Photo-degradation of synthetic and natural polyisoprenes at specific UV radiations

Synthetic and natural polyisoprenes (PI) films were exposed to different UV light sources and then analyzed by Fourier transform infrared (FTIR) and UV spectroscopy, before and after exposure to the UV source. Films of PI standards were separately irradiated by UV sources of different wavelengths: 253, 300, and 350 nm. The photo-oxidation was monitored by looking at the dependence of the IR absorbances of the CC, CO, and O-H stretching and the C-H wagging modes of cis- and trans-PI. Irradiation at 253 nm leads to cross-linking and polymer reticulation avoiding the complete oxidation of the polymer film. Under 300 nm irradiation, the chain-scission reactions occur preferentially leading to the complete oxidation of the film. Irradiation at 350 nm leads to photo-oxidation only for exposure times larger than 12 h. The natural PI rubbers, Hevea brasilienses (HB), Hancornia speciosa (HS), and Mimusops globosa (MG) were characterized by DSC and FTIR as being high cis-1,4-PI (HB and HS) and high α-trans-1,4-PI (MG). The photo-oxidation of NR presented different kinetics, with induction times smaller than those observed for the cis- and trans-PI standards. The faster photo-degradation process observed for NR films was assigned to the presence of impurities characterized by the presence of O-H and CO str bands in the FTIR spectra, even before UV irradiation.