New Insights into the Decoupling Mechanism of Ion Motion from Segmental Relaxation in Glass-Forming Polymers: The Interplay of Cohesive Energy Density

Abstract

Decoupling ion transport from segmental motion offers a promising route for solid polymer electrolytes, yet identifying its governing factor, whether the glass transition temperature (Tg), fragility index (m), or dynamic heterogeneity, remains challenging due to their inherent interdependence. Here, we systematically investigate this decoupling in poly(vinyl acetal)s by varying acetal group length, molecular weight, plasticization, and antiplasticization. It was found that decoupling strength is not dictated simply by Tg, m, or dynamic heterogeneity. Microstructurally, the dynamically heterogeneous “slow” domains are characterized by dense molecular packing and higher cohesive energy density, irrespective of plasticization or antiplasticization. This same characteristic is observed in poly(vinyl acetal) bearing short acetal groups, wherein a weak decoupling effect is also detectable as observed in the “slow” domains. Furthermore, a negative correlation between decoupling exponent and the square of solubility parameter (i.e., cohesive energy density) at room temperature was found, confirming that cohesive energy density plays a key role in governing decoupling. We suppose that the enhanced cohesive energy density impedes ion motion and confines it to proceed only in tandem with segmental relaxation. These findings not only provide new experimental evidence for the decoupling mechanism but also support relevant theoretical frameworks of glass formation.