Tuning Adhesion and Energy Dissipation in Polymer Films between Solid Surfaces via Grafting and Cross-Linking

Polymer adhesion is underpinned by a network of chain molecules forming both adhesive bonds with the surfaces and cohesive bonds among themselves. Understanding how this network propagates mechanical stress and dissipates energy under tension is a challenging but essential task for improving polymer adhesives. To this end, we present a series of coarse-grained molecular dynamics simulations of polymer films joining two parallel flat surfaces. The polymer molecules are modeled as chains of beads that can covalently bind to the surfaces and to each other via a Morse potential, allowing bond breakage and chain scission. The mechanical response of the polymer film is studied under both small oscillatory strains and large deformations, leading to complete breakup. Starting from a melt of ungrafted polymer chains and introducing one feature at a time, we find that polymer cross-linking enhances adhesion more than covalent surface grafting. Cross-linking and grafting may also act in synergy, provided the grafting density is homogeneous. Moreover, surface heterogeneity at the nanometer scale affects the viscoelastic response at small strains. The simple model used in our simulations provides a valuable platform to translate molecular-level features of the polymer network, such as chain connectivity and bond strength, into measurable quantities, such as adhesion forces and energy dissipation.