Simulations of Nanoindentation of Polymer Surfaces: Effects of Surface Cross-Linking on Adhesion and Hardness

We present a simulation methodology to chemically cross-link the surface of fully atomistic models of polymers. We simulated nanoindentation of the cross-linked model surfaces whereby nanoparticles (fullerene in this case) are used to probe different regions of the polymer surface enabling us to characterize the surface hardness of the polymer film and calculate the work of adhesion between polymer and nanoparticle. The cross-linked polyester surfaces are constructed using hexamethylene and isophorone di-isocyanate cross-linkers. Our results indicate that, despite surface cross-linking with hexamethylene di-isocyanate molecules, the polymer top surface still maintains significant softness. The hexamethylene cross-links that protrude from the surface are deformable, resulting in a strong adhesion between the surface and carbon particle. In contrast, isophorone surface cross-linking results in significantly weaker adhesion with carbon, due to the less deformable rigid structure formed at the top layer. In this case, while rigidity is imparted to the polymer top surface, the flexibility of the core region of the polymer is maintained.