Molecular level chain scission mechanisms of epoxy and urethane polymeric films exposed to UV/H2O.: Multidimensional spectroscopic studies

Step-scan photoacoustic Fourier transformed infrared (FT-IR) spectroscopy, FT-IR microscopy, and Raman chemical imaging were utilized to examine molecular level degradation processes in epoxy (EP) and polyurethane (PUR) films. A combination of these techniques allowed us to determine a three-dimensional representation of degradation processes and showed that the extent of cross-linking in EP films varies as a function of depth from the surface. When exposed to 340 nm ultraviolet (UV) radiation and water vapor condensation, EP films degrade to form cracks on the surface, which are composed of primary amines and result from a chain scission of the C-N bonds in cross-linked bisphenol A EP films. PUR films exhibit the presence of two surface domains, which are composed of PUR- and polyurea (PUA)-rich regions. When exposed to 340 nm UV radiation and water vapor condensation, PUA is converted to PUR entities. Mechanisms leading to these processes are proposed and indicate that isocyanate and polyol reactions occur in the presence of H2O and UV light.