Spatially resolved degradation in heterophasic polymers by ESR imaging and FTIR: The case of propylene-ethylene copolymers |
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Authors: | Email author" target="_blank">Shulamith?SchlickEmail author Kryzstof?Kruczala |
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Affiliation: | (1) Department of Chemistry and Biochemistry, University of Detroit Mercy, 48219-0900 Detroit, MI;(2) Faculty of Chemistry, Jagiellonian University, 30-060 Krakow, Poland |
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Abstract: | 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 ≈100 μm 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. |
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Keywords: | Hindered amine stabilizer FTIR ATR non-destructive testing thermal analysis ESR and ESR imaging stablization thermal properties thermoplastic olefins |
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