Enhancement of Solar Thermal Fuel by Microphase Separation and Nanoconfinement of a Block Copolymer

Until now, solar thermal energy storage within phase-change materials (PCMs) depends on π–π and van der Waals interactions to improve their miscibility, which often brings about finite enhancement of energy density. Here, we report one nanoconfinement approach to improve energy density using microphase separation of a block copolymer, allowing PCMs to selectively disperse into nanodomains of polymer film for use as solar thermal fuel (STF). Upon doping a certain amount of photoinert organic PCMs with liquid-crystalline block copolymer containing azobenzene as the continuous mesogenic phase, the periodic nanostructures remained due to the microphase separation, which also brings about a distinct nanoconfinement effect for the dopant PCM nanodomains, achieving crystallization temperature from 24 to −38 °C. This unique feature originates from the nanoscale microphase separation, enabling natural sunlight and environmental heat to be stored simultaneously, which can be released in several ways: cis-to-trans isomerization and isotropic-to-liquid crystal phase transition of the azobenzene-containing polymer block at a higher temperature and phase transition of PCMs at a lower temperature, even below −30 °C. Due to the improved STF performance, wearable warm fabrics and photothermal pneumatic actuators were successfully obtained. The present work paves the way for developing high-performance STF systems achieved by microphase separation (MPS) and nanoconfinement, facilitating the advancement and applications of STF materials in the fields of wearable smart materials, warm fabrics, and actuators.