Transporting protons is essential in several biological processes as well as in renewable energy devices, such as fuel cells. Although biological systems exhibit precise supramolecular organization of chemical functionalities on the nanoscale to effect highly efficient proton conduction, to achieve similar organization in artificial systems remains a daunting challenge. Here, we are concerned with transporting protons on a micron scale under anhydrous conditions, that is proton transfer unassisted by any solvent, especially water. We report that proton-conducting systems derived from facially amphiphilic polymers that exhibit organized supramolecular assemblies show a dramatic enhancement in anhydrous conductivity relative to analogous materials that lack the capacity for self-organization. We describe the design, synthesis and characterization of these macromolecules, and suggest that nanoscale organization of proton-conducting functionalities is a key consideration in obtaining efficient anhydrous proton transport.
End-functionalized poly(ethylene glycol) (PEG) and polydimethylsiloxane (PDMS) were cross-linked by a thiolene reaction with a tetra-functional thiol to create robust, tunable networks. These networks were loaded with increasing amounts of lithium bis(trifluoromethane sulfonyl imide) (LiTFSI), and their ion conductivity was measured. A wide range of salt loading was achieved, allowing the investigation of both salt-in-polymer and polymer-in-salt regimes. Thermal, mechanical, and ion conductivity properties of LiTFSI-loaded PEG and PEG-PDMS networks were measured. Even at high salt loadings, both networks maintained rubber-like characteristics, which were stable over a range of temperatures (30−90°C). The PEG network with the highest salt loading showed the greatest ion conductivity, 6.7 × 10 −4 S cm −1 at 30°C, as measured by impedance spectroscopy. This system provides a route to optimize lithium ion conduction and mechanical properties.A ccess to affordable, clean energy is a well-recognized scientific and technical challenge of this century.
Block copolymers of polystyrene‐b‐poly(vinyl benzyl trimethylammonium tetrafluoroborate) (PS‐b‐[PVBTMA][BF4]) were synthesized by sequential monomer addition using atom transfer radical polymerization. Membranes of the block copolymers were prepared by drop casting from dimethylformamide. Initial evaluation of the microphase separation in these PS‐b‐[PVBTMA][BF4] materials via SAXS revealed the formation of spherical, cylindrical, and lamellar morphologies. Block copolymers of polystyrene‐b‐poly(vinyl benzyl trimethylammonium hydroxide) (PS‐b‐[PVBTMA][OH]) were prepared as polymeric alkaline anion exchange membranes materials by ion exchange from PS‐b‐[PVBTMA][BF4] with hydroxide in order to investigate the relationship between morphology and ionic conductivity. Studies of humidity [relative humidity (RH)]‐dependent conductivity at 80 °C showed that the conductivity increases with increasing humidity. Moreover, the investigation of the temperature‐dependent conductivity at RH = 50, 70, and 90% showed a significant effect of grain boundaries in the membranes against the formation of continuous conductive channels, which is an important requirement for achieving high ion conductivity. © 2012 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1751–1760, 2013
Poly(propylene oxide)–poly(ethylene oxide)–poly(propylene oxide) (PPO–PEO–PPO) block copolymers (BCPs) with cross-linkable end groups were synthesized, blended with an ionic liquid (IL) diluent, and cross-linked to form polymer gel electrolytes. The IL prevented crystallization of PEO at high concentrations, enabling fast ion transport. In addition, the IL was selective for the PEO block, inducing strong microphase separation in what are otherwise disordered or weakly ordered BCP melts. Cross-linking the BCPs in the presence of the IL resulted in the formation of solid, elastic gels with high ionic conductivitiesgreater than 1.0 mS/cm at 25 °C for some compositions. However, it was found that neither the presence or absence of microphase separation nor the BCP composition of the microphase separated gels substantially influenced ionic conductivity. Increasing the cross-link density through the use of phase-selective PEO- and PPO-based cross-linking reagents was also evaluated. It was revealed that confinement of cross-links to the PPO rich domains through the use of PPO-based diacrylates enhanced the mechanical strength of the gels without detriment to the ionic conductivity. Conversely, cross-linking in the PEO-rich domains through the use of PEO-based acrylates significantly reduced conductivity. Isolation of cross-links within a minor nonconducting domain in a microphase separated gel is a viable strategy for mechanical property enhancement without a large sacrifice in conductivity, effectively decoupling ionic conductivity and mechanical strength. This approach yielded solid-like gel electrolytes fabricated from BCPs that can be produced inexpensively, with ionic conductivities of 0.64 mS/cm at 25 °C and a frequency independent storage modulus of approximately 400 kPa.
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