The anomalous behaviour of aliphatic fatty diamides: Chain length and hydrogen bonding interactions

A series of five linear aliphatic diamides were synthesised via facile direct coupling of stearic acid (C18) with diamines of varying carbon number (18-n-18, n = 2, 4, 6, 8 and 10) in a catalyst-free neat reaction. The compounds were investigated as potential renewable phase change materials (PCMs) for latent heat thermal energy storage. The thermal properties of the diamides were predictably related to the diamine chain length. TGA measurements indicated that the diamides evaporated before decomposition, with evaporation onsets increasing from 325 degrees C for 18-2-18 to plateau at 337 degrees C for the longer-chain diamides. The diamides display negligible supercooling and melt and crystallize at 140 degrees C-150 degrees C within a narrow transition span (< 5 degrees C). Notably, both their melting and crystallization temperatures versus n present a maximum at n = 4, explained by peak hydrogen bonding and chain length considerations. As the distance between the amide groups increases, the transition temperatures decrease exponentially owing to the subsequent decrease in intermolecular hydrogen bond strength. The latent heat of the diamides presented a sharp increase from 165 J/g for 18-2-18 to plateau at similar to 210 J/g for the other amides (n >= 4). XRD measurements support the thermal transition behaviour; it shows that all the diamides crystallize from the melt into the beta-phase, except for 18-2-18 in which the beta-phase coexisted with the less stable beta'-phase. Overall, the study shows that the diamides are suitable alternatives to classical petrochemical derived PCMs, expanding the currently limited library of organic PCMs.