Electron Paramagnetic Resonance Imaging of the Spatial Distribution of Free Radicals in PMR-15 Polyimide Resins

• Published in: Macromolecules 1997-12, Vol.30 (26), p.8318-8321
• Main Authors: Ahn, Myong K, Eaton, Sandra S, Eaton, Gareth R, Meador, Mary Ann B
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemical reactions and properties; Degradation; Exact sciences and technology; Organic polymers; Physicochemistry of polymers
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma970580t

Prior studies have shown that free radicals generated by heating polyimides above 300 °C are stable at room temperature and are involved in thermo-oxidative degradation in the presence of oxygen gas. Electron paramagnetic resonance imaging (EPRI) is a technique to determine the spatial distribution of free radicals. X-band (9.5 GHz) EPR images of PMR-15 polyimide were obtained with a spatial resolution of ∼0.18 mm along a 2-mm dimension of the sample. In a polyimide sample that was not thermocycled, the radical distribution was uniform along the 2-mm dimension of the sample. For a polyimide sample that was exposed to thermocycling in air for 300 1-h cycles at 335 °C, one-dimensional EPRI showed a higher concentration of free radicals in the surface layers than in the bulk sample. A spectral-spatial two-dimensional image showed that the EPR lineshape of the surface layer remained the same as that of the bulk. These EPRI results suggest that the thermo-oxidative degradation of PMR-15 resin involves free radicals present in the oxygen-rich surface layer.