Scaling for the inverse thickness dependence of specific penetration energy in polymer thin film impact tests

Abstract

Polymer thin films are widely used in packaging, barrier materials, and other applications where their performance under impact is of paramount importance. Recent studies on nanoscale polymeric films suggest that the specific penetration energy (Ep*), a key metric of impact performance, scales inversely with film thickness, making thinner films more resistant per unit mass. We employ coarse-grained molecular dynamics (CG-MD) to explain this counterintuitive observation. We systematically study the effect of film thickness on the impact resistance, quantifying penetration energy, stress wave propagation, and energy absorption processes. The penetration process is broken down into two stages, with Stage 1 involving initial local compression, and Stage 2 corresponding to global film deformation. Specimen thickness greatly influences which stage dominates energy dissipation, with nonlinear deformation in Stage 1 being non-negligible under all circumstances studied. When stage 1 dominates, the stage 1 penetration depth normalized by film thickness h1h is larger in thinner films. This leads to higher Ep* in thinner films, in analogy with indentation phenomena. In addition, higher energy transfer per unit mass is observed for thinner films as they deform in Stage 2. We propose that Ep* scales as the inverse of the square root of the thickness based on our nanoscale simulations. Remarkably, this scaling also agrees very well with existing impact experiments conducted at larger scales. Our findings reveal the shortcomings of Ep* as a simple metric for comparing the impact resistance of thin films, and provide important scaling arguments and molecular insights that will aid the design and interpretation of nanoscale impact tests.