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materials Article Hydrophilic Cross-Linked Aliphatic Hydrocarbon Diblock Copolymer as Proton Exchange Membrane for Fuel Cells David Julius, Jim Yang Lee and Liang Hong * Citation: Julius,D.;Lee,J.Y.;Hong,L. Hydrophilic Cross-Linked Aliphatic Hydrocarbon Diblock Copolymer as Proton Exchange Membrane for Fuel Cells. Materials 2021, 14, 1617. https://doi.org/10.3390/ ma14071617 Academic Editor: Rolando Pedicini Received: 26 February 2021 Accepted: 23 March 2021 Published: 26 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 117585, Singapore; david_julius@hotmail.com (D.J.); cheleejy@nus.edu.sg (J.Y.L.) * Correspondence: chehongl@nus.edu.sg Abstract: This study proposes a hydrophobic and hydrophilic aliphatic diblock copolymer wherein the hydrophobic block contains glycidyl methacrylate (GMA) units that are distanced by poly(acry- lonitrile) (PAN) segments to fabricate a proton exchange membrane (PEM). This diblock copoly- mer also known as ionomer due to the hydrophilic block comprising 3-sulfopropyl methacrylate potassium salt (SPM) block. The diblock copolymer was synthesized in the one-pot atom transfer radical polymerization (ATRP) synthesis. Subsequently, the membrane was fabricated by means of solution casting in which an organic diamine, e.g., ethylene diamine (EDA), was introduced to crosslink the diblock copolymer chains via the addition of amine to the epoxide group of GMA. As a result, the PEM attained possesses dual continuous phases, in which the hydrophobic domains are either agglomerated or bridged by the EDA-derived crosslinks, whereas the hydrophilic do- mains constitute the primary proton conducting channels. The in-situ crosslinking hydrophobic block by using a hydrophilic cross-linker represents the merit aspect since it leads to both improved proton conductivity and dimensional stability in alcohol fuel. To characterize the above proper- ties, Nafion® 117 and random copolymer of P(AN-co-GMA-co-SPM) were used as control samples. The PEM with the optimized composition demonstrates slightly better fuel cell performance than Nafion 117. Lastly, this diblock ionomer is nonfluorinated and hence favors lowering down both material and environmental costs. Keywords: PEM; diblock copolymer; ATRP; hydrophilic crosslink; direct methanol fuel cell 1. Introduction The development of aliphatic-based proton exchange membranes (PEMs) can benefit from rationally designed, molecularly engineered and relatively inexpensive ionomer structures. Two strategies that fall under such considerations include, firstly, well-ordered structures such as diblock [1,2], graft [3] and multiblock [4] copolymers; secondly, proton- conducting cross-linking diblock copolymer [5]. The first strategy has been extensively applied to the aliphatic-based PEMs. However, a high proton conductivity would likely have a price of dimensional stability and excessive swelling when the membranes are fully hydrated [6]. It was then reckoned that the mechanical properties of the nonfluorinated ionomer membranes could be improved by assimilating a hydrophobic cross-linkable monomer into the block copolymer chains [7]. The hydrophobic crosslinking is, however, to cause partial blockage of the hydrophilic channels to convey protons [8]. The second strategy is to make use of ionic crosslinkers such as sulfosuccinic acid (SA) so that the crosslinks of the resulting network are proton-conducting [5,9]. However, a ran- dom cross-linked membrane matrix, regardless of the ionic character of the crosslinks and the extent of crosslinking, would not provide the same connectedness as the hydrophilic channels of a well-ordered polymer structure. In addition, the ester groups, e.g., ethylene glycol dimethacrylate, in the ionic cross-links are hydrolysable by the acid moieties present Materials 2021, 14, 1617. https://doi.org/10.3390/ma14071617 https://www.mdpi.com/journal/materialsPDF Image | Hydrophilic Cross-Linked Aliphatic Hydrocarbon Diblock Copolymer
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