Thermally Activated Equilibration of Self-Suspended Hairy Nanoparticles

Elke Arenholz, CHESS

"Soft glassy materials such as colloidal suspensions and emulsions...are of interest both from a scientific perspective as model material systems for studying suspension stability, and from a practical viewpoint for their wide applications..."

Schematic time dependence of caging strength in self-suspended hairy nanoparticles upon annealing.

Schematic time dependence of caging strength in self-suspended hairy nanoparticles upon annealing. Insert top: Structure factor S(q) of unannealed and annealed hairy nanoparticles as a function of normalized wave vector qR0. R0 = average radius of the nanoparticle. The height of the first maximum in S(q) increases, whereas the peak position remains unchanged, implying more correlated particles with approximately constant interparticle spacing. Insert bottom: Illustration of the equilibration of hairy nanoparticles under geometric confinement.

What did the scientists discover?

Scientists used small-angle X-ray scattering at CHESS to investigate slow structural and mechanical evolution of a soft glassy material composed of silica nanoparticles densely grafted with poly(ethylene glycol) methyl ether (mPEG) chains, referred to as “hairy nanoparticles”. They observed a significant equilibration process that has not been reported previously and show that the process is thermally activated and associated with local rearrangements of tethered chains to their equilibrium conformations.

Why is this important?

Soft glassy materials such as colloidal suspensions and emulsions exhibit a distinct yield stress and jamming behavior and therefore are of interest both from a scientific perspective as model material systems for studying suspension stability, and from a practical viewpoint for their wide applications in advanced coatings, energy storage, and biomedical engineering. The geometrical model developed here rationalizes materials properties in terms of corona interpenetration, cage dynamics, and yielding of self-suspended nanoparticles enhancing our understanding of transport processes in these materials.

Microscopic illustration of the equilibration of hairy nanoparticles under geometric confinement
Microscopic illustration of the equilibration of hairy nanoparticles under geometric confinement, where the tethered chains migrate from initially frustrated conformations toward equilibrium conformations upon melting.

What are the broader impacts of this work?

In this study, researchers consider for the first time how caging evolves in a self-suspended nanoparticle fluid by driving the systems out of equilibrium and interrogating their structural evolution toward equilibrium using small-angle X-ray scattering (SAXS) and rheological analysis. This work provides insights into the microscopic origins of cage formation and how nanoparticle cages develop and evolve in self-suspended materials. A simple microscopic model for yielding that helps to explain the temperature dependence of the cage strength and the role that the core volume fraction plays in setting material properties.

Why did this research need CHESS?

SAXS measurements were conducted in the D1 station at CHESS uniquely optimized for small angle scattering experiments on soft materials in terms of sample environments, beamline parameters, and analysis capabilities and tools.

Collaborators:

  • Xiaotun Liu, Robert Frederick Smith School of Chemical and Biomolecular Engineering, Cornell University
  • Brooks A. Abel, Department of Chemistry and Chemical Biology, Cornell University
  • Qing Zhao, Robert Frederick Smith School of Chemical and Biomolecular Engineering, Cornell University
  • Shuke Li, Department of Materials Science and Engineering, Cornell University
  • Snehashis Choudhury, Robert Frederick Smith School of Chemical and Biomolecular Engineering, Cornell University
  • Jingxu Zheng, Department of Materials Science and Engineering, Cornell University
  • Lynden A. Archer, Robert Frederick Smith School of Chemical and Biomolecular Engineering, Cornell University

Publication Citation:

X. Liu et al., "Microscopic Origins of Caging and Equilibration of Self-Suspended Hairy Nanoparticles," Macromolecules 2019, 52, 21, 8187-8196 https://doi.org/10.1021/acs.macromol.9b01473

How was the work funded?

This research was supported by NSF award DMR−1609125. This work made use of the Cornell Center for Materials Research Shared Facilities supported through NSF DMR-1719875. The X-ray experiments were conducted at CHESS which was supported by the NSF under award DMR-1332208.