Evaluation of the plasticization of ion-selective electrode membranes by pulsed NMR analyses

The potentiometric polymeric membranes for ion-selective electrodes were evaluated by analyses of the proton spin-spin relaxation times T2 with pulsed NMR. The T2 measurements were performed using the Hahn-Echo, Solid-Echo and CPMG pulse sequences. The T2 values and fractions F of each component were obtained by analyses of the FID signals measured with the Hahn-Echo pulse sequence. The softer potentiometric polymeric membrane possessed the main fraction F(L), providing a relatively longer T(2L) value. A linear relationship existed between the weight ratio of the membrane solvent and ln T(2L) (or ln total T2×F). This analysis method could quantify the degree of hardness or softness of the potentiometric polymeric membranes with the differences in the membrane solvent weight. The normalized derivative spectra were acquired from the transverse magnetization M(t) data measured by using the Solid-Echo and CPMG pulse sequences. In the normalized derivative spectra of the potentiometric polymeric membranes, most PVC peaks in the short time region shifted to a larger area of long time regions by plasticization, and the softer potentiometric polymeric membrane incorporating more membrane solvent exhibited a relaxation peak in the relatively longer time region. Thus, the normalized derivative spectra were effective in elucidating the compatibility of the PVC with the membrane solvent.