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 (Σ < 0.15 chains/nm2) where chain entanglements are maximized. From the wrinkling–cracking method (WCM) we show that modulus (E) and yield stress (σy) are independent of nanoscale film thickness (70 < hf < 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 (r0Σ0.5 ∼ 3–6 and N/Ne ∼ 4–6). These...