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.

For pt.I see ibid., vol.22, p.1877-82 (1989). In a previous paper the morphology of a microporous material made from expanded poly(tetrafluoroethylene) was described and results presented for its mechanical behaviour. The material was shown to be highly anisotropic and exhibited a large negative Poisson's ratio. In this paper a simple model for the microstructure is described to account for this effect. The model is based on an interconnected array of anisotropic particles that deforms so as to produce a large transverse displacement under longitudinal tensile loading. Very good agreement is found between the model and experimental results, providing an explanation for the variation of Poisson's ratio with tensile strain, in terms of changes in material morphology.

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