Expanded polytetrafluoroethylene as an auxetic material: effect of extension ratio on its structure and properties

Abstract

The effect of extension ratio on the structure and property of expanded polytetrafluoroethylene (ePTFE) has been decisively studied. ePTFE is unique with its porous fibrillar network structure connected by nodes and also with its degree of orientation which increases with increases in extension ratio. It is found that the Poisson’s ratio of ePTFE is negative, and it is reduced dramatically with increased extension ratio. At the extension ratio of 30, the Poisson’s ratio reaches − 10.00. By changes of extension ratio from 1 to 10, the segmental mobility of polymer chains becomes harder, and the dynamic glass transition temperature (T g) is increased. Broad glass transition regions are revealed in tan δ versus temperature curves in polytetrafluoroethylene specimens stretched 20- (S20) and 30-fold (S30). As the extension ratio is increased, the thermal conductivity of the specimens is decreased. The thermal conductivity coefficient is dropped to 0.051 W m−1 K owing to the appearance of more pores and low degree of crystallinity at higher extension ratio. The properties of ePTFE modified by carbonyl iron powder (CIP) have been investigated. CIP has affected the structure and properties of modified ePTFE. CIP-modified ePTFE also exhibits a negative Poisson’s ratio which is larger than ePTFE at the same extension ratio. The modified ePTFE reveals higher thermal conductivity coefficient than ePTFE itself.