Abstract
This study reports a new phenomenon whereby the ionic content of a random ionomer was increased by the introduction of a hydrophobic modifier. In the current study, the ionomer synthesized from the solution polymerization of the three vinyl monomers, which are polar hydrophobic monomers acrylonitrile (AN), glycidyl methacrylate (GMA), and ionic monomer potassium 3-sulfopropyl methacrylate (SPM), encountered an early phase separation problem when the ionic content exceeded a certain threshold value. However, the addition of a strongly hydrophobic monomer, 2,2,3,3-tetrafluoropropyl methacrylate (TFPM), during the copolymerization is able to restrain this phase separation trend, consequently allowing 50% more of SPM units to be incorporated and uniformly distributed in the ionomer and achieving a random copolymer chain. The ionic clustering of the SPM units, which is the cause for the phase separation, was reduced as a result. The resulting random ionomer was demonstrated to be a superior proton conducting material over its ternary originator. This is due to the fact that TFPM possesses acidic protons, which brings about an association of TFPM with SPM and GMA via hydrogen bonding. This study could impact the synthesis of random ionomers by free radical polymerization since monitoring ionic content and improving ionic unit distribution in ionomers are issues encountered in several industries (e.g., the healthcare industry).
Highlights
Random ionomers are useful for a wide range of applications
During the synthesis of the ternary random ionomers in DMF, phase separation, which was indicated when the polymerization solution was turning turbid, occurred when the sulfopropyl methacrylate (SPM) content in the starting mixture was higher than 12 mol%
This study focuses on the topic of promoting the ionic component in a type of random aliphatic ionomer via introduction of a fluorine-containing acrylate monomer (TFPM) in the solution copolymerization system
Summary
Random ionomers are useful for a wide range of applications. The fabrication of polymer electrolyte membranes (PEMs) for DAFCs represents one of these applications [1,2,3,4]. A one-pot solution polymerization to obtain random ionomers with a sufficiently high ionic content is clearly preferable if the critical issue of phase separation during copolymerization can be satisfactorily overcome In this regard, the work of Kim and coworkers is noteworthy [19]. A relatively high ionic content (up to 20 wt.%) could be incorporated in an ionomer if a large amount of UAN was used Other than this approach, the authors of this paper are not aware of other attempts at using additives in solution copolymerization to address the phase separation problem. The assimilation of TFPM into the growing free radical chains during copolymerization could improve the distribution of the ionic SPM units in the copolymer backbone, reducing the tendency of precipitation driven by aggregation of the ionic segments. The findings here could provide a strategy for the design of random-ionomer membranes with high ionic contents
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