Deducing 1D concentration profiles from EPR imaging: A new approach based on the concept of virtual components and optimization with the genetic algorithm

Spałek, Tomasz; Kruczała, Krzysztof; Sojka, Zbigniew; Schlick, Shulamith

doi:10.1016/j.jmr.2007.08.012

Cited by 5 publications

(7 citation statements)

References 43 publications

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“…The concentration profiles used for calculation of the diffusion coefficient were determined by the virtual components and genetic algorithm (VC-GA) method that was described in detail elsewhere. 33 Representative profiles obtained by this approach and also by Fourier transform followed by Monte Carlo optimization are presented in Figure 3. The profiles were plotted only up to 2 mm (even though the sample height was ≈3 mm) because no radicals were detected beyond this region of the sample.…”

Section: Resultsmentioning

confidence: 99%

“…While the Fourier transform followed by the Monte Carlo optimization method leads to noisy concentration profiles, the approach based on virtual components and the genetic algorithm (VC-GA) allows the automatic determination of concentration profiles without noise and without prior knowledge about their shapes. The noiseless profiles make possible a more confident interpretation of results, as seen clearly in Figure , below. , …”

Section: Methodsmentioning

confidence: 94%

“…The 1D concentration profiles were determined based on the concept of virtual components and on the genetic algorithm (GA) that enables optimization of the results. 33 In this approach the concentration profile P(r) is expressed as a superposition of several Gaussian functions and two Boltzmann functions, which allow a better representation of the changes in concentration at the edges of the sample, as in eq 2. The Boltzmann (B) and Gaussian (G) functions are given in eqs 3 and 4.…”

Section: Methodsmentioning

confidence: 99%

“…The noiseless profiles make possible a more confident interpretation of results, as seen clearly in Figure 3, below. 26,33 The 2D spectral-spatial ESR images were reconstructed from a complete set of projections, typically 256, obtained as a function of the magnetic field gradient, using a convoluted backprojection algorithm, as described elsewhere. 18,25 In the first reconstruction stage, the projections at the missing angles, typically 93 out of 256, were assumed to be identical to the projection measured at the largest available angle.…”

Section: Methodsmentioning

confidence: 99%

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

Kruczała

Schlick

2010

Macromolecules

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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 (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.

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Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 94%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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

Kruczała

Schlick

2010

Macromolecules

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“…The genetic algorithm (GA) was used for minimization of the difference between simulated and experimental 1D images; this procedure allowed the best fit to be chosen automatically. 26,27 The 2D spectral-spatial ESR images were reconstructed from a complete set of projections, typically 128-256, collected as a function of the magnetic field gradient, using a convoluted back-projection algorithm. 9,10 In the first reconstruction stage, the projections at the missing angles were assumed to be identical to the projection measured at the largest available angle.…”

Section: Methodsmentioning

confidence: 99%

Stabilization and Early Degradation of UV-Irradiated Heterophasic Propylene−Ethylene Copolymers Based on ESR, ESR Imaging, UV−Vis, and DSC: Effect of Ethylene Content and UV Wavelength

2005

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Heterophasic propylene−ethylene copolymers (HPEC) containing 1 wt % bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770) as a hindered amine stabilizer (HAS) were aged by exposure of ≈3 mm thick plaques to UVA and UVB sources in weathering chambers at 318 K. Two types of copolymers, HPEC1 and HPEC2, were examined, which contained 25 and 10 wt % ethylene (E), respectively, as ethylene/propylene rubber (EPR). Spatial and temporal effects of the aging process were studied by electron spin resonance (ESR) and ESR imaging (ESRI) of HAS-derived nitroxide radicals (HAS-NO), by comparison of thermal transitions in original and aged samples using DSC, and by measuring the UV−vis absorption of HPEC films as a function of their thickness. ESR spectra indicated the presence of nitroxide radicals in two amorphous sites, fast (F) and slow (S). The spatial distribution of HAS-NO was obtained by 1D ESRI. Nondestructive (“virtual”) slicing of the 2D spectral−spatial ESR images resulted in a series of ESR spectra that reflected the variation of the F/S intensity ratio with sample depth. HAS-NO was detected on the nonirradiated side due to the small, but not negligible, intensity of UV light transmitted through the plaque; on this side the nitroxide concentration remains high because it is not consumed in stabilization processes, as the local rate of degradation is low. Both ESRI and DSC experiments indicated a higher degradation rate in HPEC2, which contained more polypropylene (PP), 90 wt %; in these samples the degradation was accompanied by an increase in the extreme separation in the ESR spectra, indicating slower dynamics as a result of the increased degree of crystallinity. The degradation was also more advanced in samples exposed to UVB radiation compared to UVA due to the higher UV absorption coefficient in the shorter λ range. The different effect of EPR content in thermal and UV aging of HPEC samples was discussed.

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Electron Paramagnetic Resonance Spectroscopy and Forward Recoil Spectrometry

Kruczaa

Szajdzińska‐Piętek

2014

Characterization of Polymer Blends

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Deducing 1D concentration profiles from EPR imaging: A new approach based on the concept of virtual components and optimization with the genetic algorithm

Cited by 5 publications

References 43 publications

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

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

Stabilization and Early Degradation of UV-Irradiated Heterophasic Propylene−Ethylene Copolymers Based on ESR, ESR Imaging, UV−Vis, and DSC: Effect of Ethylene Content and UV Wavelength

Electron Paramagnetic Resonance Spectroscopy and Forward Recoil Spectrometry

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