Understanding Interlayer Adhesion in Lamellar Polymer Composite Materials via Computation

Personal body armor applications often rely on high-performance polymers, such as poly(p-phenylene terephthalamide) (PPTA) and ultrahigh-molecular-weight polyethylene (UHMWPE). While these materials have seen success, improvements can be made by utilizing composites, thereby combining desirable properties from several different polymers into layered materials with variable thickness, weight, and rigidity. Herein, we use fully atomistic molecular dynamics (MD) simulations, as a complement to peel test and imaging experiments, to explore strategies for modifying the PPTA/UHMWPE interface with an aim to improve adhesion and ballistic performance. Specifically, we investigate how the adhesion between PPTA and UHMWPE is impacted by plasma treatment and/or by coating with polypyrrole (PPy) in the form of either nanoclusters or nanodomains. Our MD simulations, coupled with peel test experiments, demonstrate that while plasma-treatment of UHMWPE results in stronger adhesion to PPTA, it is not as effective as coating with PPy. Insight from density functional theory (DFT) electronic structure calculations shows that pyrrole and benzene moieties have a relatively high binding energy due to the polar nature of pyrrole. However, H-bond interactions are even stronger. Thus, the interfacial adhesion appears to depend on both the specific interactions between different polymer monomers and the accessible surface area at each polymer-polymer interface. This finding is also evidenced by the poor performance of PPy nanodomains versus nanofibers, demonstrated in our peeling experiments. Therefore, achieving an optimal balance between the interaction strength and the density of interaction sites is crucial for improving adhesion between these polymer interfaces.