluoroisopropylidene)diphenol, 4,4′-thiodiphenol, 4,4′-dihydroxybenzophenone, p-cresol and phenol. The nature of the para-position phenolic substituent appears to have an impact on the oxidation process affecting the degrees of substituted benzoquinone formation. Reasons for the differing rates of oxidation are discussed, as well as secondary reactions that occur as a result of photooxidation.