# Using Torsion for Controllable Reconfiguration of Binary Nanoparticle Networks

## Collaborators

Tao Zhang, Badel L. Mbanga, Victor V. Yashin, and Anna C. Balazs$^*$

Chemical Engineering Department, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
$^*$E-mail: balazs@pitt.edu

## Motivation

Mechanical deformation can potentially provide an effective means of controlling the nanoscale morphology in hybrid materials. The challenge, however, is establishing optimal couplings of the deformation and mechano-responsive components within the material to achieve nanoscopic structural reorganization without causing catastrophic damage. In hybrid materials, polymers introduce the needed degree of flexibility and mutability; when combined with the appropriate mechanical deformation, this mutability could provide a means of reorganizing solid nanoparticles into useful structures.

Here, we take inspiration from the early Rubik’s cube, where individual colored blocks were held together by elastic bands; by twisting the structure, the blocks moved to form a new, stable arrangement of elements in the cube. To design an analogous mechano-responsive system, we focus on polymer-grafted nanoparticles (PGNs), where each rigid nanoparticle core is decorated with a corona of polymer chains. The free ends of these chains encompass reactive functional groups that allow the polymers to form “arms” between neighboring particles and thus, interconnect the PGNs into a network. In effect, these polymer arms act as the elastic bands and the nanoparticles correspond to the individual blocks in the toy. Using computational modeling, we have undertaken preliminary studies to show that by applying torsion to this material, we can achieve significant control over the arrangement of a binary mixture of nanoparticles and hence, tailor the nanostructure of the composite.

## Approach

Herein, we introduce both A- and B-type nanoparticles, which encompass different reactive groups at the free ends of the grafted chains. The energies of the labile bonds formed between the same types of particles, $U_{AA}$ and $U_{B}$, are taken to be different than the bond energies between dissimilar types of particles, $U_{AB}$. For simplicity, it is assumed that $U_{AA}=U_{BB}≡U_{AA,BB}$ . We focus on the behavior of the binary PGN networks under applied torsional deformations.
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## Acknowledgments

We gratefully acknowledge support from the DOE through CBES, an EFRC at Northwestern University, and the AFOSR.