One-pot preparing EVA elastomer auxetic foam through stress relaxation of molecular chains by heat annealing

It is a challenge to prepare closed-cell auxetic foam by a simple method without destroying the properties of the foam. In this paper, auxetic foams were prepared directly from ethylene-vinyl acetate copolymer (EVA) resin in one step by annealing immediately after sc-CO2 foaming. The effects of annealing time and foaming temperature on the cell structure and Poisson's ratio were investigated using scanning electron microscopy (SEM) and Poisson's ratio testing. The reasons for the formation of EVA foams with negative Poisson's ratio were analyzed by dynamic mechanical thermal analysis (DMA) and differential scanning calorimetry (DSC). It was found that annealing treatment maintained the high movement ability of molecular chains and provided sufficient time for the molecular chains to relax, so that the stress generated by the foaming process can relax quickly and reach a new stable state, thus weakening the elastic shrinking of EVA foam. Under the action of the pressure difference between the inside and outside of the cell formed by the different diffusion rates of air and CO2, the cell concaves inward to produce a reentrant structure, and then the foam shape is fixed with the continuous improvement of crystallization. The obtained EVA auxetic foam has excellent resilience. Finally, it is proved that the method has good universality for the preparation of auxetic foams with most elastomers, so this study has important implications for the preparation of closed-cell elastomer polymer auxetic foams.