Flexible and Inherently Photothermal Waterborne Polydopamine/Polyurethane/Phase Change Material Foams for Light-to-thermal Energy Conversion and Thermal Energy Storage

Flexible, nanoparticle-free, industrially adaptable waterborne polyurethane (WPU) foams with light-to-thermal energy conversion and latent heat storage capacity are presented. WPU particles were coated in dispersion with polydopamine (PDA), a photothermal polymer, to create an inherently photothermal polymer matrix. The resulting aqueous PDA-WPU dispersions with light-to-thermal energy conversion capability prepared at varying PDA ratios were converted into open-cell foams via simple physical mixing with a thickener and surfactants. It was found that the temperature of the PDA-WPU foam synthesized with a 6 mg/mL dopamine concentration reached 172.6 and 70.4 °C under 30 s near-infrared (NIR) laser light and 20 min solar-light exposure, respectively. Polyethylene glycol (PEG), a phase change material, was directly incorporated into the foams at varying weight ratios by physically mixing the aqueous PDA-WPU dispersion and PEG at the foam preparation stage. The melting and solidifying enthalpies of the PDA-WPU/0.5PEG composite foams prepared at PDA-WPU:PEG weight ratios of 1:0.5 were calculated to be 55.2 and 50.9 J/g, respectively. The composite foams retained their shape stability throughout 60 consecutive heating/cooling cycles. When irradiated with solar light for 5 min, the temperature of the PDA-WPU/0.5PEG composite foam heated significantly more than the control WPU foams without PDA and reached 71.2 °C. The composite foams were also demonstrated to exhibit a slower cooling rate than the control PDA-WPU without PEG when the solar irradiation stopped due to the latent heat storage capacity of the composite foams arising from the phase transition of the PEG component. The form-stable, flexible, industrially applicable, and durable foam-type composites, which can efficiently harvest and store sunlight, have been shown to have strong potential as solar-driven thermoregulating materials.