The Projectile Perforation Resistance of Materials: Scaling the Impact Resistance of Thin Films to Macroscale Materials.

From drug delivery to ballistic impact, the ability to control or mitigate the puncture of a fast-moving projectile through a material is critical. While puncture is a common occurrence, which can span many orders of magnitude in the size, speed, and energy of the projectile, there remains a need to connect our understanding of the perforation resistance of materials at the nano- and microscale to the actual behavior at the macroscale that is relevant for engineering applications. In this article, we address this challenge by combining a new dimensional analysis scheme with experimental data from micro- and macroscale impact tests to develop a relationship that connects the size-scale effects and materials properties during high-speed puncture events. By relating the minimum perforation velocity to fundamental material properties and geometric test conditions, we provide new insights and establish an alternative methodology for evaluating the performance of materials that is independent of the impact energy or the specific projectile puncture experiment type. Finally, we demonstrate the utility of this approach by assessing the relevance of novel materials, such as nanocomposites and graphene for real-world impact applications.

Controlling toughness of polymer-grafted nanoparticle composites for impact mitigation.

Toughness in an entangled polymer network is typically controlled by the number of load-bearing topological constraints per unit volume. In this work, we demonstrate a new paradigm for controlling toughness at high deformation rates in a polymer-grafted nanoparticle composite system where the entanglement density increases with the molecular mass of the graft. An unexpected peak in the toughness is observed right before the system reaches full entanglement that cannot be described through the entanglement concept alone. Quasi-elastic neutron scattering reveals enhanced segmental fluctuations of the grafts on the picosecond time scale, which propagate out to nanoparticle fluctuations on the time scale 100s of seconds as evidenced by X-ray photon correlation spectroscopy. This surprising multi-scale dissipation process suggests a nanoparticle jamming-unjamming transition. The realization that segmental dynamics can be coupled with the entanglement concept for enhanced toughness at high rates of deformation is a novel insight with relevance to the design of composite materials.

Using microprojectiles to study the ballistic limit of polymer thin films.

The dynamic impact between a particle and a planar material is important in many high impact events, and there is a growing need to characterize the mechanical properties of light-weight polymeric materials at dynamic loading conditions. Here, a laser-induced projectile impact test (LIPIT) is employed to investigate the ballistic limit (V0) and materials properties at impact velocities ranging from 40 m s-1 to 70 m s-1. An analytical expression describing the various energy dissipation mechanisms is established to estimate the yield stress and elasticity for polycarbonate thin films. This measurement approach demonstrates the utility of using low sample mass for discovery of materials for impact mitigation, as well as high-throughput mechanical characterization at dynamic loading rates.

pH-Controlled Electrochemical Deposition of Polyelectrolyte Complex Films.

Polyelectrolyte complex (PEC) films made from oppositely charged polymer chains have applications as drug-delivery vehicles, separation membranes, and biocompatible coatings. Conventional layer-by-layer techniques for polyelectrolyte coatings are low-throughput and multistep processes that are quite slow for building films on the order of micrometers. In this work, PEC films are electrochemically deposited using a rapid one-pot method, yielding thick (1 µm) films within short experimental time scales (5 min). This rapid electrodeposition is achieved by exploiting the reduction of hydrogen peroxide at mild electrode potentials that avoid water electrolysis yet trigger the pH-responsive self-assembly of a PEC film composed of poly(acrylic) acid and poly(allylamine) HCl. In situ rheology using an electrochemical quartz crystal microbalance quantified the shear modulus-density product of the deposited layer to be on the order of 107 Pa g/cm3 at a frequency of 15 MHz, with a viscoelastic phase angle of approximately 50°. This electrodeposition scheme furthers the development of PEC coatings for more high-throughput applications, where a fast and efficient single-step approach would be desirable for obtaining coatings.