Deformation Behavior of Polystyrene-Grafted Nanoparticle Assemblies with Low Grafting Density

Matrix-free, polymer-grafted nanoparticle (PGN) assemblies show promise for a wide array of structural, photonic, and electrical applications. We examine the modulus, yield strength, and crazing of assemblies of polystyrene grafted Fe3O4 (Fe3O4-PS) at low graft density (Sigma < 0.15 chains/nm(2)) where chain entanglements are maximized. From the wrinkling-cracking method (WCM) we show that modulus (E) and yield stress (sigma(y) ) are independent of nanoscale film thickness (70 < h(f) < 250 nm) and graft molecular weight (30 kDa < MW < 370 kDa) and in good agreement with predictions from effective medium theory. Furthermore, thin film craze observations from TEM imply two critical length scales for maximum deformability of matrix-free PGN assemblies: (1) PGN core size should be less than the critical length scale of the craze microstructure, and (2) near-neighbor entanglements are optimized for the lowest graft length (N) where the PGN architecture exhibit intermediate graft densities (r(0)Sigma(0.5) similar to 3-6 and N/N-e similar to 4-6). These findings provide a foundation for optimizing designs that simultaneously maximize inorganic volume fraction, processability, and mechanical robustness for PGN thin film assemblies.