Effect of High‐Pressure Carbon Dioxide on the Isothermal Crystallization of Thermoplastic Polyurethane

ABSTRACT Thermoplastic polyurethane (TPU) foams are widely used because of their unique properties, while the crystallization behavior and crystal structure of TPU are critical to determining foam cellular structure and final performance. However, the role of high‐pressure CO 2 in the isothermal crystallization of TPU remains unclear. In this work, the isothermal crystallization of TPU under high‐pressure CO 2 at different temperatures was investigated using both high‐pressure and regular differential scanning calorimetry (DSC). The results showed that the effect of CO 2 pressure on TPU crystallization strongly depended on isothermal temperature. At low temperatures, the crystallinity first increased and then decreased with increasing CO 2 pressure, whereas at high temperatures it generally increased with pressure. These changes were governed by the combined effects of CO 2 plasticization and supercooling degree. The synergistic effects of temperature and CO 2 pressure on the crystallization of hard segment domains were further clarified for TPUs with different initial crystallinity. More importantly, the results indicate that the CO 2 ‐controlled crystallization behavior can alter the perfection, packing state, and growth mode of hard‐segment crystals, which is expected to further affect cell nucleation, cell growth, and foam stability during TPU foaming. Therefore, this study provides a useful basis for understanding and regulating foam structure and properties through crystallization control under high‐pressure CO 2 .