A molecular dynamics study of the effects of hydrogen bonds on mechanical properties of Kevlar (R) crystal
SC Chowdhury and JW Gillespie, COMPUTATIONAL MATERIALS SCIENCE, 148, 286-300 (2018).
Effects of loading direction and hydrogen bonds on the anisotropic elastic and strength properties of p-phenylene terephthalamide (PPTA) crystal (i.e. Kevlar (R)) are studied using molecular dynamics simulations with reactive force field ReaxFF. Simulations of monotonic tension, compression and shear loadings of the crystal indicate that the elastic properties exhibit orthotropic symmetry but strengths in the various crystal directions are anisotropic and highly dependent on the direction of loading. Tensile strength in the chain axis of the crystal fails by chain scission at a stress level that is twenty times higher than the compression strength governed by chain buckling. To study the effects of hydrogen bonds, simulations are repeated by removing the hydrogen bonding interactions from the ReaxFF force field parameter file. Simulation results indicate that hydrogen bonds do not affect the axial properties that are dominated by covalent bonds in the chains. However, in the absence of hydrogen bonds, transverse tensile, compressive and shear moduli (40-44%) and strengths (47-69%) are reduced significantly. The absence of hydrogen bonding on changes in failure modes for the various loading directions are identified. The results presented in this study provide insight into moisture induced property degradation mechanisms of Kevlar (R) reported in the literature attributed to reduction in hydrogen bonding.
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