Unusual Nonlinearity and Nonexponentiality in Glass-Forming Polymers Governed by Ionic Interactions

The glass transition is dynamically heterogeneous with non-Arrhenius, nonexponential, and nonlinear features, and those parameters are mutually correlated in most cases. This study systematically investigates these interrelated features through thermodynamic analysis of polyvinylpyrrolidone (PVP)/salt complexes, employing enthalpy relaxation parameters, specifically the heat capacity jump (Δ<i>C</i>_p) at <i>T</i>_g and enthalpy hysteresis (Δ<i>H</i>_R). Notably, the introduction of ionic interactions induces simultaneous reductions in dynamic fragility (<i>m</i>), nonexponential parameter (β), and nonlinear parameter (<i>x</i>), thereby disrupting their previously established empirical correlations. Through the framework of the Adam-Gibbs model, we demonstrate that the concurrent decrease in <i>x</i> and <i>m</i> primarily originates from thermodynamic factors, as evidenced by enhanced Δ<i>C</i>_p. The observed anomaly where <i>x</i> exceeds β suggests that physical inhomogeneities in retardation time distributions contribute significantly to the nonexponential nature of cooperative motions. This phenomenon likely stems from the relatively lower elementary activation energy associated with reversible ionic interactions, as supported by increased Δ<i>C</i>_p and Δ<i>H</i>_R. These findings not only reinforce the crucial connection between thermodynamic properties and dynamic fragility but also establish a novel thermodynamic framework for interpreting heterogeneous dynamics in polymeric glass formation.