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

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 work, the effect of n-alkyl chain length in fatty-acid-functionalized polyamines on crosslinked epoxy-amidoamine systems is studied using a combined experimental and simulation-based approach. 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 T_g 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 T_g insensitivity. This work demonstrates a new way to significantly decrease the density of a thermoset polymer without compromising desirable properties such as high T_g and low coefficient of thermal expansion, and therefore may provide sounder rationale for molecular-based design of epoxy-based coatings.