Novel Method for Preparing Auxetic Foam from Closed-Cell Polymer Foam Based on the Steam Penetration and Condensation Process.

Auxetic materials are a class of materials possessing a negative Poisson's ratio. Here, we established a novel method for preparing auxetic foam from closed-cell polymer foam based on the steam penetration and condensation (SPC) process. Using polyethylene (PE) closed-cell foam as an example, the foams treated by the SPC process presented a negative Poisson's ratio during stretching and compression testing. The effect of steam-treated temperature and time on the conversion efficiency of negative-Poisson's ratio foam was investigated, and the mechanism of the SPC method for forming a reentrant structure was discussed. The results indicated that the presence of enough steam within the cells was a critical factor for the negative Poisson's ratio conversion in the SPC process. The pressure difference caused by steam condensation was the driving force for the conversion from conventional closed-cell foam to the negative-Poisson's ratio foam. Furthermore, the applicability of the SPC process for fabricating auxetic foam was studied by replacing PE foam by polyvinyl chloride foam with a closed-cell structure or replacing water steam by ethanol steam. The results verified the universality of the SPC process for fabricating auxetic foams from conventional foams with a closed-cell structure. In addition, we explored the potential application of the obtained auxetic foams by the SPC process in the fabrication of shape-memory polymer materials.