Shear response in crystalline models of poly(p-phenylene terephthalamide)

The high anisotropy of polymer-based fibres confers them high tensile strength, but also makes them more vulnerable against non-uniform mechanical loads. This is even more important for Kevlar® fibres, which are made up of individual fibrils containing crystalline domains at different orientations. In this work, crystals of poly(p-phenylene terephthalamide), or PPTA, are subject to shear strain and their response simulated in atomic detail. For shear deformations involving movements orthogonal to the chains' axis, an originally defect-free crystal fully recovers its native contacts and original shear strength after repeated failures. Full recovery of crystalline contacts proceeds over tens of nanoseconds, demonstrating the importance of sampling realistic strain rates. For shear deformations involving movements parallel to the chains' axis, the crystal accumulates an increasing number of defects that lower its shear strength. Although the same types of intermolecular forces make up the response of a PPTA crystal to each shear mode, the relative contributions of these modes in a specific type of applied load will affect profoundly how Kevlar® fibrils and fibres fail under shear. The shear stress–strain profiles here computed will ultimately benefit the development of quantitative mechanical models of Kevlar® as well as new polyamide materials.