Investigation of Domain Size in Polymer Membranes Using Double-Quantum-Filtered Spin Diffusion Magic Angle Spinning NMR

Solid-state ¹H magic angle spinning (MAS) NMR was used to investigate sulfonated Diels−Alder poly(phenlylene) polymer membranes. Under high spinning speed ¹H MAS conditions, the proton environments of the sulfonic acid and phenylene polymer backbone are resolved. A double-quantum (DQ) filter using the rotor-synchronized back-to-back (BABA) NMR multiple-pulse sequence allowed the selective suppression of the sulfonic proton environment in the ¹H MAS NMR spectra. This DQ filter in conjunction with a spin diffusion NMR experiment was then used to measure the domain size of the sulfonic acid component within the membrane. In addition, the temperature dependence of the sulfonic acid spin−spin relaxation time (T₂) was determined, providing an estimate of the activation energy for the proton dynamics of the dehydrated membrane.