Functional poly(urethane-imide)s containing Lewis bases for SO2 detection by Love surface acoustic wave gas micro-sensors

Functional polymers containing Lewis bases are known as promising materials for the elaboration of highly sensitive micro-sensors for acid gas detection. In this work, poly(urethane-imide)s (PUIs) with Lewis bases were synthesized in two steps only from various functional diols (N-methyldiethanolamine, N-tert-butyldiethanolamine, N-phenyldiethanolamine and 1,4-diethanolpiperazine) containing one or two tertiary amine groups with different chemical structures. The synthesis led to high yields, high molecular weights and a good control of the PUI amine content. These good film-forming polymers were then used as SO2 sensitive coatings on Love surface acoustic wave (L-SAW) micro-sensors. The physical and mechanical properties required for an optimal micro-sensor design were determined by High Resolution Brillouin Spectroscopy of PUI thin layers. Three-layer micro-sensors were developed by respecting the conditions for the L-SAW generation. The three-layer structure included Quartz ST-90 as the piezoelectric substrate, a ZnO guiding layer and a PUI sensitive layer. These micro-sensors were weakly sensitive to temperature and thus compatible with the targeted application. The experimental results for SO2 detection showed that all the PUI coatings greatly improved the sensitivity compared to a micro-sensor without polymer coating. The basicity of the amine groups was not determining for the micro-sensor sensitivity to the SO2 acid gas and the key factor was their steric hindrance. Finally, the micro-sensor sensitivity increased with the accessibility of the amine groups in the order Piperazine-diol < tBu-DEA ≅ Ph-DEA ≪ MDEA.