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Materials 2021, 14, 1617 13 of 20 tion of the hydrophilic blocks. It is noteworthy to point out that the decreased mobility of the hydrophilic blocks could also impede the segmental motion of the hydrophobic blocks; contravening the increase in hydrophobic mobility due to the ethanol solvation effect. For example, at high ethanol concentrations (8 M) the reduced water content in the hydrophilic domains constrains indirectly the segmental motion of the hydrophobic blocks, and inhibited the transport of ethanol molecules through the hydrophobic domains of the diblock ionomer as a result. However, at low ethanol concentrations (1 M), the water content in the solution is sufficient to cause excessive membrane swelling due to the hydrophilic-block-induced segmental movement of the hydrophobic blocks. Although the presence of water in PEM is critically important for proton transport, excess water can cause undue swelling and increase alcohol permeability. Hence swelling is more severe with the higher the hydrophilicity of the ionomer (at a high sulfonic acid content). In general, alcohol permeability and water uptake of ionomer membranes are positively correlated. Nafion membranes, for instance, have been known to have high alcohol permeability and significant water uptake. High alcohol permeability could, in principle, be suppressed by designing an ionomer structure that minimizes water uptake and enhances the cohesive forces in the hydrophobic domains to oppose swelling. Nevertheless, for the P(AN-co-GMA)-b-SPM diblock ionomer membranes, an alcohol permeability lower than that of Nafion was still possible at water uptakes much higher than Nafion (Table 2). These observations suggest that the diblock ionomer membranes have stronger intrinsic alcohol resistance, not only due to the use of alcohol-resistant constituents (PAN and acrylates) and hydrophilic covalent cross-links, but also a domain structure through the diblock design). The domain structure imparted more regularity in the membrane structure through the formation of connected hydrophilic channels. Each domain consists of several AxGy blocks, in which strong association of the AN segments and covalent cross-linking bonds between the GMA units provided mechanical stability as well as low alcohol permeability. It has been demonstrated that a membrane which is cross-linked to constrain swelling can effectively also reduce alcohol permeation [11,23]. Such was the benefit of hydrophilic covalent cross-linking in the current ionomer design. 3.3. Microstructures of the Dual Phase Membrane Electron microscopy was used to examine the microstructures of membrane, and from which to infer some the property changes in ionomers with different hydropho- bic/hydrophilic ratios. From Table 2 one can conclude that room temperature proton conductivity and water uptake decreased with the increase in hydrophobic/hydrophilic ra- tio. A longer hydrophobic block, such as that in A150G4S-10, is expected to form larger and more rigid hydrophobic domains and more likely to cause discontinuities in the hydrophilic domains. On the contrary, the A50G4S-10 membrane with a shorter hydrophobic block would form less rigid hydrophobic domains and indirectly improved the connectivity and continuity of the hydrophilic domains. The hypothesis that the hydrophobic/hydrophilic ratio affected the formation of different ionic aggregates was tested by examining the cross-sections of the diblock membrane with FE-SEM (Figure 11). The images show the nano-domains (≈30–50 nm) morphology in all the membrane specimens. It is also worthy of note that A100G4S-10 presents the finest phase separation morphology, meaning that GMA units are most uniformly distributed in the hydrophobic A100G4 block. Thus, the crosslinking could most effectively limit the aggregation of AN segments, and hence the highest boundary region where the hydrophilic S block locates as illustrated (yellow) in the inset of Figure 12. There are both boundaries and necks between domains [24]. The boundary phase is regarded as primary hydrophilic channels whereas the necks as the secondary hydrophilic channel and the hydrophobic coalescing joints. In addition, with the increase in the dose of the hydrophobic block to x = 100, the boundary phase charac- teristic fades away and the domain sizes reduce, implying more hydrophilic coalescence occurs. Finally, when x = 150, the membrane reveals pits where the loci are formed of the aggregated hydrophilic blocks because the dilution of hydrophilic crosslinks. AdditionalPDF Image | Hydrophilic Cross-Linked Aliphatic Hydrocarbon Diblock Copolymer
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