Session: 04-28-02: Modeling and Experiments in Nanomechanics and Nanomaterials

Paper Number: 120015

120015 - The Role of Interchain Friction on the Nanoscale Energy Dissipation in Amorphous Polymers During Ballistic Impact 

Polymer and polymer nanocomposites are good candidates for lightweight protection against high-velocity ballistic threats due to their high strength-to-weight ratios and enhanced energy dissipation under high strain rates. Additionally, recent micro-particle impact tests have shown that polymer thin films can absorb remarkable amounts of energy during ballistic impacts compared to their macroscale counterparts, indicating a size scale dependence on energy dissipation. However, little is known about the molecular-level energy dissipation mechanisms resulting in a large knowledge gap that limits our understanding and application of nanomaterials for ballistic impact mitigation. In modern polymer dynamics theory, chain diffusion and relaxation are regulated by the interchain friction coefficient, which is thought to contribute significantly to impact energy absorption in polymers. The complexity of the chain friction versus chain dynamics relationship is compounded during high-pressure confinement, as the relationships between morphology, chain dynamics, temperature, and pressure become more interdependent. To better design and optimize novel ballistic resistant materials, these complex structure-property relationships need to be better understood. 

 In this work, we use large-scale Molecular Dynamics (MD) simulations to study the influence of changing intrachain and interchain potential energies and chain lengths during ballistic impact. Intrachain interactions govern the behavior of the chain backbone and contribute to chain stiffness and to torsional energy barriers that must be overcome during chain alignment. Interchain interactions, i.e., van der Walls forces, govern the chain-to-chain behavior and are mainly responsible for the strength of the friction coefficient. We present impact simulations conducted on a polyethylene-like amorphous polymer tested under various impact velocities while arbitrarily modifying the intrachain and interchain interactions to change the chain backbone properties and the friction coefficient. The intrachain interactions were changed by modifying the bond, angle, and dihedral coefficients of the atomic forcefield to induce various potential energies within the chain. The interchain interaction was modified by adjusting the van der Waals parameter to increase the potential energy between non-bonded atoms. A parametric study was conducted to determine the relative influence of each parameter on the energy dissipation. Two energy dissipation stages were identified during the ballistic impact: 1) energy dissipated during initial impact resulting in a highly compressive state of the polymer before perforating the film, and 2) energy dissipated after perforation culminating in the large stretching of chains. Overall, the van der Waals parameter was shown to be the most influential potential energy parameter, working in tandem with increasing chain length to provide the most energy absorption during impact. The dihedral angle parameter also has a strong influence, particularly in stage 1, while bond strength has a large influence in stage 2 due to the stretching of chains.  

Permission to publish was granted by Director, Geotechnical & Structures Laboratory.

Distribution A: approved for public release, distribution unlimited.

Presenting Author: Andrew Bowman U.S. Army Engineer Research and Development Center

Presenting Author Biography: Dr. Andrew L. Bowman is a Research Mechanical Engineer in the Structural Mechanics Branch (SMB) within the Geotechnical and Structures Laboratory (GSL) of the U.S. Army Engineer Research and Development Center (ERDC). He earned a Ph.D. from Mississippi State University in Mechanical Engineering (2019) and holds bachelor’s degrees in Biological Sciences (2010) and Mechanical Engineering (2014).
Dr. Bowman was awarded the Department of Defense (DoD) Science, Mathematics and Research for Transformation (SMART) Scholarship in 2016. His research interest includes multiscale modeling of viscoelastic materials with focus on characterizing and modeling high rate damage and plasticity in polymers.

Authors:

Andrew Bowman U.S. Army Engineer Research and Development Center
Caleb Miller Liberty University
William Pisani U.S. Army Engineer Research and Development Center