Mikhail V. Motyakin

Electron spin resonance imaging of polymer degradation and stabilization

Abstract We present the application of one-dimensional (1D) and two-dimensional (2D) electron spin resonance imaging (ESRI) for the study of photo- and thermal degradation of polymers containing hindered amine stabilizers (HAS). The method is based on the formation of stable nitroxide radicals derived from HAS during polymer treatment, and on encoding spatial information in the ESR spectra via magnetic field gradients. The imaging technique allowed nondestructive profiling of the HAS-derived nitroxide radicals. The intensity profile in the sample depth was deduced by 1D ESRI, and the spatial variation of the ESR line shapes (‘spectral profiling’) was determined by 2D spectral-spatial ESRI. Application of this approach to the photodegradation of polypropylene and to the photo- and thermal degradation of poly(acrylonitrile–butadiene–styrene) will be described. The method can be used to detect spatial heterogeneities in the degradation process and to identify morphological domains that are selectively degraded.

Thermal degradation at 393 K of poly(acrylonitrile-butadiene-styrene) (ABS) containing a hindered amine stabilizer: a study by 1D and 2D electron spin resonance imaging (ESRI) and ATR–FTIR

Electron spin resonance imaging (ESRI) was developed in our laboratory as a method for spatial and spectral profiling of radicals formed during polymer degradation. We present the application of this approach to the study of thermal degradation at 393 K of poly(acrylonitrile-butadiene-styrene) (ABS) containing 1 or 2% w/w Tinuvin 770 as the hindered amine stabilizer (HAS). The spatial distribution of the HAS-derived nitroxide radicals, obtained by 1D ESRI, was homogeneous at short treatment times but became heterogeneous after treatment times ⩾ 70 h. The spatial variation of the ESR line shapes with sample depth was visualized by 2D spectral–spatial ESRI. Nondestructive (“virtual”) slicing of the 2D images resulted in a series of ESR spectra along the sample depth, which were used to deduce the relative intensity of nitroxide radicals present in two distinct sites. The two sites represent radicals located in butadiene-rich and SAN-rich domains, respectively. Taken together, 1D and 2D ESRI allowed the determination of the extent of degradation within morphologically-distinct domains as a function of sample depth and treatment time. The conclusions from the ESRI experiments were substantiated by attenuated total reflectance (ATR) FTIR spectroscopy of the outer layer (500 μm thick) of the polymer. Comparison of the two techniques suggested that the advantage of the ESRI method is its ability to provide mechanistic details on the early stages of the ageing process. ESRI and FTIR data indicated that the larger Tinuvin 770 content in the polymer, 2%, leads to less efficient stabilization.

Spectral profiling of radicals in polymer degradation based on electron spin resonance imaging (ESRI)

Abstract We have developed 1D and 2D electron spin resonance imaging (ESRI) in order to deduce the spatial variation of radical properties in polymers exposed to UV radiation in the presence of oxygen, or thermally treated. The accelerated degradation of poly (acrylonitrile-butadiene-styrene) (ABS) containing Tinuvin 770 as the hindered amine stabilizer (HAS) was studied in a weathering chamber equipped with a Xe source that mimics the spectral range of sunlight. The HAS-derived nitroxides detected by ESR occupy two sites and were assigned to radicals located in domains differing in their monomer composition. The spatial distribution of the radical intensity obtained by 1D ESRI indicated that nitroxide radicals are produced initially only on the irradiated side, but the intensity of radicals on the opposite side increases with irradiation time. By contrast, the spatial distribution of nitroxides produced during thermal degradation at 600C is spatially homogeneous. Via 2D ESRI it was possible to visualize the spatial variation of the ESR spectra and to deduce the relative intensity of the nitroxides in the two distinct sites along the selected spatial coordinate. The spectral profiling made possible by ESRI provided spatial details that can be used to deduce morphology-sensitive chemical processes along the sample depth.

1D and 2D Electron Spin Resonance Imaging (ESRI) of Transport and Degradation Processes in Polymers

Paul Lauterbur was the first to propose, in his notebook entry of 2 September 1971, the use of magnetic field gradients in order to “tell exactly where an NMR signal came from”; his first proton density map appeared soon afterwards (Lauterbur, 1973). He named the technique zeutmatography, from the Greek word for “joining together”, meaning to join the magnetic field gradient and the corresponding radiofrequency in a nuclear magnetic resonance (NMR) experiment. This connection allowed the encoding of spatial information in the NMR spectra. The use of magnetic field gradients to separate the resonant frequencies corresponding to the various spatial elements led to the development of NMR imaging (NMRJ) or, in current language, magnetic resonance imaging (MRI). Against a back-drop of general skepticism, NMRI has blossomed into an essential, and routine, diagnostic procedure in medicine that provides an image of previously hidden anatomic parts. Applications of NMRI to Materials Science and other important disciplines, although not as dramatic as the medical applications, are extensive, ubiquitous, steadily developing, and well documented (Blumler et al., 1998). The wonderful story on the discovery of NMR imaging has been told recently (Kevles, 1997).

Electron Spin Resonance Imaging (ESRI) of Degradation and Stabilization Processes in Polymers

Polymeric materials, exposed to heat, mechanical stress and ionizing or UV irradiation, undergo degradation in the presence of oxygen due to the formation of reactive intermediates such as free radicals R., RO. and ROO., and hydroperoxides ROOH.1, 2, 3, 4, 5, 6 The degradation process can be accelerated by chromophores, free radicals and metallic residues from the polymerization reactions. Often the deleterious effects are not immediately detected, but develop over longer periods. The gradual changes in the polymer properties observed in many systems, including polyolefins, and the ultimately grave results are due to trapped radicals that react slowly, to peroxy radicals that decompose in time with formation of reactive radicals and gas molecules, and to trapped gases that lead to local stresses and to cracking.2,5 While the timescale of these changes may vary, the final results are dramatic: degradation of the structure and collapse of the mechanical properties. The accelerated rate of ozone depletion in the stratosphere due to environmental factors is expected to raise the level of UV-B radiation (≈290–320 nm), thus adding severity to the problem of degradation and urgency to the need for solutions.