Role of the interphase dynamics in the induction time of the iPP thermo-oxidation

This work completes an earlier study on the influence of molar mass and microstructure on the thermal stability of low molar mass isotactic polypropylenes (iPP). The relative dependence of induction time (ti) on both parameters has been assessed for new metallocene iPP samples (M-PP) with molar masses from 65,000 to 182,000. The new M-PP series includes one metallocene ethylene–propylene copolymer (M-EP) with an ethylene content of 2.6 mol%.When the new ti data are considered together with those previously reported, neither the molar mass nor the microstructure, taken as sole parameters, is able to explain the global ti evolution of the M-PP samples.On the basis of the results corresponding to the M-EP copolymer, it is proposed that local chain dynamics occurring at high free volume regions, associated to propylene segment interruptions, play a main role in the ability of the interphase to initiate the oxidation. The correlation of the characteristic interphase chain dynamics, as measured by DMTA, with ti data supports this suggestion. This hypothesis provides a more unified insight about the actual origin of the iPP thermo-oxidation, as it integrates the influence of parameters which have been found to drive partially the thermal stability of iPP, in particular, molar mass and microstructure.