Achieving Continuous Anion Transport Domains Using Block Copolymers Containing Phosphonium Cations

Abstract

Triblock and diblock copolymers based on isoprene (Ip) and chloromethylstyrene (CMS) were synthesized by sequential polymerization using reversible addition− fragmentation chain transfer radical polymerization (RAFT). The block copolymers were quaternized with tris(2,4,6-trimethoxyphenyl)phosphine (Ar 3 P) to prepare soluble ionomers. The ionomers were cast from chloroform to form anion exchange membranes (AEMs) with highly ordered morphologies. At low volume fractions of ionic blocks, the ionomers formed lamellar morphologies, while at moderate volume fractions (≥30% for triblock and ≥22% for diblock copolymers) hexagonal phases with an ionic matrix were observed. Ion conductivities were higher through the hexagonal phase matrix than in the lamellar phases. Promising chloride conductivities (20 mS/cm) were achieved at elevated temperatures and humidified conditions. ■ INTRODUCTIONAEMs have attracted increasing interests due to numerous applications across many fields, such as photosynthesis, 1 water purification, 2,3 gas separation, 4,5 and alkaline fuel cells. 6−10 Compared to the extensive studies on proton exchange membranes (PEMs), 11 fundamental understanding of the structure−performance relationship in AEMs is urgently needed. This requires well-defined ionic polymers for AEM preparation and investigation. The most widely used strategy of AEMs synthesis is through chemical modification of engineering polymers, such as polysulfone, 12−23 poly(ether ether ketone), 24 polyphenylene, 25−27 and poly(phenylene oxide). 28−33 Other strategies include direct polymerization to synthesize homopolymers 34 or random copolymers. 35−39 In addition, cross-linked materials have also been reported, where better mechanical properties, higher ion content, controlled water sorption, and reduced fuel crossover could be achieved. 40−48 However, although good membrane performance has been achieved, the disordered nature of these materials still hinders a complete understanding of the anion transport mechanism.As demonstrated in PEMs, microphase separation can promote ion transport and water diffusion. 49,50 Recent research has discovered similar effects in AEMs. A few examples of multiblock poly(arylene ether)s were prepared by polycondensation. 51−54 Faster anion transport was observed in ordered structures compared to random copolymer counterparts. Similarly, triblock copolymers with polysulfone as the midblocks were prepared by polymer coupling and displayed improved anion conductivities. 55 Modifying precursor block copolymers via chloromethylation or bromination and subsequent quaternization resulted in ionic copolymers with promising conductivities. 56−58 Knauss et al. 59 and Xu et al. 60 reported block and graft copolymers from end-functionalized poly(phenylene oxide), and good mechanical strengths were achieved together with remarkable anion conductivities. Direct polymerization was utilized to synthesize ionic liquid based poly(methyl methacrylate) block copolymers. 61,62 It was found that in weakly segregated block copo...

Highly conductive and chemically stable alkaline anion exchange membranes via ROMP of trans -cyclooctene derivatives

“…Recent studies have shown that cyclic ammonium (24)(25)(26)(27) and tetraalkyl ammonium cations, specifically those with long alkyl chains (18)(19)(20)(28)(29)(30), have better alkaline stability than traditional benzyltrimethyl ammonium cations (31)(32)(33). Other cations, such as imidazolium (16,21,(34)(35)(36)(37)(38)(39)(40)(41)(42)(43), phosphonium (44)(45)(46)(47)(48)(49), and metal cations (50)(51)(52)(53)(54), have also been widely applied in various AAEMs. More specifically, pentasubstituted imidazolium (34)(35)(36)(37)(38), tetrakis(dialkylamino)phosphonium (44)(45)(46), and cobaltocenium (50,51) have shown superior alkaline stability.…”

Pendant side‐chain sterics against electrostatic forces: Influencing short‐range ordering in random polyelectrolytes

“…Due to the ubiquitous application of charged polymers in polymer science, the study of their structure–property relationships has received considerable attention in the literature . Understanding the correlation between structure and polyelectrolyte properties could be used to design better ion conducting and mechanically robust polyelectrolyte membranes .…”

Amelioration in physicochemical properties and single cell performance of sulfonated poly(ether ether ketone) block copolymer composite membrane using sulfonated carbon nanotubes for intermediate humidity fuel cells

“…This block copolymerization has been more effective than the conventional hydrocarbon membranes, physically blended membranes, and random copolymers owing to their continuous microphase-separated morphology, and improved thermal, mechanical, and electrochemical properties. 18,19 Highlights • An organic-inorganic hybrid membrane was exploited for intermediate humidity fuel cell applications. • Sulfonated CNTs improved the water retention and other physicochemical properties of composite membranes.…”

“…The hydrophilic segments provide the broad interconnected hydrophilic networks responsible for the increased proton conductivity while the hydrophobic segments prevent the excessive swelling of membranes, thereby maximizing their overall performances. This block copolymerization has been more effective than the conventional hydrocarbon membranes, physically blended membranes, and random copolymers owing to their continuous microphase‐separated morphology, and improved thermal, mechanical, and electrochemical properties …”