One pot stimuli-responsive linear waterborne polyurethanes via Diels-Alder reaction

Different strategies were used to obtain waterborne polyurethanes containing Diels-Alder moieties. In the first one (two pot), furan and maleimide end capped waterborne polyurethane dispersions were synthesized with the aim of obtaining a stimuli-responsive material after drying a mixture of both dispersions. However, the maleimide hydrolysis that took place at basic pH prevented the use of this strategy. The only way to introduce maleimide groups in the furan capped dispersions was after water removal, which limits the application. In the second strategy (one pot), a Diels-Alder adduct was introduced in the polyurethane chain. Using this strategy, stable waterborne polyurethane dispersions were successfully synthesized. Both strategies allowed the obtaining of films that according to FTIR, DSC and NMR experiments showed cyclic temperature responsive behaviour, as at high and low temperature the Diels-Alder adduct was opened and regenerated respectively. As a consequence of this reaction, the temperature increase provoked chain scission reducing both the polymer molecular weight and the viscosity. Using the one pot strategy, a Diels-Alder adduct containing polyurethane film was obtained for the first time from waterborne systems. This can be of great interest in order to obtain functional self-healing coatings with reduced volatile organic compounds (VOC).