Measuring Diffusion Coefficients of Nitroxide Radicals in Heterophasic Propylene−Ethylene Copolymers by Electron Spin Resonance Imaging

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Heterophasic propylene-ethylene copolymers (HPECs), also known as impact-modified polypropylene copolymers (IPC), are an important class of materials because of their attractive mechanical properties and low cost. Their chemical composition and phase heterogeneity have important effects on the physical and mechanical properties of these systems. Commercially used HPECs usually contain stabilizers such as hindered amines, which can be lost by diffusion We present the measurement of the diffusion coefficient of nitroxide radicals by electron spin resonance imaging (ESRI); the radicals were obtained by oxidation of the commercial Tinuvin 770 hindered amine stabilizer (HAS) with m-chloroperoxybenzoic acid and consist of a mixture of mono- and biradicals. The diffusion rates were measured in two polymers, HPECI and HPEC2, which differed in the ethylene (E) to propylene (P) ratio, 25:75 and 10:90 wt %, respectively. One dimensional (1D) ESRI allowed the visualization of the radical distribution (profiles) within the sample as a function of storing time at 393 K. Simulation of this distribution led to the determination of the macroscopic diffusion coefficients, D, of the paramagnetic tracer in HPEC1 and HPEC2. The results indicate that the main factor determining the diffusion rates in these polymers is the amount of the crystalline phase, mostly composed of isotactic polypropylene (iPP). The D value was the same within experimental error in both systems, 1.42 x 10 -8 cm 2 s -1 . Comparison with literature data of the temperature variation of D for the same diffusant in polyethylene (PE) and polypropylene (PP) suggest that in both HPEC and HPEC2 the diffusion takes place in the amorphous phase restrained by the proximity of the crystalline domains. The results also indicate that ≈20% of the diffusant is lost by evaporation (blooming). Results of spectral-spatial 2D ESR1 show that the ESR spectra of the nitroxides do not change significantly along the sample length during diffusion, suggesting that the loss of stabilizer is due to diffusion and not to chemical reactions.