Low-Temperature Ionic Conductivity Enhanced by Disrupted Ice Formation in Polyampholyte Hydrogels

Abstract

The phase behavior of water in hydrogels has a broad impact on many health and energy applications. Our previous study showed that polyampholyte hydrogel has the potential to be used as an aqueous gel electrolyte in electrochemical storage devices at −30 °C due to enhanced low-temperature conductivity. In this study, we detail the impact polymer structure has on this enhanced conductivity, explaining this finding with a model charge-balanced polyampholyte, poly(4-vinylbenzenesulfonate-co-[3-(methacryloylamino)propyl]trimethylammonium chloride), a hydrogel whose polymer and water structures are probed by variable-temperature SAXS, WAXS, and solid-state NMR spectroscopy. SAXS results at room temperature indicate a networked globule structure in the charge-balanced polyampholyte hydrogels. The globular radius of gyration is ∼2.5 nm, whereas the globular size and its clustering structure are dependent on synthesis parameters. Variable-temperature SAXS data reveal a temperature-dependent structure evolution of the polyampholyte hydrogel. An interconnected globular network structure of polymer-rich phase at low temperature, observed by electron microscopy, is suggested to preserve ion-conducting channels of nonfrozen water molecules at low temperatures. This hypothesis is further supported by solid-state NMR spectroscopy. Together these findings provide macromolecular-and molecular-level insight that may be used to design gel electrolytes for enhanced low-temperature performance.

Direct visualization of nano and microscale polymer morphologies in as-prepared and dialyzed polyampholyte hydrogels by electron microscopy techniques

“…Here, microscopic cellular structures were observed in the DIW-dialyzed sample, whereas there is no visible macroscopic structure in the KOH-dialyzed sample. This morphologic observation is consistent with high mechanical strength [21,22] and high low-temperature ionic conductivity, [16,20] respectively, reported in previous literatures.…”

“…[19] Recently, we found a strong evidence that such globular network structure exists in hydrogels; our SAXS results indicated a networked globule structure in the charge-balanced polyampholyte hydrogels, where the unit globules have an average radius of gyration of ∼2.5 nm. [20] Such globular structure is consistent with theoretical prediction based on Debye-Huckel fluctuation-induced attraction model. [12,13] However, the existence of the globules have not been supported by direct microscopic visualization techniques to the best of our knowledge.…”

“…Although it should be noted that FE-SEM is not a reliable method to determine nanoscopic size of polymeric objects, the diameter of the spherical particles (globules, to our model interpretation) is in a reasonable agreement with the expected average diameter of ∼5.2 nm according to SAXS fitting. [20] (It is notable that the radius of a sphere is 5/3 √ times larger than the radius of gyration.) The spherical shape of the globules are attributed to the ionic coacervation of charged polymer chains.…”

“…In the supporting information, the SAXS fitting (Fig. S1) shows that the nanoscopic globular structures found in PA-10-2.1 sample [20] is also found in K-PA-10-2.1 sample. It is, however, notable that K-PA-10-2.1 samples showed whitening and stiffening after 6 M KOH dialysis, which may suggest the dissociation of amide group in MPTC monomeric unit in strong basic solution.…”

“…The protocol of polyampholyte synthesis was described in previous works. [20,22] Briefly, 1 M sodium 4-vinylbenzenesulfonate (NaSS) and 1 M [3-(methacryloylamino)propyl] trimethylammonium chloride (MPTC) with Irgacure 2959 (photoinitiator, 0.25 mol%, compared with the total concentration of NaSS and MPTC) and NaCl were dissolved in DIW to form the precursor solution (see Table I for the sample details). In the precursor solution, NaCl concentration is 10 wt.%.…”