Tailoring adhesion characteristics of poly(L-lactic acid)/graphene nanocomposites by end-grafted polymer chains: An atomic-level study

Tethered polymer chains are known to have a great potential in tailoring interfaces and promoting interfacial adhesion in polymer-based nanocomposites. In this context, this paper is aimed to explore the adhesion characteristics of poly(L-lactic acid)/graphene (GR) samples having a modified interface, where the PLLA chains at different grafting densities are attached on the GR surface. This is achieved through conducting pull-off tests employing all-atom molecular dynamics simulations. Monitoring the evolution of interfacial energy and pull-off force combined with the microstructural studies indicates that there exists an optimum coverage ratio (i.e., 0.002), beyond which the interfacial interactions would be disrupted. This is supported by the trend observed for variations of the adhesion characteristic such as interfacial normal strength and work of adhesion with the grafting density. To deeply analyze this issue, we further proceed to design composite systems of different architectures referred to as mushroom and brush-like conformations. It is demonstrated that the most effective molecular morphology is the one providing the highest degree of polymer chains entanglement. Also, failed attempts to reinforce adhesion by increasing the grafting density suggest that to have a beneficial chain arrangement, one should control several factors such as diffusion conditions, manufacturing methods, and grafting techniques.