The high frequency loss tangent data also reveal another peak, especially visible near 70 °C at 100 kHz. It suggests the presence of a sub-Tg local β relaxation, which would thus play an important role in the dielectric enhancement measured near RT, by allowing a degree of dipole mobility below Tg. Such transitions have been previously observed in polyurethanes and attributed to the movements of the polar carbonyl groups of the urethane hydrogen-bonded to water molecules in these particularly hygroscopic materials. Understanding and controlling these phenomena will be key to further enhancing the energy storage performances of these highly promising bio-based polymers.