Long Functional Graft Polymer Induced Multiscale Morphological Inversion and Enhanced Anion Transport Efficiency in Radiation-Grafted Anion Exchange Membranes

Abstract

We comprehensively investigate the hierarchical structures of the previously reported radiation-grafted anion-exchange membranes (AEMs), consisting of p-(2-imidazoliumyl) styrene graft-polymers onto poly(ethylene-co-tetrafluoroethylene) (ETFE) films with different grafting degrees (GD), using the partial scattering function (PSF) analysis. We first confirm the existence of a characteristic GD (GD_c) of 20–25%, over which AEMs show enhanced anion transport efficiency. To clarify the structure features in correlation with GD_c, two AEMs with GD of 14% and 42% (AEM14 and AEM42) are selected for PSF analysis, where AEMs are treated as a 3-component system, containing EFTE base polymer (BP), graft polymer (GP), and water (W). Our results reveal the two different microphase inversions across GD_c in tens of nanometers and a few nanometers ranges, respectively. In the large scale (>10 nm), at GD < GD_c (i.e., AEM14), the hydrophilic GP/W domains and dry GP domains coexist together, which are spherically shaped with an average radius of ∼11.5 nm, dispersing in the BP matrix. At GD > GD_c (i.e., AEM42), a network structure is formed by irregularly shaped BP domains with a radius of gyration of ∼9.5 nm dispersed in GP/W matrix (hydrophilic/hydrophobic phase-inversion compared to AEM14). In the small scale (<10 nm), ion-channel inversion from water-in-polymer to polymer-in-water in the hydrophilic region is confirmed by the bicontinuous local structures shown in AEM14 and AEM42, with GP or W being the rich phase, respectively. The extraction of these unique structure factors enables the prediction of scattering intensity profiles of AEMs with GD > 42%.