Molecular weight changes during the photooxidation of isotactic polypropylene

The photooxidation of unstabilised isotactic polypropylene (PP) samples ((M) over bar(n) similar to 53 kg mol(-1), (M) over bar(w) = 274 kg mol(-1)) was studied under accelerated ageing conditions (Xenotest, lambda greater than or equal to 300 nm; T = 70 degrees C black panel) by infrared (IR) spectrophotometry and size exclusion chromatography. The measurements were made on microtome sections parallel to the exposed surface, in order to establish the degradation profiles due to diffusion control of oxidation kinetics. From (M) over bar(n) and (M) over bar(w) values, the numbers n of chain scissions and x of crosslinks were calculated. From IR spectra, the concentrations of carbonyls [CO] absorbing at 1715 cm(-1) and of hydroxyls [OH] absorbing at 3400 cm(-1) were determined. The time variations of these values are discussed in terms of oxidation mechanisms. The variation of polydispersity is discussed in terms of reaction homogeneity.