PNNL

Abstract

The micro-sized pore parameters, such as pore size and distance between pores in a series of model EPDM rubbers, were determined in situ under the pressure of 500 psi using 129Xe nuclear magnetic resonance (NMR) techniques: spin-lattice (T1) and spin-spin (T2) relaxation measurements, pulsed-field gradient (PFG) NMR, and 2-dimensional exchange spectroscopy (2D EXSY). The T1/T2 (>>1) ratio for the xenon confined in the pores is larger than for non-confined free xenon. This suggests that almost the entire pore surface interacts with xenon atoms like a closed pore. While these pores still connect each other through very narrow diffusion/exchange channels, it is possible to observe the echo decay in PFG-NMR and cross-peaks in 2D EXSY. The results show that both diffusion (Dpore ˜ 2.1 × 10-10 m2/s) and exchange (ex-change rate, texch = a few tens of milliseconds) of xenon between a pore within the material and outer surface are prolonged. The exchange distances (l), which corresponds to the xenon gas penetration depth, were estimated to be 70 – 100 µm based on the measured diffusion coefficients and exchange rate (1/texch). NMR diffraction analysis reveals that the pore sizes (a) and the pore distances (b) are on the order of magnitude of micrometers and tens of micrometer while the diffusion coefficients of xenon gas in the diffusion channels (Deff) is about10-8 m2/s. Overall, this study suggests that the pores with a few micrometers and connected through very narrow flowing channels with the length of several tens of micrometers are developed from 70 - 100 µm below the rubber surface. Furthermore, the overall steady-state diffusion of xenon is slower, approximately two orders of magnitudes, than the diffusion in the channel between the pores. The pore and exchange distances correlated with the composition of rubbers showed that the properties of EPDM rubber as a high-pressure gas barrier could be improved by reducing the size of cracks and the depth of gas penetration by the addition of both carbon black and silica fillers.