New Insights into the Interfacial Interactions and Segmental Dynamics in Restricted Poly(vinyl acetate): a Comparative Study of Hydrogen-Bonded Nanocomposites versus Ion-Dipole Interacted Ionomers

Although extensive investigations have confirmed the crucial role of polymer–filler interfacial interactions in mediating segmental dynamics within nanocomposites, the fundamental mechanisms underlying alterations in the segmental fragility index (m) and its correlation with the glass transition temperature (Tg) remain poorly understood. Inspired by the dynamics observed in ionomers exhibiting similar interfacial interactions and restricted mobility, we conducted a comparative study on poly(vinyl acetate)/SiO2 nanocomposites with hydrogen-bonded interfaces and poly(vinyl acetate)/salt ionomers featuring ionic clusters. Increasing the SiO2 concentration was found to increase both the bulk m and bulk Tg concurrently. Conversely, within the interfacial regions, m decreased as the interfacial Tg increased. Furthermore, the interfacial regions consistently exhibited higher Tg and lower m compared with the bulk areas of both nanocomposites and ionomers, irrespective of whether the bulk relaxation dynamics were slowed or accelerated. Based on the generalized entropy theory (GET) of glass formation, we propose that the reduced fragility observed in interfacial regions originates from their higher polarity and cohesive energy density in both nanocomposites and ionomers. These results not only provide comprehensive mechanistic insights into how interfacial interactions tailor the fragility index but also offer fundamental experimental validation for predictions derived from GET and molecular dynamics simulations of glass-forming polymers.