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So far, negative Poisson’s ratio foam (auxetic foam) is usually prepared by the transformation of conventional polymer foam, and it is a challenge to prepare auxetic foam directly from polymer resin. In this work, a novel method based on the synergism between the phase transition of water and the permeability rate difference of the CO2blowing agent from air in the foaming process is put forward to prepare auxetic foam directly from the polymer resin. Using this method, herein, nylon elastomer (NE) foam with auxetic behavior is successfully prepared from NE resin by a one-pot CO2foaming method inside an autoclave aided by water; the obtained auxetic foam has a Poisson’s ratio value of −1.29 and has excellent tensile cycle stability and energy absorption performance. This method breaks through the bottleneck for preparing auxetic foams from the polymer resin directly, which is also green and environment-friendly. It is believed that the abovementioned method also applies to other thermoplastic polymers, which paves a way for designing and preparing multifunctional auxetic foam materials in the future via a one-pot CO2foaming process. Furthermore, this work first demonstrates that the transformation between auxetic foam and positive Poisson’s ratio foam is reversible due to the closed cell structure, which provides an opportunity to reversibly adjust the performance and the shape of polymer foam materials.

How to realize the recycling of materials is an important topic for polymer materials, especially crosslinked polymers. In this work, by means of the reaction of zinc oxide (ZnO) and maleic anhydride group, ethylene-vinyl acetate copolymer (EVA)/ZnO composites with reversible ion-crosslinking structure were prepared by melt mixing method. The gel content and the degree of ion crosslinking in EVA were tracked using the THF extraction method. The presence of ion-crosslinking structure strongly affected the foaming behavior of EVA composites in supercritical CO2(sc-CO2) molding foaming process. The influence of the ion crosslinking on the structure and properties of EVA foams was studied by means of SEM and compression set test etc. The results showed that the appropriate degree of ion-crosslinking was beneficial to increase the volume expansion rate and cell density of the obtained foams, and reduce the average cell diameter and compression set of the foams. The obtained EVA composite foams showed good resilience and melt-reprocessing performance. However, too high ion-crosslinking degree severely limited the growth of cells due to the difficulty of molecular chain movement. At the same time, the excessive ZnO in the matrix had an adverse effect on the compression set of EVA foam.