Electron Paramagnetic Resonance: Volume 20 (Specialist Periodical Reports - Electron Paramagnetic Resonance) - Hardcover

Hardcover

ISBN 10: 085404325X ISBN 13: 9780854043255

Publisher: Royal Society of Chemistry, 2006

Synopsis

Reflecting the growing volume of published work in this field, researchers will find this book an invaluable source of information on current methods and applications.

About this title

From the Back Cover

Electron Paramagnetic Resonance (EPR) highlights major developments in this area, with results being set into the context of earlier work and presented as a set of critical yet coherent overviews. The topics covered describe contrasting types of application, ranging from biological areas such as EPR studies of free-radical reactions in biology and medically-related systems, to experimental developments and applications involving EPR imaging, the use of very high fields, and time-resolved methods. Critical and up-to-the-minute reviews of advances involving the design of spin-traps, advances in spin-labelling, paramagnetic centres on solid surfaces, exchange-coupled oligomers, metalloproteins and radicals in flavoenzymes are also included. As EPR continues to find new applications in virtually all areas of modern science, including physics, chemistry, biology and materials science, this series caters not only for experts in the field, but also those wishing to gain a general overview of EPR applications in a given area.

Excerpt. © Reprinted by permission. All rights reserved.

Electron Paramagnetic Resonance Volume 20

By B C Gilbert, M J Davies, D M Murphy

The Royal Society of Chemistry

Copyright © 2007 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-0-85404-325-5

Contents

ESR Imaging Beyond Phantoms: Application to Polymer Degradation Shulamith Schlick and Mikhail V. Motyakin, 1, 
Advances in Spin Trapping J.-L. Clément and P. Tordo, 29, 
Site-Directed Spin-Labelling (SDSL) Applications in Biological Systems Jimmy B. Feix and Candice S. Klug, 50, 
Quantum Chemical Approaches to Spin-Hamiltonian Parameters Frank Neese, 73, 
Getting an Inside View of Nanomaterials with Spin Labels and Spin Probes Victor Chechik and Agneta Caragheorgheopo, 96, 
EPR, ENDOR and EPR Imaging of Defects in Diamond M.E. Newton, 131, 
EPR of Exchange Coupled Oligomers David Collison and Eric J.L. McInnes, 157, 
Biological Free Radicals and Biomedical Applications of EPR Spectroscopy Simon K. Jackson, John T. Hancock and Philip E. James, 192, 
Progress in High-Field EPR of Inorganic Materials Peter C. Riedi, 245, 

CHAPTER 1

ESR Imaging Beyond Phantoms: Application to Polymer Degradation

BY SHULAMITH SCHLICK AND MIKHAIL V. MOTYAKIN

1 Introduction and Motivation

1.1 Polymer Degradation. – Polymers undergo degradation when exposed to heat, mechanical stress, and ionizing or UV irradiation in the presence of oxygen, due to the formation of reactive intermediates such as free radicals R· and ROO·, and hydroperoxides ROOH. Exposure to environmental factors leads to profound changes in polymer properties, on both molecular and macroscopic levels. The chemical structure is modified due, for example, to chain scission and cross-linking, resulting in changes of the elastic properties and of the degree of crystallinity. The chemistry of degradation is complicated because even small amounts of chromophores, free radicals, and metallic residues from polymerization reactions can introduce additional reaction pathways that usually enhance the rate of degradation. Polymer degradation is equivalent to corrosion in metals, a fundamental problem with important practical ramifications.

Electron spin resonance (ESR) methods have been used extensively for detecting and identifying the radicals formed, clarifying the degradation mechanism, and simulating the variation of the ESR spectra with temperature. Simulations of ESR line shapes for specific models of dynamics have been developed for the study of oxidative degradation of polymers due to ionizing radiation. Irradiation in vacuo has enabled the study of the type and mobility of alkyl radicals R· derived from the polymer. These studies were initially performed on polytetrafluoroethylene (Teflon) and other perfluorinated polymers, because perfluoroalkyl radicals can be stabilized even at ambient temperature; in these polymers mid-chain and end-chain alkyl radicals have been detected. Admission of oxygen led to the formation of the corresponding peroxy radicals, ROO·. The motional mechanism of the peroxy radicals was deduced by simulation of the temperature dependence of the spectra; in this way a correlation between dynamics and reactivity has been established. This approach has also been extended to protiated polymers, for instance polyethylene and polypropylene.

Direct ESR and spin-trapping experiments have identified oxygen radicals as well as membrane-derived fragments in Nafion, a perfluorinated ionomer used as a proton exchange membrane (PEM) in fuel cells, exposed to oxygen radicals produced in the Fenton reaction or by UV irradiation of hydrogen peroxide. Identification of the radicals was possible by variation of sample preparation methods, temperature used for spectra acquisition, and annealing conditions.

1.2 Polymer Stabilization by Hindered Amines. – Recent research efforts on the effects of radiation and thermal treatment on polymeric materials have two main goals: understanding the degradation mechanism and predicting polymer lifetimes, and development of protective additives. Hindered amine stabilizers (HAS) rank among the most important recent developments for stabilization of polymeric materials. Nitroxides and amino ethers are major products of reactions involving HAS. The HAS-derived nitroxides (HAS-NO) are thermally stable, but can scavenge free radicals to yield diamagnetic species; the amino ethers can regenerate the original nitroxide, thus resulting in an efficient protective effect. Some of these events are shown in Figure 1, where >NH denotes the amine, >NO· the nitroxide, and R· ROO· and ROOH the reactive intermediates derived from polymer chains exposed to oxygen and irradiation or heat. The intermediate radical N-peroxy radical >NOO· has been detected by ESR. The most stable is >NO·. Though stable, however, HAS-NO can react with alkyl radicals derived from polymeric precursors, and stabilize the polymer. In spite of numerous studies, important information on the degradation steps and kinetics is still incomplete or missing altogether.

1.3 Motivation for ESRI Studies of Polymer Degradation. – The concept of diffusion-limited oxidation (DLO) has greatly contributed to the understanding of the mechanism for polymer degradation: if oxygen diffusion is slow compared to the rate of degradation, as in accelerated degradation in the laboratory, only thin surface layers in contact with air are degraded, while the sample interior is little, if at all, affected; this is the DLO regime.

The presence of oxygen is crucial for oxidative degradation of polymers. In the diffusion limited oxidation regime, the distribution of oxygen in the polymer can be described by equation (1),

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

where the first term on the right describes the rate of oxygen diffusion, and D is the oxygen diffusion coefficient; the second term describes the rate of oxygen consumption due to degradation, assuming a first order reaction, and k is the rate constant for oxygen consumption. For steady state conditions, the rate of oxygen consumption is equal to the oxygen supply by diffusion:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

For the boundary conditions [O2] = [O2] at x = 0 and [partial derivative][O2]/[partial derivative]x = 0 at x = 1, the solution of equation (2) is (l is half sample thickness):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

Assuming that all oxygen in the polymer reacts and the oxygen-containing products are not lost by diffusion, equation (3) describes the distribution of oxygen-containing products such as hydroxyl, carbonyl, and nitroxide. For large values of x equation (3) can be approximated as

C/C0 = exp [-(k/D)1/2 x] (4)

where C0 and C are the concentrations of oxidation products on the surface and at depth x, respectively.

The parameter a = (D/k)1/2, known in the literature as the degradation depth, shows the depth where most (90%) of oxygen-containing products are located. The degradation profile for two values of a, and a sample depth of 4 mm are shown in Figure 2. For a degradation depth comparable with the sample depth, a = 3.4 mm, the degradation profile is essentially homogeneous; this is the case of low k, low oxygen consumption, and low degradation rates. For a degradation depth smaller than the sample depth, a = 0.4 mm in Figure 2, the profile is heterogeneous; this is the DLO regime, with high k, high oxygen consumption, and high degradation rates.

The DLO concept implies that lifetimes of polymeric materials deduced from the study of average properties of samples involved in accelerated degradation cannot be used to estimate the durability of polymers in normal exposure; profiling methods, which determine the variation of the extent of degradation with sample depth, are needed.

The primary motivation for ESR imaging (ESRI) studies was to develop methods for spatially-resolved degradation: to visualize degradation profiles and variation of degradation processes within sample depth. We have developed 1D and 2D spectra-spatial ESRI for the study of heterophasic systems such as poly(acrylonitrile-butadiene-styrene) (ABS) and heterophasic propylene-ethylene copolymers (HPEC) containing bis(2,2,6,6-tetramethyl-4 -piperidinyl) sebacate (Tinuvin 770) as the stabilizer, and exposed to thermal treatment and UV irradiation. The HAS-NO provided the contrast necessary in the imaging experiments. The major objectives were to examine polymer degradation under different conditions; to assess the effect of rubber phase, polybutadiene in ABS and ethylene-propylene rubber (EPR) in HPEC, on the extent of degradation; and to evaluate the extent of stabilization by HAS. The repeat units in ABS and the formula of Tinuvin 770 are shown in Figure 3.

Recent advances in the ESRI field, for instance the choice of pulsed vs continuous-wave (CW) experiments, combined ESR/NMR imaging, progress in resonators, and applications in various disciplines have been described in detail. The focus of this Chapter is on ESRI experiments and applications that have been used for, and are relevant to, polymeric systems. The experimental details reflect the imaging system in the Detroit laboratory.

This Chapter is organized as follows. In Section 2 we describe ESR spectra in the presence of magnetic field gradients, review applications of ESRI to polymeric systems, and describe crucial experiments that led to the development of ESRI methods in our laboratory. Section 3 describes selected experimental details on sample preparation and treatment, and on the determination of the nitroxide intensity profile (by 1D ESRI) and spectral profile (by 2D spectral-spatial ESRI). The ESR spectra of HAS-NO in the heterophasic polymers are described in Section 4. 1D and 2D ESRI experiments are described in Section 5, which includes results for the ABS and HPEC systems, a comparison of ESRI and FTIR methods, and our experience with the effect of microtoming on crystalline polymers. Conclusions and prospects are presented in Section 6.

2 ESR Imaging and Applications to Polymer Systems

2.1 ESR Spectra in the Presence of Magnetic Field Gradients. – ESR spectroscopy can be transformed into an imaging method, ESRI, by measuring ESR spectra in the presence of magnetic-field gradients. In ESR imaging experiments the microwave power is absorbed by the unpaired electrons located at point x when the resonance condition is fulfilled:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

In equation (5), n is the microwave frequency, Bres is the resonant magnetic field, Gx is the linear magnetic field gradient (in Gauss cm-1) at x, and the other parameters have their usual meaning. As in NMR imaging, the field gradients produce a correspondence between spin location and Bres and allow the encoding of spatial information in the ESR spectra. If the sample consists of two point samples, for example, the distance between the samples along the gradient direction can be deduced if the field gradient is known. In this way it is possible not only to verify the existence of the paramagnetic species in a sample, but also "to tell exactly where the signal came from". The ESRI methodology is similar to that of NMR imaging (NMRI, or magnetic resonance imaging, MRI).

The challenges in the application of the gradient approach to ESRI are numerous. First, higher gradients are needed compared to NMRI, usually 100-1000 times larger. Second, the ESR spectra are often complex, with multiple lines due to hyperfine interactions and g-value anisotropy; these signals complicate the imaging experiments but in most cases do not add additional information. Third, most systems do not contain stable paramagnetic species on which imaging is based. ESRI experiments are usually performed on paramagnetic transition metal ions, radicals produced by irradiation, or stable nitroxide radicals as dopants; in some experiments triarylmethyl ("trityl") radicals are used as probes, because their ESR spectra consists of a single narrow line, of width typically ≈ 50 mG in the absence of oxygen.

The feasibility of ESRI was first demonstrated in 1979 by Hoch and Day, who described the distribution of colour centres in natural diamonds. The instrumentation, software, and applications of ESRI have been described in a 1991 monograph and updated in recent reviews. The early efforts described the type and stability of the gradients necessary for specific ESRI experiments and the software necessary for image reconstruction in spatial and spectral dimensions. These studies also investigated the feasibility of ESRI experiments in a variety of "phantom" samples, and discussed and estimated the spatial resolution. Because of the short relaxation times of the electron spins, most ESRI experiments are performed in the CW mode, unlike MRI which is used in the pulsed mode. In vivo studies are usually performed at lower frequencies, typically below 2 GHz, which can accommodate large samples with high water content. Most experiments in materials science are performed at X band, ≈ 9 GHz. Gradients can be applied in the three spatial dimensions, and a spectral dimension can be added by the method of stepped gradients. The widening scope of ESRI studies was highlighted at the 2004 ESR Symposium in Denver, with focus on spatially-resolved degradation and software development for applications to polymers, and in vivo studies at 300 and 700 MHz for the detection of local oxygen profiles and the study of radical involvement in oxidative diseases.

2.2 ESR Imaging of Polymeric Systems. – While most ESRI efforts are directed to biological applications, a small number of studies on polymeric systems have appeared. Information on the spatial distribution of paramagnetic molecules deduced from ESRI experiments has been used for measuring macroscopic translational diffusion. Diffusion coefficients, D, of paramagnetic diffusants can be deduced from an analysis of the time dependence of the concentration profiles along a selected axis of the sample. The determination of D for spin probes in liquid crystals and model membranes, and the effect of polymer polydispersity, have been described in a series of papers by Freed and coworkers. In our laboratory the diffusion coefficients of paramagnetic guests in ion-containing polymers, polymer solutions, cross-linked polymers swollen by solvents and self-assembled polymeric surfactants have been determined by 1D ESRI. These papers represent an effort to extract quantitative information from ESRI experiments. Moreover, in some cases these experiments allow the comparison of macroscopic diffusion coefficients in the presence of a concentration gradient (measured by ESRI) with the microscopic D values (measured by pulsed field-gradient NMR).

Some of these studies have resulted in measurements of diffusion coefficients that can be deduced only by ESRI. An example is the measurement of D at 300 K for nitroxides probes that differ in their hydrophobicity, and were doped in the various phases (micellar, hexagonal, lamellar, and reverse micellar) of the triblock copolymers poly(ethylene oxide)-b -poly(propylene oxide)-b-poly (ethylene oxide), EOm POnEOm, (commercial name Pluronics). The self-assembling is due to the different hydrophobicity of the two blocks, PEO and PPO, in water as solvent. Ionic, neutral and hydrophobic probes select specific sites in the self assembled system, and these sites are reflected in the rate of transport of the probes: in the value of D. The cationic probe 4-(N,N, N-trimethyl)ammonium-2,2,6,6-tetramethyl-piperidine-1-oxyl iodide (CAT 1) and the hydrophobic probe 5DSE, the methyl ester of doxylstearic acid (5 indicates the carbon atom to which the doxyl group is attached) exhibited a contrasting transport behaviour in aqueous solutions of Pluronic L64, EO13PO30EO13. Although the molecular masses M are similar, 340 for CAT1 (213 for the cation) and 414 for 5DSE, the corresponding D values at each polymer content are very different. The ratio DCAT1/D5DSE is 35 in the micellar phase (polymer contents 20 and 35 % w/w), 11 in the hexagonal phase (polymer content 50 % w/w), and 3.1 in the mixed Lα (lamellar) and L2 (reverse micellar) phase (polymer content 80% w/w). The range of D values measured in this study was 1.0 x 10-5 cm2 s-1 - 1.0 x 10-7 cm s-1. These results indicate that ESRI is the method of choice for the determination of diffusion coefficients for guests present in low concentrations and located in various regions of self-assembled systems; this conclusion is relevant for drug delivery systems.

2.3 ESR Imaging of Polymers Stabilized by Hindered Amines. – Degradation processes can be studied by ESRI in polymers containing hindered amine stabilizers (HAS); this approach was originally suggested by Ohno, who presented 2D spectral-spatial ESRI images of radicals in polypropylene (PP) containing two different stabilizers, but no detailed analysis. The method is based on the formation of stable nitroxide radicals derived from HAS, HAS-NO, during UV- or thermal treatment. ESRI based on HAS-NO represents an important step in the development of ESRI beyond phantoms, because the nitroxides are part of the system, and participate in, and reflect, degradation processes.

(Continues...)

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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2006
• Language: English
• ISBN 10: 085404325X
• ISBN 13: 9780854043255
• Binding: Hardcover
• Number of pages: 284