Free-volume hole size evaluated by positron annihilation lifetime spectroscopy in the amorphous part of poly(ethylene terephthalate) degraded by a weathering test

The degradation of poly(ethylene terephthalate) (PET) was investigated through a weathering test by using a xenon weather meter. The degraded structure was evaluated by means of gel permeation chromatography (GPC), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and positron annihilation lifetime spectroscopy (PALS). The GPC results indicated a decrease of number average molecular weight and a broadening of polydispersity with the degradation of PET, probably due to chain scission and some networking reactions. The DSC results indicated that the crystallinity index was changed through the weathering test. However, the change of crystallinity was poorly correlated with the progress of degradation. In the Tan δ measurement by DMA, suppression of molecular mobility during the degradation was implied by the increase in Tg with increasing exposure time. The PALS results revealed that the free-volume hole size decreased with increasing exposure time. The decrease in the free-volume hole size can be attributed to the interaction between functional groups produced by hydrolysis or photochemical chain scission. The change of properties and structure in the amorphous part of PET was found to be correlated with the progress of the degradation in the weathering test.