Effect of alkyl chain length on properties of epoxy-amidoamine systems
Authors:   , Sridhar Arun Srikanth , Abrams Cameron F. 
Published: July 2021

Polyamine crosslinkers functionalized with pendant fatty acids are commercially used in epoxy coatings in part to enhance water barrier properties for corrosion prevention. However, a systematic understanding of the links between monomer molecular structure and material properties of such systems remains elusive, which limits our ability to design newer and more environmentally friendly coating systems. In this dissertation, the effect of n-alkyl chain length in fatty-acid-functionalized polyamines on the properties of crosslinked epoxy-amidoamine systems is studied using molecular dynamics (MD) simulations. Simulations are compared with experiments whenever available. Both experiments and simulations show that density decreases and volume expansion coefficients increase with increasing pendant chain length. Furthermore, it is shown that the trends in density and coefficient of volume thermal expansion with chain length obtained from simulations are consistent with an ideal mixing approximation. Interestingly, however, the glass-transition temperature Tg is found to be insensitive to chain length. Molecular simulations reveal that increasing the alkyl chain length from four to ten carbons does not introduce new flexibility mechanisms to the dense thermosets which explains the Tg insensitivity. The mechanical properties of epoxy-amidoamine systems were also analyzed. The glassy-state Young's modulus was estimated using non-equilibrium molecular dynamics 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. Poissons ratios of all systems predicted from the simulations were insensitive to n, indicating a lack of anisotropy. The yield stress and strain hardening modulus were predicted for two systems with and without the alkyl chain (n=10). It was found that the thermoset with alkyl chain had lower yield stress and strain hardening modulus compared to the one without. Stress partitioning revealed that LJ (Lennard-Jones) and covalent interactions contribute significantly to the yield stress and the strain hardening modulus with electrostatic interactions having no significant effect. It was found that the lower yield stress is due to the dilution effect. The decrease in strain hardening modulus is due to both dilution effect and LJ (Lennard-Jones) interactions of methylenes in the network. This dissertation demonstrates some new and generic ways to assess the role of inter and intra-molecular interactions of molecular constituents on the properties of the crosslinked networks using molecular dynamics which are otherwise difficult or impossible to obtain from experiments and therefore may provide sounder rationale for molecular-based design of epoxy-based coatings.