Changes in molecular mass characteristics in the autooxidation of isotropic and oriented isotactic polypropylene films: Primary initiation of the kinetic chain

Abstract

Autooxidation of polymers is accompanied by the destruction of the macromolecule, which leads finally to the deterioration of the polymer. In several publications1 − 7 it was pointed out that the autooxidation kinetics of isotropic isotactic polypropylene (isotropic IPP) differ considerably from those of the oriented material, the greatest difference being observed at the beginning of oxidation, the induction period of the latter being much longer than that of the former. With increasing degree of oxidation, the kinetic differences decrease or disappear. It was also pointed out2,3 that, at identical conversion, a greater proportion of initial strength remains in the oriented, than in the isotropic, samples and this proportion increases with the degree of stretching (λ). These data led to the conclusion that stretched macromolecules ‘resist’ destruction and oxidize more slowly than macromolecules having the compact conformation of the amorphous phase. These observations inspired the present work in which the oxidative destruction of macromolecules was directly followed, using the GPC method to investigate the change of the molecular mass distribution (MMD). The method was expected to yield information about the processes which take place during the induction period of IPP oxidation, during which the rate of oxygen uptake and the accumulation of product are too small to be followed by the usual methods. That is why the reason for the existence of the induction period has not been clarified so far in the uninhibited oxidation of the polyolefins. Any supplementary knowledge about the so far unexplored processes which take place during the induction period is essential in order to elucidate the nature of the phenomenon.