Micro-layered film and foam alternating structures: A novel passive daytime radiative cooling material

This study investigates the confined foaming mechanisms and passive cooling performance of Micro-/Nano-Layered (MNL) film/foam alternating structures. These structures consist of solid poly(carbonate) (PC) film layers and foamed poly(methyl methacrylate) (PMMA) layers. We examine the impact of confinement effects, layer thicknesses, and foaming conditions on cell morphology, cell nucleation, growth, and bulk expansion behavior. Samples ranging from 1 to 513 layers were foamed at 20 MPa and temperatures between 70 and 150°C. We identified three distinct cell structures—multiple, double, or single row(s) of cells—in MNL film/foam structures. We observed that cell size decreases and cell nucleation density increases with increasing layer count. The highest cell nucleation density (1.1×1013 cells/cm3) and smallest cell size (400 nm) were achieved in a 513-layer sample foamed at 70°C and 20 MPa. MNL samples primarily expand vertically, with the apparent expansion ratio ranging from 2.3 to 11 times as layers increase. Samples with double or single row(s) of cells exhibited tensile strengths (~30 MPa) comparable to their solid counterparts and thermal conductivities (29.7 mW/m·K) comparable to air. Our samples exhibited high solar radiation reflection (93.5%) and strong infrared emissivity (91.2%) in the atmospheric transparent window. Mid-day passive cooling experiments showed an average temperature of 17.7°C lower under our sample than ambient conditions. Given these combined properties and the cost-effective nature of the manufacturing method, our samples are readily applicable for building applications, offering significant energy savings and contributing to the mitigation of global warming.