Spatially resolved degradation in heterophasic polymers by ESR imaging and FTIR: the case of propylene-ethylene copolymers.

Heterophasic propylene-ethylene copolymers (HPEC) containing bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770) as a hindered amine stabilizer (HAS) were thermally aged at 393 and 433 K. Two types of HPEC were examined, containing 25% and 10% ethylene (E), respectively, as ethylene/propylene

rubber (EPR). Electron spin resonance (ESR) spectra of nitroxide radicals in HPEC were studied in the temperature range 100-433 K; the nitroxides were derived from the HAS and are termed HAS-NO. The results were compared with ESR spectra of the same radicals obtained first by oxidation of Tinuvin 770 and then were doped in HPEC and related homopolymers, polyethylene (PE) and polypropylene (PP); these nitroxides are termed "spin probes." ESR spectra indicated that HAS-NO and the spin probes reside in a range of amorphous sites differing in their dynamic properties. The relative population of the sites was explained by assuming that the crystalline domains exert a restraining effect on chains located in vicinal amorphous domains. Spatial and temporal effects of the aging process were studied by ESR and ESR imaging (ESRI) of HAS-derived nitroxide radicals, and by FTIR of films prepared by compression molding. 1D ESRI enabled the visualization of an outer region of thickness [approximately equal to]100 [micro]m that contained a lower amount of nitroxides, and is believed to result from the loss of the stabilizer by diffusion ("blooming") and possibly also in chemical reactions during aging. Two-dimensional spectral-spatial ESRI indicated the presence of nitroxide radicals in two amorphous sites, fast and slow; the corresponding relative intensity varied with sample depth. Both ESRI and FTIR experiments suggested a faster degradation rate in HPEC containing 25% E, as compared to 10% E; moreover, a larger Tinuvin 770 content in the polymers led to less efficient stabilization. FTIR spectra indicated increased ordering of polypropylene segments in HPEC during aging at 433 K.

Keywords: Hindered amine stabilizer, FTIR, ATR, non-destructive testing, thermal analysis, ESR and ESR imaging, stablization, thermal properties, thermoplastic olefins

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Heterophasic propylene-ethylene copolymers (HPEC) systems, known commercially as impact polypropylene copolymers (IPC), are an important class of polymers, due to their attractive mechanical properties and low cost. (1,2) The polymers consist of crystalline polypropylene (PP) modified by an elastomeric component, typically ethylene-propylene rubber (EPR), and are prepared by the polymerization of propylene in the presence of catalysts, and the sequential polymerization of a propylene-ethylene mixture with the same catalysts. (3) The resulting polymeric materials are heterophasic, but the specific morphology depends on the preparation method and monomer ratio. Many studies have demonstrated the presence of four phases in HPEC: crystalline PP, amorphous PP, crystalline EPR (mostly polyethylene, PE), and amorphous EPR. (4-7) The morphology of HPEC is of considerable interest because processing them at high temperatures can lead to morphological changes. Therefore, an understanding of the morphology and of the temperature dependence of domain size has enormous practical importance. For samples prepared by injection molding of PP/EPR blends, depth-profiling studies have revealed a skin consisting of several layers ("stratification"), whose composition is different in comparison to the bulk phase. (8,9) The effect was explained by the post-processing temperature variations in the sample upon cooling. Similar effects had been detected earlier in PP samples and were assigned to flow, shear, and temperature variations during and processing. (10)