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

Abstract

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 (blooming). 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, HPEC1 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 Â 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 HPEC1 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 ESRI 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.

Effect of UV absorbers and hindered amine light stabilizers on the photodegradation of ethylene–octene copolymer

“…Then, the reactions between the radicals generated the gel. 27 Correspondingly, the UV stabilizers, whose photostabilizing mechanisms were shown in Scheme 1, [45][46][47] were able to decrease the gel formation. Figure 3 revealed the gel content of samples before and after UV irradiation.…”

“…It was reported that the UV stabilizers used here could inhibit the occurrence of the photodegradation reactions of polymers. 27 Correspondingly, the UV stabilizers, whose photostabilizing mechanisms were shown in Scheme 1, [45][46][47] were able to decrease the gel formation. The bigger the value of the gel content was, the more serious photodegradation the sample suffered from.…”

Characterisation of Solute Mobility in Hypercross‐Linked Resins in Solvents of Different Polarity: Two Promising Supports for Catalysis

“…[29] Spin labels attached to the polymers are widely used to assess the mobility of the polymeric chains, and spin probes diffusing into the polymers are used to evaluate the mobility of solutes. [30][31][32][33][34] We previously used spin probes in correlation with inverse steric exclusion chromatography (ISEC), to assess the "pore" size of gel-type resins. [35][36][37] This work is aimed at evaluating the nanoporosity and the molecular accessibility of the HYPRs after the transformation of a commercial gel-type polymeric support (GT) into a hypercross-linked material (HGT) and then after the introduction of strongly acidic sulphonic groups (HGS), useful for anchoring metal ions as precursors of catalytically useful metal nanoclusters.…”

Spectral spatial electron paramagnetic resonance imaging as a tool to study photoactive dimethacrylate-based dental resins

“…They originated from hindered amines used as stabilizers. Lucarini et al carried out detailed studies on PP using 1D EPRI [23][24][25], Marek et al extended the work to polystyrene [26], while Schlick and collaborators used both 1D EPRI and 2D SSI to study the degradation of heterophasic system such as poly(acrylonitrile-butadiene-styrene) and propylene-ethylene copolymers [27,28], or very recently to measure diffusion coefficients of nitroxide radicals in propylene-ethylene copolymers [29] (for review on this topic see Refs. [30][31][32] by Schlick).…”