Covalent Crosslinking of High-Density Polyethylene Trimethylammonium-Based Radiation-Grafted Anion Exchange Membranes for Reduced Water Uptakes and Swelling

Abstract

Anion exchange membranes (AEMs) are polymer membranes of varying thickness (10 - 50 µm) with covalently bound quaternary ammonium cations, which have been evaluated in fuel cells (AEMFCs) and green hydrogen electrolysis.1,2 Radiation-grafted AEMs made from high-density polyethylene (RG-HDPE) with ca. 30 microns hydrated thickness exhibit high hydroxide conductivities, promising mechanical stabilities, and high AEMFC performances.1,3 The enhanced water transport properties is hypothesized to be from nano and microscale level structural and morphological changes.3 Our best performing ultrathin RG-HDPE films exhibited excellent conductivity and AEMFC performance retention over the 400 h.3 However, the water uptake of RG-HDPE film is ca. 150%, hence, it is important to add a controlled amount of crosslinking.4-7 In previous studies of radiation-grafted membranes for use in reverse electrodialysis, crosslinking + amination was achieved by using a heated amination fabrication step involving a mixture of liquid, high boiling point mono-amines (such as N-methylpiperidine) and a crosslinking diamine (such as N,N,N',N'-tetramethyl-1,6-hexane diamine - TMHDA).8 However, a thermal method is not viable or safe when using the commonly supplied aqueous solutions of dissolved trimethylamine (TMA) gas. This presentation will discuss a variety of approaches, to target controlled TMHDA-crosslinking of TMA-based RG-AEMs, targeting reduced water uptakes and swelling with retention of ion conductivity (Figure 1). The most promising approach involved two steps with the initial amination of the poly(VBC)-grafted HDPE intermediate membranes using a sub-stoichiometric amount of TMHDA. Square wave voltammetry analysis of the solution recovered from amination step 1 confirmed complete reaction of the added TMHDA. In the second step, the membranes were then quickly [important] reacted with aqueous TMA to complete the amination process. TMA can displace TMHDA-based head-groups after long step 2 amination times, hence, we emphasize the use of short TMA amination times in the second step.