Thermal conductivity of low-density micro-and nanocellular poly(methyl-methacrylate) (PMMA): Experimental and modeling

Nowadays, finding new materials with enhanced thermal insulation properties has become a mandatory task to reduce energy consumption and CO2 emissions. In recent years, nanocellular polymers have aroused great attention due to their very interesting combination of properties, which include reduced conduction through the gas phase thanks to the Knudsen effect. There are plenty of theoretical works hypothesizing the thermal insulation performance of nanocellular polymers. However, there is a lack of experimental results, especially at low densities. In the present work, the thermal conductivity of low-density microcellular and nanocellular poly(methyl-methacrylate) (PMMA) was measured to evaluate the different heat transfer mechanisms acting on these structures. PMMA foamed sheets with relative densities ranging from 0.09 to 0.18 and cell sizes between 400–4000 nm were produced by gas dissolution foaming using CO2 as a physical blowing agent. Samples were measured at various temperatures, resulting in thermal conductivities between 37.4 and 46.6 mW/(m·K) at 10 °C. Experimental results have been analyzed to build a semi-empirical model able to predict the thermal conductivity and each heat transfer mechanism contribution. To do this, a novel method to determine the solid structure factor from the slope of the thermal conductivity versus the temperature curve is introduced.