How Do Nanofillers Influence the Glass Transition Temperature of Polymer Nanocomposites?: Insights from Local Characterizations of Interfacial Adsorption

<br/>Polymers play a foundational role in shaping the modern world, with critical applications in healthcare, energy, and the environment. Their remarkable tunability affords avenues towards enhanced properties via the incorporation of a second phase. However, the consequences of introducing a second phase remain inadequately understood in several key aspects. Particularly, the effect of nanofillers on the glass transition temperature (Tg) of polymer nanocomposites remains unclear, with conflicting reports observing Tg enhancement, depression, or no perturbation. The underlying cause of these inconsistencies is unclear because conventional characterization techniques lack spatial resolution, preventing a comprehensive understanding of how nanofillers influence local polymer properties and, consequently, the overall system behavior.<br/><br/>One route towards understanding the phenomenon of perturbed Tg in nanocomposites is glassy physics. It has been reported in the framework of glassy physics that polymers annealed at elevated temperatures can irreversibly adhere to adjacent substrates, thereby forming adsorbed polymer layers. Once considered "dead" due to restricted chain conformations, adsorbed layers are now recognized as regions of active dynamics, as revealed by the occurrence of a glass transition in adsorbed 2D nanofilms. Conventional investigations of 2D adsorbed nanofilms are limited in their ability to distinguish perturbations between the substrate interface and the free interface, but these studies highlight the important implications of irreversible adsorption in polymer nanocomposites. Polymer nanocomposites are routinely prepared via high-temperature annealing and feature a large interfacial area. Accordingly, it can be expected that they feature a significant degree of overall chain adsorption at the polymer-nanofiller interface. Nevertheless, conventional techniques pose technical obstacles to understanding the evolution and impact of irreversibly adsorbed layers in nanocomposites, as approaches such as bulk differential scanning calorimetry provide data averaged across the entire system.<br/><br/>In this study, we present a novel and comprehensive approach that overcomes the limitations of conventional techniques, thereby allowing us to characterize the structural evolution and glassy properties of nanocomposites' adsorbed layers. Our approach leverages a stepwise assembly of nanocomposites: Nanoparticles are well dispersed within a polymer matrix, then annealed for various durations to induce interfacial adsorption from the matrix. Subsequent isolation of these nanoparticles allows for direct investigation of the adsorbed polymer structure through low-dose cryo-TEM imaging. Furthermore, by incorporating covalently attached fluorescent and dielectric labels into the adsorbed polymer, we can reintegrate them into an analogous unlabeled polymer matrix. This integration enables targeted in-situ measurements of the adsorbed layer properties within the nanocomposite. Importantly, our approach unravels the intricate relationships between local structure and properties in nanocomposites, elucidating the role of processing conditions that give rise to interfacial adsorption—an often overlooked factor with significant explanatory power for resolving discrepancies in the literature and for advancing material design strategies.