Paper Number
GG19 

Session
Out of Equilibrium Systems: Gels and Glasses

Title
Anisotropic self-assembly and percolation of nanoparticles in nanoparticle-incorporated supramolecular hydrogels

Presentation Date and Time
October 22, 2019 (Tuesday) 11:30

Track / Room
Track 7 / Room 306C

Authors (Click on name to view author profile)

1. Song, Jake (Massachusetts Institute of Technology, Department of Materials Science and Engineering)
2. Rizvi, Mehedi H. (North Carolina State University, Department of Materials Science and Engineering)
3. Ilavsky, Jan (Argonne National Laboratory, Advanced Photon Source)
4. Mankus, David (Massachusetts Institute of Technology, Koch Institute for Integrative Cancer Research)
5. Tracy, Joseph B. (North Carolina State University, Department of Materials Science and Engineering)
6. Holten-Andersen, Niels (Massachusetts Institute of Technology, Department of Materials Science and Engineering)
7. McKinley, Gareth H. (Massachusetts Institute of Technology, Department of Mechanical Engineering)

Author and Affiliation Lines (in printed abstract book)
Jake Song¹, Mehedi H. Rizvi², Jan Ilavsky³, David Mankus⁴, Joseph B. Tracy², Niels Holten-Andersen¹, and Gareth H. McKinley^5
1Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA; ²Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC; ³Advanced Photon Source, Argonne National Laboratory, Lemont, IL; ⁴Koch Institute for Integrative Cancer Research, Massachusetts Institute of Technology, Cambridge, MA; ⁵Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA

Speaker / Presenter 
Song, Jake

Text of Abstract
Nanoparticle incorporation has recently emerged as a novel strategy to control the mechanical rigidity of supramolecular hydrogels; however, the microstructural origin of the resulting mechanical reinforcement in such nanoparticle-incorporated supramolecular hydrogels (NISH) remains relatively unexplored. Here, we investigate this relationship by systematically incorporating metallic nanoparticles into a metal-ion-coordinated polymer hydrogel matrix. Using a combination of ultra-small-angle x-ray scattering, scanning electron microscopy, and oscillatory shear rheology, we show that the nanoparticles also participate in metal-coordination with the polymer matrix, and undergo preferential self-assembly within the hydrogel. We show that this self-assembly is anisotropic and hierarchical across multiple length-scales, which in turn leads to mechanical percolation at ultralow concentrations, as evidenced by the dramatic reinforcement of the viscoelastic properties of the supramolecular hydrogel. Surprisingly, we find that the presence of metal-ion/ligand coordination in the hydrogel matrix is essential to the low percolation threshold of the nanoparticle assembly: by occupying a substantial number of associating groups in the hydrogel, the metal ions stabilize the nanoparticle network from otherwise collapsing via precipitation. These results provide unique insights into the intriguing self-assembly behavior of nanoparticles that are incorporated into supramolecular hydrogels, and a rational way to engineer their mechanical properties through such means.