Abstract
Electron spin resonance (ESR) spectra of nitroxide radicals formed in thermally treated heterophasic propylene−ethylene copolymers (HPEC) containing bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770) as a hindered amine stabilizer (HAS) were studied in the temperature range 100−433 K; the nitroxides are 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 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 in HPEC and homopolymers reside in a range of amorphous sites differing in their dynamical properties. Evidence for the various sites was obtained from the ESR line widths, the temperature variation of the extreme separation in the ESR spectra, and the effect of stress on the ratio of the fast and slow components in the spectra of HAS−NO in thermally treated HPEC. The relative population of sites was explained by assuming that the crystalline domains exert a restraining effect on chains located in vicinal amorphous domains; in PE the restraining effect was more pronounced in the polymer with the higher crystallinity (HDPE vs LDPE). Additional support for this assumption was provided by Fourier transform infrared (FTIR) spectra of HPEC and related polymers and by differential scanning calorimetry (DSC). The dynamically restrained chains evidenced by the spin probes are thought to be located in a rigid amorphous phase, which was described in the literature. In HPEC the fast spectral component represents unrestrained ethylene−propylene rubber (EPR) chains, and the slow spectral component represents amorphous PP and EPR chains in the rigid amorphous phase. This study has demonstrated the exceptional sensitivity of ESR spectra from nitroxide radicals to polymer morphology and degree of crystallinity.
Published Version
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