Continuous Wave and Time-Resolved Electron Paramagnetic Resonance Study of Photoinduced Radicals in Fluoroacrylic Porous Polymer Films

Fluoroacrylic polymers with inherent micro/nanoporosity are promising media for incorporation of fluorescent molecules and following application as pressure-sensitive paints (PSPs), and UV photostability of PSPs is critically important for their long-term performance. Although photodegradation mechanisms of fluoroacrylic polymers have been studied previously in solutions, they have never been addressed in practically relevant for PSPs solid-state porous films. In this work we combined continuous wave (CW) and time-resolved (TR) electron paramagnetic resonance (EPR) to study UV photodegradation of thin porous films of a few representative fluoroacrylic polymers. Different types of spectra were detected using CW and TR EPR and assigned to the species formed on the inner surface of the pores and in the bulk of the polymer, respectively. The radical pairs formed in the bulk are short-lived, as is evidenced by TR EPR, and most likely recombine back to the initial polymer. On the contrary, the radicals formed on the surface of the pores are metastable in the absence of oxygen; they can be studied by CW EPR and clearly assigned to the radicals of type ·C­(CH3)­CH2– (so-called propagating radicals) formed via the cleavage of the C–C bond of the ester side chains and consecutive β-scission. Remarkably, their CW EPR spectra closely resemble solution-state spectra, indicating that these radicals are localized in the pores where the mobility of methyl and methylene protons is not suppressed. Thus, based on complementary results of CW and TR EPR, we conclude that UV photodegradation of porous fluoroacrylic polymer films mainly occurs on the inner surface of the pores, which needs to be considered in future development of this type PSPs.