Auxetic Closed Cell Nylon Foams Produced by Vacuum and Mechanical Compression (VMC)

This research presents a method to convert a conventional Nylon closed cell foam, with a density of 48 kg m^{- 3}, into auxetic metamaterials with densities ranging between 68 and 138 kg m^{- 3}. The samples are produced by applying vacuum and mechanical compression techniques. The study reports on the effect of processing parameters such as vacuum time, temperature, and mechanical pressure. Under optimized conditions, the resulting auxetic foams exhibit a tensile Poisson's ratio as low as -0.86, while the minimum compressive Poisson's ratio for the same sample reached -0.16. Based on morphological analyses via scanning electron microscopy (SEM), structural changes induced by the treatment are determined to relate with tensile and compressive properties, including modulus and strength, as well as the analysis of the stress level for different strains. The characterizations also include differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) for the Nylon foam before and after conversion. These findings underscore the potential applications of these auxetic foams in sports and military protective gears, as well as energy dissipation systems.