Collaborative Research: Integrated experiments and simulations to understand the mechanism and consequences of polymer adsorption in films and nanocomposites

NON-TECHNICAL SUMMARY:

From lightweight materials to flexible solar panels, the materials opening the door to tomorrow’s technologies frequently exhibit “nanostructure”: they are comprised of two finely intermixed domains only hundreds to thousands of atoms across. In many cases, one of these domains consists of polymers, which are long chains of molecules that plastics, rubber, and many biological materials are made of.

In parallel, the second domain often consists of tiny inorganic nanoparticles – rigid regions that can dramatically enhance the polymers’ properties. Over the past decade, scientists have found evidence that something strange happens at the interfaces between these domains: the polymer molecules become tightly ‘glued’ to the particles at the molecular level.

This process, known as “irreversible adsorption”, seems to dramatically alter these materials’ properties, with the potential to imbue tolerance of higher temperatures, to alter permeability, and perhaps to enhance mechanical strength. However, the cause of this effect – or even why it should occur at all – remains unknown. Even more practically, there is little understanding of how to control this irreversible adsorption phenomenon in order to obtain the best possible properties for next-generation materials.

This collaborative project (co-supported by the Polymers Program and the Condensed Matter and Materials Theory Program in the Division of Materials Research) will combine experiments and computer simulations to understand why this adsorption effect occurs and how scientists and engineers can control it to optimize material properties. Experiments will employ a nanoscale characterization method wherein fluorescent probe molecules, localized to the nanoscale domain near the interface, report on the properties of the adsorbed layer and how it forms.

Molecular simulations performed on supercomputers will zoom in to the molecular scale to understand how molecules move and evolve during irreversible adsorption, making it possible to link changes in material properties with underlying causes in molecular structure and motion. Together, these approaches aim to provide the fundamental scientific understanding needed to enable more rational engineering and design of these materials, with relevance to economic sectors ranging from infrastructure to energy.

This research will be coupled with a new high-school internship program that will support broadening the pipeline of students moving into STEM professions. TECHNICAL SUMMARY:

In polymer films and nanocomposites, the formation of an irreversibly adsorbed layer from the polymer melt can dramatically alter the properties of the interfacial domains that dominate the overall properties of these materials. Unlike in polymer adsorption from solution, which is driven by a combination of an energetic mismatch and an entropic size asymmetry between solvent and polymer, the thermodynamic mechanism of adsorption from the melt (where these factors are absent) remains unresolved.

Moreover, numerous properties are reported to co-evolve during adsorption, challenging the development of a theory of adsorption accounting for all of them. A central challenge has been the difficulty of probing the evolution of near-substrate and near-particle properties in a temporally and spatially resolved manner during adsorbed layer formation.

To overcome these challenges, this work will employ fluorescence experiments to locally probe the evolution of multiple properties near substrates and particles during adsorption. These experiments will be combined with molecular dynamics simulations that will provide spatially resolved insight into how segmental packing, chain conformations, and polymer dynamics co-evolve during adsorbed layer formation.

Synergistic experiments and simulations will take an integrated approach to systematically probe the layer formation process, the behavior of isolated adsorbed layers, and the ultimate impact of the adsorbed layer presence on material properties, all across a matrix of key controlling variables. This strategy will establish an understanding of how multiple mechanisms may interact to drive adsorbed layer formation and mediate its impact on polymer properties.

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This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.