Recent high-rate microballistic studies of amorphous homopolymers have shown that their fracture properties are directly related to the entanglement density. Inspired by this insight, we pose the following question, "What is the relationship between polymer entanglements and fracture behavior for matrix-free polymer grafted nanoparticle (PGN) systems?" In this work, we begin to answer this question by studying the high-rate fracture behavior of PGN films via laser-induced projectile testing (LIPIT). The PGN films consist of poly(methyl acrylate) (PMA) chains grafted onto 14 nm diameter silica nanoparticles synthesized via surface initiated reversible addition-fragmentation chain transfer polymerization. By systematically varying the molecular mass of the PMA (from 31 kg/mol. to 184 kg/mol.) at a fixed grafting density, we find that kinetic energy loss due to puncture by the microprojectile is strongly dependent on the molecular mass of the polymer but displays a non-monotonic trend that is unlike other polymeric systems. By estimating the extent of chain interdigitation using a recently developed polymer brush scaling theory, we show that the molecular mass dependent enhancement in kinetic energy loss is attributed to the length- and time-scale defined mechanisms of fracture for entangled polymers.

Using microprojectiles to study the fracture of polymer-grafted nanoparticle films

ACS Spring Meeting 2021

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