Molecular Origins of Adhesion at Aramid Fiber Interfaces: Polyvinyl Butyral and Polypyrrole.

Aramid fibers are one of the most important ballistic polymer materials owing to their strength, stability, and low density. However, the reliability of such fibers can suffer with exposure to UV and moisture, and these problems can be difficult to address due to the relatively inert nature of the fiber surface. It is therefore important to understand the adhesion between polymeric materials in order to improve the mechanical performance and durability of composite systems. In this paper, we investigate the adhesion of thermoplastic polymer coatings, polypyrrole (PPy) and poly(vinyl butyral) (PVB), and how the strength of adhesion is determined by the molecular interactions at the interface. Both peeling experiments and simulations reveal that PVB and PPy adhere significantly more strongly to PPTA than polyethylene (PE), a nonpolar polymer with relatively weak interactions. Molecular calculations show that PPy has the greatest adhesion strength due to the strong dipole moment of the pyrrole moiety, with the ring interactions appearing to dominate the adhesion strength. The composite strength depends not just on the molecular structure but also on the surface morphology, as we find that PVB with PPy nanofibers yields a larger peeling force compared to PPy nanodomains. Thus, design of polymer composites with strong interfacial interactions depends upon a careful consideration of the intermolecular interactions at the interface as well as the morphology of the surface coatings.