Re-entrant transformation methods in closed cell foams

Open-cell foams have been transformed in prior studies, to a re-entrant (inward bulging) cell structure which gives rise to a negative Poisson's ratio. Different methods were used for polymer foams with and without a softening point. In this study, methods based on air pressure were developed for the transformation of closed cell polymer foams. Low density polyethylene foam was successfully transformed, however polymethacrylimide foam was densified without transformation.