The effect of alkyl chain length on mechanical properties of fatty-acid-functionalized amidoamine-epoxy systems

The effects of pendant alkyl chain length n ranging from n=0 to n=10 in amidoamine crosslinkers on mechanical properties of epoxy thermosets are explored in this work. The glassy-state Young's modulus was estimated using non-equilibrium molecular dynamics (MD) simulations and compared with experiments. Both simulations and experiments showed that Young's modulus decreases with increase in n. Stress partitioning based on molecular interaction types showed that both Lennard-Jones and covalent bond interactions were responsible for this sensitivity, with Coulombic interactions playing no significant role. The dependence of Young's modulus on n was strain-rate dependent in the simulations, with moderate to high strain rates showing no sensitivity. A strong correlation was observed between Young's modulus from non-equilibrium MD and volume fraction of methylenes estimated from equilibrium MD. Poisson's ratios of all systems predicted from the simulations were insensitive to n, indicating a lack of anisotropy. The information revealed here on the roles of various intermolecular interactions on the mechanical properties of these thermosets could be useful for design of crosslinkers.