MOLECULAR-WEIGHT CHANGES DURING THE PHOTOOXIDATION OF ISOTACTIC POLYPROPYLENE

The photooxidation of unstabilised isotactic polypropylene (PP) sample s ((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, la mbda greater than or equal to 300 nm; T = 70 degrees C black panel) by infrared (IR) spectrophotometry and size exclusion chromatography. Th e measurements were made on microtome sections parallel to the exposed surface, in order to establish the degradation profiles due to diffus ion control of oxidation kinetics. From (M) over bar(n) and (M) over b ar(w) values, the numbers n of chain scissions and x of crosslinks wer e calculated. From IR spectra, the concentrations of carbonyls [CO] ab sorbing 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 disc ussed in terms of reaction homogeneity.