Open Photoacoustic Cell study of thermal diffusivity of Nafion® as a function of water content

Abstract

The Dupont Nafion® (acid polymer perfluorsulfonic/PTFE copolymer in an acid form H + ) is derived from Teflon® (PTFE) and both present great similarities. In order to study thermal properties of the Nafion®, we used theopen photoacoustic cell (OPC) to measure the longitudinal α⊥ and the thermal lens technique (TLS) for the planar direction, α H . From OPC experiment it was found an averaged (Nafion® in nature) value α⊥ = (6.5 ′ 0.4) x 10 - 4 cm 2 /s whilst TLS gave α H = (7.0 ′ 0.2) x 10 - 4 cm 2 /s. The value found, α⊥ ∼ α H , means that there is no significant anisotropy in the Nafion® foil. Water content dependence of thermal diffusivity for ten samples dipped in water showed a minimum near 4 h (∼ 4.5 x 10 - 4 cm 2 /s) and goes to a saturating value after 28 h (∼ 5.5 x 10 - 4 ). On the other hand, in experiment where the sample is left to dry in the ambient air, α⊥ present a transitional behavior near 5 h off water (∼ 6.0 x 10 - 4 ) and diminish monotonically after 28 h. It is believed that thermal diffusivity can be related with water content and it may be helpful in the understanding of permeation in the Nafion® membrane.