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Materials 2021, 14, 1617 6 of 20 flow rate of 50 cc/min. The DMFC performance was evaluated at two temperatures: 30 and 50 ◦C. As part of the cell conditioning, the cell was rested in the open circuit condition for 30 min before measurements were taken. 3. Results and Discussion 3.1. Characteristics of the Diblock Ionomer Membranes It has been hypothesized that high proton conductivity and good mechanical prop- erties of many of the state-of-the art PEMs are consequential upon an ordered polymer structure [14]. In Nafion, for example, the perfluorosulfonate side chains are grafted onto the poly(tetrafluoro ethylene)-main chains at nearly regular intervals. This unique chain structure, with large hydrophobicity-hydrophilicity difference between different parts of the polymer, drives phase separation at nanoscale upon hydration, forming ex- tensively connected proton-conducting channels throughout the membrane matrix [15]. Similarly, alternative aliphatic PEMs prepared from an ordered polymer structure, such as the diblock, multiblock or branch/graft structure, also showed good PEM properties as a result of hydrophobicity–hydrophilicity-driven phase separation [16]. The extensive- ness of the connectivity of the hydrophobic and hydrophilic domains is determined by the extent and the length scale of phase separation which should be dependent on the hydrophobicity–hydrophilicity difference in the ionomer. There are two hurdles in replicating a Nafion-like architecture in nonfluorinated hy- drocarbon polymers such as aliphatic diblock ionomers with alternating hydrophobic and hydrophilic blocks. Firstly, the perfluorocarbon chains of Nafion are far more hydrophobic than any other hydrocarbon chains. Hence the hydrophilicity–hydrophobicity driven phase separation in hydrocarbon-based ionomers would not be as extensive and would not occur at the same small length scale as that in Nafion. The proton conducting channels in the former are therefore more tortuous and there is more likelihood for partial blockages [17]. Secondly, the hydrophilic segment of Nafion is much shorter than its perfluoro block and the overall molecular weight of Nafion is also far smaller than what hydrocarbon ionomers usually should have, so that Nafion molecules could readily undergo self-assembling to generate highly divided double continuous phases. To circumvent the hurdles, the P(AN-co-GMA)-b-SPM diblock ionomer (Figure 2), synthesized by an ATRP technique in an EC/DMSO dual-solvent system, lays out the bicontinuous hydrophilic and hydrophobic domains due to the orderliness in a block structure with alternating hydrophobic and hydrophilic properties. In-situ crosslinking was introduced during membrane casting to form a covalently cross-linked network with hydrophilic character (Figure 3). A designated cross-linker such as an organic diamine [R(NH2)2], EDA, was added to the casting solution. Upon drying the cast membrane, the reaction between the diamine molecules and the pendant epoxide groups of GMA established a covalently bonded network between the hydrophobic (poly(acrylonitrile) (PAN)-co-GMA) blocks, allowing freestanding membranes to be made. Acronym: AxGyS–10; Acrylontrile Glycidyl methacrylate Sulfopropyl methacrylate, where x and y are millimole numbers with respect to 10 mmole S. The in-situ crosslinking is a key feature in this PEM design. The cross-linked network helps to integrate neighboring hydrophobic blocks into a continuum, thus strengthening the membrane matrix. By comparison, the ionomers prepared from diblock P(AN-b-SPM) without GMA failed to form free-standing membranes (Table 1). The reaction between the epoxide group of GMA and diamine formed not only the covalent aminoethanolic linkages but also hydrophilic –OH and –NH– groups that imparted a hydrophilic character to the crosslinks. The association of the hydrophilic crosslinks formed secondary hydrophilic channels (“the waterways canals”) in the integrated hydrophobic domains to support the transport of protons and water molecules across the hydrophobic domains (Figure 3).PDF Image | Hydrophilic Cross-Linked Aliphatic Hydrocarbon Diblock Copolymer
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