Anisotropic polyurethane foam

Anisotropic polymer foams have been prepared, which exhibit a Poisson's ratio exceeding 1, and ratios of longitudinal to transverse stiffness exceeding 50. The foams are as much as 20 times stiffer in the longitudinal direction than the foams from which they were derived. The transformation process involved applying a uniaxial stress sufficient to produce 25% to 45% axial strain to open-cell polyurethane foam, heating above the softening point, followed by cooling under axial strain. 1 . Introduction A cellular material is one made up of an interconnected network of solid struts or plates which form edges and faces of cells. It may be viewed as a composite consisting of a solid phase and empty space or a fluid phase such as air. Cellular solids have served structural roles in nature as honeycombs, bone and coral skeletons, for millions of years. Man, on the other hand, has only recently begun to realize the potential of these materials and has made an attempt to utilize their structure-property relations in practical applications. Cellular solids, including foams, are very efficient structures in terms of optimizing strength and stiffness with respect to weight. Foam materials have often been called "nature's equivalent of the I-beam" [1], and are commonly employed in cushioning, insulating, padding and packing et al. Encouraged by the engineering potential of cellular materials, one is motivated to understand the mechanical behavior of the cellular solid. All commonly known cellular materials (naturally existing and man-made), have a convex cell shape and exhibit a positive Poisson's ratio which is defined as the negative of the lateral strain divided by the axial strain when a load is applied in an axial direction. Such materials undergo a lateral contraction in response to an axial stretch, and a lateral expansion when subjected to axial compression. Therefore, for all ordinary materials, Poisson's ratio has a positive value. For reference, typical Poisson's ratios for some common material are 0.5 for rubbers, 0.33 for aluminum, 0.27 for most steels, 0.1 to 0.4 for typical polymeric foams, and nearly zero for cork [2]. The theoretical allowable range of Poisson's ratio for isotropic materials in three dimensions is -1 to 0.5 as demonstrated by energy arguments [3]. An isotropic material with negative Poisson's ratio, however, was not believed to exist until recent work by Lakes [4]. The fabrication was achieved through a transformation of the cell structure from a convex polyhedral shape to a concave shape. These types of foam samples having a negative Poisson's ratio have been termed "re-entrant" due to their macrostructural appearance and behavior. Prior experiments in preparing and studying re-entrant polyurethane foam (Scott industrial foam) [4, 5, 6], silicone rubber foam [5] and metal foam [4, 5, 7] dealt with open-cell foam. Different techniques were used for each of these materials. In this study, an open-cell polyurethane foam was strained in tension at elevated temperature, resulting in characteristic permanent transformations. Scott Industrial foam with a pore size 0.4 mm (65 pores per inch (ppi)) was used. This is similar, except for pore size, to the Scott foam (25 ppi) used in earlier studies of creation of negative Poisson's ratio foam [4, 5, 6]. This procedure resulted in completely different properties than those achieved in compressed