Poly(lactic acid) (PLA) is a kind of bio-based and biodegradable polymers. PLA foams with high expansion ratio have great application potential in heat insulation, adsorption and other areas. However, due to its low melt strength and slow crystallization rate, linear PLA has poor foaming ability, so it is hard to prepare PLA foams with high expansion ratio. Although chain branching, blending, and other methods have been utilized to improve PLA's melt strength and foaming ability, they easily destroy PLA's biodegradability, cause chemical pollution, and raise production costs. In this study, a new supercritical fluid foaming process, based on pre-isothermal cold crystallization, was proposed to prepare PLA foams with high expansion ratio. To improve PLA's melt strength and foaming ability, a pre-isothermal treatment was applied to induce sufficient cold crystallization. According to the SEM results, the cold crystallization of PLA becomes more sufficient and the crystal morphology becomes more perfect with the pre-isothermal treating. The DSC and WAXD results confirm that, the pre-isothermal treatment remarkably promotes the PLA's cold crystallization, and endows the PLA sample higher crystallinity and more perfect crystalline structure. Moreover, the high-pressure rheology testing results indicate that the pre-isothermal treatment improves the PLA's melt viscoelasticity significantly. Finally, the foaming results show that the pre-isothermal treatment significantly enhances the PLA's foaming ability. With the pre-isothermal treatment, the PLA's maximum expansion ratio increases from 6.40-fold to 17.7-fold, and the uniformity of cellular structure is also improved obviously. The new process provides a green, flexible, and low-cost way to prepare fully biodegradable PLA foams with high expansion ratio.

聚乳酸（PLA）是一种生物基可生物降解的聚合物。具有高膨胀率的PLA泡沫在隔热，吸附等领域具有巨大的应用潜力。但是，由于线性PLA的熔融强度低，结晶速度慢，因此发泡能力差，因此难以制备膨胀倍率高的PLA泡沫。尽管已采用链支化，共混和其他方法来提高PLA的熔体强度和发泡能力，但它们很容易破坏PLA的生物降解性，引起化学污染并提高生产成本。在这项研究中，提出了一种基于预等温冷结晶的超临界流体发泡新工艺，以制备具有高膨胀率的PLA泡沫。为了提高PLA的熔体强度和发泡能力，进行预等温处理以引起足够的冷结晶。根据SEM结果，通过等温预热处理，PLA的冷结晶变得更加充分，并且晶体形态变得更加完美。DSC和WAXD结果证实，等温预处理显着促进了PLA的冷结晶，并赋予了PLA样品更高的结晶度和更完美的晶体结构。此外，高压流变测试结果表明，等温预处理可以显着提高PLA的熔体粘弹性。最后，发泡结果表明预等温处理显着增强了PLA的发泡能力。经过预等温处理，PLA的最大膨胀率从6.40倍增加到17.7倍，细胞结构的均匀性也明显提高。新工艺提供了一种绿色，灵活且低成本的方法来制备具有高膨胀率的可完全生物降解的PLA泡沫。