Understanding multi-scale dynamics in polymer nanocomposites through novel experimental techniques, model systems, and theoretical frameworks
Polymer nanocomposites are industrially relevant, as the addition of the nanoscopic fillers can change the glassy and melt state properties of the matrix polymer in a wide variety of ways. These range from simple mechanical reinforcement to changes in optical, transport, and electrical properties. Modern polymer nanocomposite materials are currently a billion dollar market, primarily through industrial use of carbon nanotube composites and nano-clay composites to improve thermal and mechanical properties. Car tires can also be considered nanocomposites, as the addition of nanoscopic carbon black to the rubber matrix substantially improves the performance of the tire. Most of the improved properties in nanocomposites can be directly related to the distribution of heterogeneities in the polymer matrix and soft-hard material interfaces.
The ultimate vision of the work that will be presented in my thesis is to provide the groundwork for understanding the multi-scale dynamics of a polymer system based on the dynamics of the neat polymer, polymer-nanofiller interaction strength, and geometry. This would help fundamentally bridge over two decades of sometimes contradictory research into a robust physical model. This model could then be applied to the formulation of new materials, providing guidance for principled design of polymer nanocomposites in industrial settings.
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1) Rationalizing the Composition Dependence of Glass Transition Temperatures in Amorphous Polymer/POSS Composites
Walter W. Young, Joseph P. Saez, Reika Katsumata. ACS Macro Lett. 2021, 10, 1404–1409. https://doi.org/10.1021/acsmacrolett.1c00597
One of the most important properties of a polymer nanocomposite is its “glass transition temperature”, the temperature at which a polymer goes from a rigid, tough state to a soft, malleable state. By developing a model system with well-controlled molecular structure and geometry, we precisely isolate the structural factors which increase the glass transition temperature of nanocomposites. In particular, we find a strong relationship between the number of filler-polymer interactions and the change in glass transition temperature; more strong interactions lead to a greater increase in glass transition temperature. The insights from this work provide clues for how to better understand and harness the unique properties of polymer nanocomposites.
2) Relating the Degree of Nanofiller Functionality to the Glass Transition Temperature and Structure in a Polymer–Polyhedral Oligomeric Silsesquioxane Nanocomposite
Walter W. Young*, Rui Shi*, Xiang-Meng Jia*, Hu-Jun Qian, Reika Katsumata. Macromolecules 2022, 55, 12, 4891–4898. https://doi.org/10.1021/acs.macromol.2c00646
Building on our previous publication, we wanted to determine whether changing the functionality of the nanofiller would have the same effect on the glass transition temperature. In addition to experiments, we collaborated with simulation experts from Jilin University, who showed that the number of hydrogen bonds in the system is strongly temperature dependent. Also intuitive but interesting is the fact that the amines on lower functionality POSSs have a higher probability of forming a hydrogen bond compared to amines on the high functionality POSSs, due to the conformational penalty of monomers interacting with neighboring sites.
3) A Hidden Relaxation Process in Poly(2-vinylpyridine) Homopolymers, Copolymers, and Nanocomposites
The glass transition can be studied very accurately using a technique called dielectric spectroscopy. While performing these measurements on our samples to study the glass transition our nanocomposites, our collaborators noticed an anomalous relaxation process occurring in the material. Although this relaxation process had been observed previously, it had not been studied systematically. We observed that this “slower” relaxation process becomes exponentially slower with increasing POSS loading. Further work has focused on studying how the “slower process” manifests in the mechanical response of the nanocomposites.
