Phase Behavior and Thermal Properties of Precision Polyelectrolyte Blends: The Dilute Charge Regime

In this work, miscibility and thermal properties were studied in blends of poly(ethylene oxide) (PEO) and a precision polyanion of linear polyethylene hosting a phenyl-4-[sulfonyl(trifluoromethylsulfonyl)imide-Li] at every fifth carbon (p5). Blends were found to be miscible at low to moderate PEO concentrations ranging from 10 to 80 wt % PEO. However, in blends with high PEO weight fraction (≥90 wt % PEO), visible, macroscopic phase separation was observed. Differential scanning calorimetry was used to study trends in the primary and secondary phase transitions of each blend to better understand the phase boundaries and interactions underpinning phase behavior. A single glass transition temperature (Tg) was identified for each blend composition. Within the miscible range, the Tg values were well fit by the Kwei model and a modified Fox model with a single fitting parameter, but significant disagreement between models and experiment was observed for two-phase blends (>80 wt % PEO). In miscible semicrystalline blends, PEO melting point depression was clearly observed due to the presence of p5 as a diluent. In contrast, the PEO melting temperature of two-phase blends was a nonmonotonic function of p5 content. This evidence agrees well with predictions from theory and modeling by Sing and Olvera de la Cruz (10.1021/mz500202n) for blends with strong ion correlations, indicating that charge plays a significant and complex role in blend phase behavior especially in the dilute charge regime. Furthermore, a modified Fox equation was developed to model blend Tg as a function of blend composition that performed comparably to the Kwei model (a model with 2 adjustable parameters), despite only requiring one fitting parameter. Both findings provide key insight into the prospective design of polyanion-based polymer blend electrolytes as miscibility and flexibility are essential in these materials.