Failure Mechanisms in Polymer Grafted Nanoparticle Thin Films Under High Strain Rate

Recent studies on amorphous homopolymers have shown that the high-rate microballistic fracture behavior of the polymer is directly related to its entanglement density. Based on this insight, we ask the question, “What is the relationship between chain entanglements and fracture behavior of matrix-free polymer grafted nanoparticle films?” In this work, we address this question by using laser-induced projectile testing (LIPIT) to study the fracture behavior of poly(methyl acrylate)-grafted silica nanoparticle thin films. By systematically varying the molecular mass of the poly(methyl acrylate), we find that kinetic energy loss, as well as the ballistic limit, is strongly dependent on the molecular mass of the polymer grafts that displays a non-monotonic trend. We show that the maximum energy absorption occurs when increase in molecular mass causes a transition from a close-packed concentrated polymer brush with no entanglements to a semi-dilute polymer brush with chain interdigitation. Thus the failure mechanism here is attributed to be pullout of short chain segments. Finally, we show that chain scission is the dominant failure mechanism above this transition point.