The process of open-porous structure development in high-density polyethylene (HDPE) films during uniaxial deformation in supercritical carbon dioxide (SC-CO₂) fluid at 35 °C and 10 MPa has been studied and visualized by means of atomic force microscopy. We suggest that the supercritical fluid act as adsorption-active medium, and the porous structure is developed via the crazing mechanism due to the increasing the distance between of lamellae and the formation of oriented separate fibrils in the intercrystallite space. Effective bulk porosity of the films has been up to 40%. Small-angle X-ray scattering studies and ethanol permeability measurements have revealed that the pores and fibrils are about 10 nm in diameter. The prepared nanoporous materials exhibit good vapor permeability. Structural and mechanical behavior of the prepared porous films has been investigated. Large reversible deformation (up to 80%) of HDPE in the SC-CO₂ has been observed. Repeated drawing of the shrunk films in air under ambient conditions has led to the open-porous structure recovery.

中文翻译：

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超临界二氧化碳拉伸过程中高密度聚乙烯纳米孔结构的演变

高密度聚乙烯（HDPE）薄膜在超临界二氧化碳（SC-CO ₂）已通过原子力显微镜对35°C和10 MPa的流体进行了研究和可视化。我们认为，超临界流体起着吸附活性介质的作用，并且由于层间距离的增加和晶间空间中定向分离纤丝的形成，多孔结构是通过裂纹机理形成的。薄膜的有效本体孔隙率已高达40％。小角X射线散射研究和乙醇渗透性测量表明，孔和原纤维的直径约为10 nm。制备的纳米多孔材料表现出良好的蒸气渗透性。已经研究了所制备的多孔膜的结构和机械行为。HD-PE在SC-CO _2中发生大的可逆形变（高达80％）已经观察到。在环境条件下在空气中反复拉伸收缩薄膜会导致开孔结构的恢复。