Low-density and structure-tunable microcellular PMMA foams with improved thermal-insulation and compressive mechanical properties

Polymer foams play an increasingly significant role in the field of thermal insulation due to their low thermal conductivities. As thermal insulation materials, their performances are primarily determined by the cellular structure. However, the correlation between the foam's properties and its cellular structure is still not fully understood. This greatly limits the development of high-performance polymer foams with optimal cellular structures. Hereby, we report the fabrication of poly (methyl methacrylate) (PMMA) foams with widely tunable cellular structures by using CO2 as the blowing agent. In particular, the microcellular PMMA foam with a void fraction of 0.956 and with an average cell size of 4.7 pm was obtained, which, to the best of knowledge, is by far the largest void fraction of polymer foam with a cell size of less than 5 gm. The PMMA foam presents an excellent thermal-insulation behaviour with a thermal conductivity of as low as 29.9 mW/m K. Meanwhile, the PMMA foam exhibits excellent mechanical properties due to its extremely small cell sizes. Moreover, the dependences of thermal and mechanical properties on cellular structure are obtained by independently analyzing the effects of cell size and void fraction. All these results demonstrate a promising method to fabricate environmentally friendly and economical thermal insulation materials with improved thermal-insulation and compressive mechanical properties.