Manipulating the cellular microstructure is an important means to alter the mechanical properties and expand the application of polymer foams. Herein, a vacuum-counter pressure-assisted supercritical CO foaming (V-CP foaming) process was developed to fabricate polypropylene (PP) foams with closed re-entrant cell structures. The re-entrant cellular structure was realized by the combined effect of the cell over expansion during the vacuum treatment stage right after the depressurization, and the rapid cell contraction under the counter pressure of the refilled air. The morphology of the re-entrant structure could be tuned by adjusting the key processing parameters. By using butanol and scCO as the co-blowing agent, the over expansion and rapid contraction of cells were greatly amplified due to the plasticizing, vaporization, and liquidation of the butanol in the V-CP process. The fabricated PP re-entrant foams that have a negative Poisson's ratio possess prominent compressive properties and impact energy absorption performance.

中文翻译：

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真空反压辅助超临界CO2发泡实现高吸能聚丙烯再入泡沫

调控泡孔微观结构是改变聚合物泡沫力学性能和扩大其应用的重要手段。在此，开发了真空反压辅助超临界CO发泡（V-CP发泡）工艺来制造具有闭孔结构的聚丙烯（PP）泡沫。通过减压后真空处理阶段细胞过度膨胀和在重新填充空气的反压下细胞快速收缩的综合作用，实现了再入细胞结构。凹状结构的形态可以通过调整关键加工参数来调整。采用丁醇和 scCO 作为共发泡剂，由于 V-CP 过程中丁醇的塑化、汽化和液化，细胞的过度膨胀和快速收缩被大大放大。所制备的具有负泊松比的PP可再入泡沫具有突出的压缩性能和冲击能量吸收性能。