AbstractThe modification of isotactic poly(propylene) (PP) foils by reactive (O2) and inert (He) gas plasma treatment was studied with relevance to surface structure and thermo‐oxidative stability for different treatment times. The following effects were investigated: change of non‐helical (atactic or amorphous) content in the surface region and formation of vinyl and vinylidene groups by frustrated multiple internal reflection Fourier transform infrared spectroscopy (FMIR‐FTIR), formation of peroxyl redicals by electron spin resonance spectroscopy (ESR) and change of the invariant activation energy of degradation by thermogravimetric analysis (TGA). For the latter a new kinetic evaluation method was used based on a selection of multiple step model to determine the invariant Arrhenius constants, e.g., activation energy and preexponential factor, for the weight loss during thermoxidative degradation of untreated and, for the first time, of plasma‐treated PP. Several thermogravimetric curves under different temperature scanning conditions have been evaluated with different solid state models as well as with a selection of different multiple step reaction pathways. With a limitation to total weight loss of 60% of the mass for an untreated PP and applying a most probable kinetic model consisting of one consecutive and one parallel reaction step, the activation energies were found to be 84.2 ± 1.0 and 73.2 ± 1.0 kJ/mol for the initial and the consecutive reaction and 137.8 ± 3.0 kJ/mol for the parallel reaction. The corresponding data of the plasma‐treated samples are 80 ± 1.0 and 71.0 ± 1.0 kJ/mol for the initial and the consecutive reaction, while the activation energy for the parallel reaction remained almost unchanged. Unexpectedly, we found the He plasma treatment to decrease the activation energies of the first and consecutive reaction nonlinearly and within the first 600 s to a larger extent than O2plasma treatment. Based on this kinetic model, a modified mechanism for the thermo‐oxidative degradation of PP is proposed in terms of elementary degradation reactions and will be discussed with reference to