Relationship between long-range diffusion and diffusion in the ZIF-8 and polymer phases of a mixed-matrix membrane by high field NMR diffusometry

Abstract

Pulsed field gradient (PFG) NMR is applied to resolve different modes of ethene self-diffusion inside a mixed-matrix membrane (MMM) consisting of ZIF-8 particles embedded in 6FDA–DAM polymer: (i) diffusion inside the ZIF-8 particles, (ii) diffusion inside the surrounding polymer matrix, and (iii) diffusion under conditions of an exchange between the particles and the polymer over length scales smaller than and comparable with the membrane thickness. This resolution is achieved using C-13 PFG NMR studies at a high magnetic field of 17.6T and large magnetic field gradients up to 30T/m. C-13 PFG NMR was also applied to measure ethene diffusion in the constituent components used in the MMM formation: a bed of ZIF-8 particles and a pure 6FDA–DAM polymer. It is shown that the comparison of all the measured diffusion data elucidates details of sorbate diffusion and diffusion mediated exchange dynamics between the ZIF-8 particles and the polymer in the MMM. An exchange model is applied to connect the long-range diffusivity measured for displacements larger than the size of ZIF-8 particles in the MMM with the diffusivities measured inside the ZIF-8 particles and the polymer.