Tuning the Dynamic Fragility of Polymers by Metallic Ions: The Interplay of Coordination Strength

Abstract

High-Tg polymers generally exhibit high dynamic fragility (m). Here, we propose to design striking polymeric materials with low m but high Tg by introducing ionic coordination. The enthalpy relaxation of poly(vinylpyrrolidone)/metal chloride complexes was investigated to elucidate the influences of the coordination strength on Tg and m. Findings reveal that Tg increases with the addition of metallic ions, while m decreases for weakly coordinated complexes (Li+) and moderate systems (Zn2+ and Mg2+). Meanwhile, m increases continuously for strongly coordinated systems (La3+). Further experiments revealed that m is well correlated to heat capacity jump ΔCp and enthalpy hysteresis ΔHR as predicted by Adam–Gibbs theory. Ionic coordinated polymers with low m but high Tg can be achieved only when ΔHR changes more than Tg·ΔCp, suggesting that the enthalpy change plays a leading role in the activation free energy of segmental cooperative rearrangement. These results not only add new evidence of the thermodynamic link to dynamic fragility but also help clarify the mechanism of the independent tuning of Tg and m.