Microsized Pore Structure Determination in EPDM Rubbers UsingHigh-Pressure Xe-129 NMR Techniques

Microsized pore parameters, such as pore size anddistance between pores in a series of model EPDM rubbers, weredeterminedin situunder the pressure of 500 psi using129Xe nuclearmagnetic resonance (NMR) techniques: spin-lattice (T1) andspin-spin (T2) relaxation measurements, pulsed-field gradient(PFG) NMR, and two-dimensional exchange spectroscopy (2DEXSY). TheT1/T2(???1) ratio for the xenon confined in the poresis larger than that for nonconfined free xenon. This suggests thatalmost the entire pore surface interacts with xenon atoms like aclosed pore. While these pores still connect each other throughvery narrow diffusion/exchange channels, it is possible to observethe echo decay in PFG-NMR and cross-peaks in 2D EXSY. Theresults show that both diffusion (Dpore approximate to 2.1x10-10m2/s) andexchange (exchange rate,Tau exch= a few tens of milliseconds) of xenon between a pore within the material and outer surface areprolonged. The exchange distances (l), which correspond to the xenon gas penetration depth, were estimated to be 70-100 mu mbased on the measured diffusion coefficients and exchange rate (1/Tau exch). NMR diffraction analysis reveals that pore size (a) andpore distance (b) are on the order of magnitude of micrometers and tens of micrometers, while the diffusion coefficients of xenongas in the diffusion channels (Deff) are about 10-8m2/s. Overall, this study suggests that the pores with a few micrometers connectedthrough very narrowflowing channels with the length of several tens of micrometers are developed 70 to 100 mu m below the rubbersurface. Furthermore, the overall steady-state diffusion of xenon is slower, approximately 2 orders of magnitudes, than the diffusionin the channel between the pores. The pore and exchange distances correlated with the composition of rubbers showed that theproperties of EPDM rubber as a high-pressure gas barrier could be improved by reducing the size of cracks and the depth of gaspenetration by the addition of both carbon black and silicafillers.