Molecular Dynamics Simulations of Surface and Interfacial Tension of Graft Polymer Melts

Understanding the surface properties of polymer melts is crucial for designing new polymeric coatings, adhesives, and composites. Here, we study the effect of molecular architecture on surface and interfacial tension of melts of graft and linear polymers by molecular dynamics simulations. In particular, we elucidate the effect of the degree of polymerization of the side chains n_sc and their grafting density 1/ n_g on the surface tension of the graft polymer/vacuum interface, γ_G, and the interfacial tension of the interface between graft and linear polymer melts, γ_GL. For the case of the graft polymer/vacuum interface, our simulations confirm that the surface tension is a linear function of the fraction of the backbone ends f_be and side chain ends f_se, γ_G = γ_∞ - γ_be f_be - Δγ f_se, where γ_∞ is the surface tension of the system of graft polymers with infinite molecular weight and γ_be and Δγ are surface tension contributions from backbone ends and difference between contributions coming from the side chain ends and grafting points, respectively. This dependence of the surface tension highlights the entropic origin of the surface tension corrections associated with the redistribution of the grafting points and ends at the interface. However, the interfacial tension of the interface between graft and linear polymer melts does not show any significant dependence on the molecular structure of the graft polymers, thus pointing out the dominance of enthalpic contribution to the interfacial tension.