Molecular dynamics simulations of ultrathin PMMA films

Abstract

The mechanical properties of an ultrathin film made from a thermoplastic differ from the bulk due to the presence of the free surface. Here, molecular dynamics simulations are used to explore the thickness dependence of uniaxial and equi-biaxial tensile responses of polymethyl methacrylate (PMMA) films. The sensitivity of deformation response to temperature, molecular weight and the degree of side-branching is determined. We find that the tensile failure strain decreases with decreasing film thickness, temperature, and with decreasing molecular weight. The degree of side-branching plays a secondary role in dictating the tensile response. Failure is by the initiation of voids at the free surface, followed by the expansion of the voids in the thickness direction. Recent solid−state nanofoaming experiments and models suggest that the attainable porosity of nanofoams is less than that of macro−scale foams due to the reduced ductility of the cell walls of the nanofoam. Our results provide a physical explanation for this observation.