Waterborne Polydopamine-Polyurethane/Polyethylene Glycol-Based Phase Change Films for Solar-to-Thermal Energy Conversion and Storage

Form-stable phase change films composed of a polydopamine-polyurethane polymer matrix with photothermal conversion properties and polyethylene glycol (PEG) are presented. Surfaces of environmentally friendly waterborne polyurethane (WPU) particles in aqueous dispersions were coated with polydopamine to create a stable waterborne dispersion of a polydopamine-polyurethane (PDA-WPU) matrix, which intrinsically presents significant photothermal conversion properties, and PEG was directly integrated into the PDA-WPU matrix by simple mixing in the dispersion form. Successful film formation was achieved at PDA-WPU to PEG ratios of 1:1 and higher by weight, resulting in form-stable, homogeneous PDA-WPU/PEG phase change films. Incorporation of PEG into the amorphous PDA-WPU matrix was demonstrated to impart a semicrystalline character to PDA-WPU films, which also increased their thermal stability and thermal conductivity. Young's modulus of PDA-WPU/PEG films increased while the tensile strength and elongation at break values decreased as a function of PEG content, yet all films showed a flexible behavior. For the films prepared with the highest amount of PEG (PDA WPU:PEG 1:1), the melting and solidifying enthalpies were calculated to be 81.1 and 77.9 J/g, respectively, and enthalpies remained the same over 60 consecutive heating-cooling cycles. The temperature of the PDA-WPU:PEG 1:1 film reached 74.8 degrees C under 20 min of solar irradiation at 150 mW/cm(2) with a solar-to-thermal energy conversion efficiency of 72.9%. In a cold environment, PDA-WPU/PEG films and their surroundings were shown to heat up more than controls under solar light and stay warmer after the solar irradiation was stopped. The temperature of the environment surrounded with the PDA-WPU/PEG film increased 10 degrees C more than the temperature of the control environment under 30 min of sunlight irradiation. Upon switching the sunlight irradiation off, the PDA-WPU/PEG environment cooled down to ambient temperature 10 min later than the control environment, demonstrating that these form-stable, flexible, and durable films can efficiently harvest and store sunlight and have strong potential as solar-driven thermoregulating materials.