Weight loss mechanism in the photooxidation of polypropylene

The photooxidation (fluorescent lamp; lambda(max) = 365 nm; intensity = 7 x 10(18) photons cm(-2) h(-1); temperature: 40 degrees C) of photochemically unstabilised polypropylene films (thickness 100 microns) was studied by gravimetric and LR spectrophotometric methods. After a pseudo-induction period, the sample mass increases in a sigmoidal way, reaches a maximum and then decreases abruptly. From IR spectrophotometric data, we tried to estimate the weight gain due to oxygen incorporation (hydroperoxides and carbonyl groups). From the weight balance, the weight loss kinetics were established. It was first supposed that weight loss results from a random chain scission with evaporation of the small fragments. A simple kinetic model reveals, however, that experimental data are inconsistent with this hypothesis. Heterogeneity of oxidation cannot account for the observed behaviour. Finally, on the basis of previously published analytical data on volatile products, it is suggested that weight loss could essentially result from a chain 'backbiting' process capable of propagating along the macromolecule as a zip depolymerisation.