Projectile Impact Shock-Induced Deformation of One-Component Polymer Nanocomposite Thin Films

Abstract

Matrix-free assemblies of polymer-grafted nanoparticles (PGNs) enable mechanically robust materials for a variety of structural, electronic, and optical applications. Recent quasi-static mechanical studies have identified the key parameters that enhance canopy entanglement and promote plasticity of the PGNs below T g . Here we experimentally explore the high-strain-rate shock impact behavior of polystyrene grafted NPs and compare their energy absorption capabilities to that of homopolystyrene for film thicknesses ranging from 75 to 550 nm and for impact velocities from 350 to 800 m/s. Modeling reveals that the initial shock compression results in a rapid temperature increase at the impact site. The uniformity of this heating is consistent with observations of greater kinetic energy absorption per mass (E p *) of thinner films due to extensive viscoplastic deformation of molten film around the penetration site. Adiabatic heating is insufficient to raise the temperature at the exit surface of the thickest films resulting in increased strain localization at the impact periphery with less melt elongation. The extent and distribution of entanglements also influence E p *. Structurally, each NP acts as a giant cross-link node, coupling surrounding nodes via the number of canopy chains per NP and the nature and number of entanglements between canopies anchored to different NPs. Load sharing via this dual network, along with geometrical factors such as film thickness, lead to extreme E p * arising from the sequence of instantaneous adiabatic shock heating followed by visco-plastic drawing of the film by the projectile. These observations elucidate the critical factors necessary to create robust polymer-nanocomposite multifunctional films.

Simulation of the Coronal Dynamics of Polymer-Grafted Nanoparticles

“…The dynamics of polymer chains play an important role in many physical properties of polymeric materials such as the characteristics of the stress relaxation modulus G ( t ), fracture mechanics, transport phenomena, energy dissipation, and many others. − When polymers do not entangle, their dynamics can be described in a couple of ways. The Rouse description of the dynamics is based on a friction coefficient for segments of the polymer chain (ζ p ) that scales linearly with the number of monomers in that segment (i.e., ζ p = p ζ 0 ).…”

Extreme Tribological Characteristics of Copolymers Induced by Dynamic Rheological Instability

“…Due to low phase transition temperatures and ratedependent dynamics of macromolecules, the UHR tribological and rheological properties of polymers can exhibit exceptionally diverse nonlinearities and to date are not well-understood. When a polymer microparticle (μP) collides with a stationary target substrate, its kinetic energy is converted to heat due to adiabatic shock compression 12 and plastic deformation within a short duration (<100 ns). Combined with the low thermal conductivity of polymers, high-speed collision of polymeric μPs leads to an effectively adiabatic situation, which leads to the tribological nonlinearity of the material at the interface.…”

Layered Thin Film Deposition via Extreme Inter-Brush Slip in a Lamellar Block Copolymer

“…29 Hydrostatic pressure generally raises the T ODT due to an increase in the system volume upon mixing, while extensive layer deformation lowers the T ODT due to the unfavorable chain conformations. Hydrocode simulations of particle impact to a glassy homopolymer PS at these velocities show that the compressive shock wave creates a several GPa step jump in pressure that is released within ∼100 picosecond 13 . The initial jump in temperature due to shock compression slightly lags that of the pressure rise.…”

“…The laser-induced projectile impact test (LIPIT) has recently emerged as a useful small-scale ultrahigh strain rate test . LIPIT has several loading configurations including particle-specimen impacts onto rigid substrates. − Quantitative studies are performed by monitoring the changes in the specimen-projectile speed and shape along with post-impact cross-sectional imaging of the specimen interior morphology. A simple initial microstructural state allows detailed analysis of the deformation mechanisms via post-impact microscopy of the deformation texture and displacement field of the material.…”