New insights into the photodegradation of aromatic polyurea: A spectro-thermo-mechanical characterization study

• Spectro-thermo-mechanical response of virgin and UV-exposed polyurea elastomers • Hydroperoxide decomposition and changes in hydrogen-bonding were connected to increased surface reduced modulus • UV radiation also affected thermal transitions assed by thermogravimetric and calorimetric measurements • Spectral investigations revealed the underlying molecular mechanisms connected to photodegradation of polyurea Polyurea emerged as a strategic material for protective coatings in harsh environments, with widespread adoption in civilian infrastructure and defense-related applications. However, polyurea, including an aromatic backbone that provides structural robustness, is susceptible to environmental stressors during extended deployment, most notably UV radiation. The objective of this research is to provide new insights into the mechanical, thermal, and physicochemical performance of polyurea after extended exposure to ultraviolet irradiation. Polyurea samples exhibited surface cracking and an increase in reduced modulus upon UV exposure, as probed using nanoindentation. The increase in reduced modulus also extends in depth, and the depth at which the elastic modulus increases is proportional to the exposure duration. FT-IR analysis indicated that multiple chemical structures were altered by UV exposure. Most prominent is the cleavage of ester bonds connecting soft and hard segments, as well as the cleavage of aliphatic chains. In addition, in UV-exposed polyurea, hydroperoxide decomposition led to crosslinking via radical recombination, and new hydrogen-bonding structures formed within the hard segments. These phenomena contribute to the increased surface reduced modulus. UV exposure of polyurea also decreased its thermal decomposition temperature, as determined by thermogravimetry and calorimetry. These outcomes challenge previous experimental results and provide novel mechanistic insights that accelerate the development of next-generation protective materials and structures with prolonged deployment durations.