Microporous materials with negative Poisson's ratios. I. Microstructure and mechanical properties

Abstract

A microporous, anisotropic form of expanded polytetrafluoroethylene has been found to have a large negative major Poisson's ratio. The value of Poisson's ratio varies with tensile strain and can attain values as large as -12. The microporous structure of the material is described and the mechanisms that lead to this large negative Poisson's ratio are identified. Micro-rotational degrees of freedom are observed, suggesting that a micropolar elasticity theory may be required to describe the mechanical properties.

Negative Poisson's Ratio Behavior Induced by an Elastic Instability

“…Examples of this are provided by the discovery that metals with a cubic lattice [8] , natural layered ceramics [9] , ferroelectric polycrystalline ceramics [10] and zeolites [11] may all exhibit negative Poisson's ratio behavior. Moreover, several geometries and mechanisms have been proposed to achieve negative values for the Poisson's ratio, including foams with reentrant structures [1] , hierarchical laminates [12] , polymeric and metallic foams [13] , microporous polymers [14] , molecular networks [15] and manybody systems with isotropic pair interactions [16] . Negative Poisson's ratio effects have also been demonstrated at the micron scale using complex materials which were fabricated using soft lithography [17] and at the nanoscale with sheets assemblies of carbon nanotubes [18] .…”

Effect of heterogeneity on the elastic properties of auxetic materials

“…6 Conversely, some auxetic materials can produce a Poisson's ratio as large as Ϫ12. 7 In 1987, Lakes 8 fabricated a polymeric foam that exhibited a negative auxetic behavior. There are many other materials ͑real and hypothetical͒ that can also exhibit auxetic behavior.…”

Effect of nodule shape for modeling of auxetic microporous polymers

“…Ever since the pioneering works on auxetic materials by Lakes et al [1,2], Evans et al [3,4] and Wojciechowski et al [5,6], the field of auxetic materials has gained considerable prominence. The various geometrical models, experimental verifications and proposed applications have been summarized by Lim .…”