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Manufacture of thermoplastic foams with a fine cellular structure (a higher expansion ratio, a higher cell density, and smaller cell sizes) is challenging work due to the weak viscoelastic behavior and the unsuitable crystallization behavior of common thermoplastic materials. In this work, a novel method of making microcellular foams with micro‐/nano‐fibrillar reinforced polymeric composites (M/NFC) is introduced, which shows various advantages compared to conventional foams. The M/NFC foams have improved cellular structures, excellent mechanical properties, and enhanced thermal insulation properties, which make them popular candidates for structural applications and insulative products. Various methods to manufacture of M/NFC foam are summarized. To understand the fundamental mechanisms of the foaming enhancement by incorporating micro‐/nano‐size fibrils, the rheological and crystallization behavior of the M/NFC are analyzed. It is shown that the micro‐/nano‐fibrils can strengthen the melt strength, induce faster crystallization, and increase the number of crystals. Due to the improvement of the cell morphology and the stiffness of the cell walls, the reinforced foams have superior mechanical properties. A hierarchically porous structure in high expansion ratio reinforced foams has also been developed. It is believed that the nano‐size holes in the cell walls can further reduce the thermal conductivity of the foams.

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