Exploiting aromaticity in fatty terephthalate diesters to enhance melting point and prevent polymorphism

Abstract

Saturated aliphatic diesters sourced from vegetable oils present phase change properties particularly suitable for latent heat thermal energy storage (LHTES). However, inherent ester group flexibility, weak intermolecular attractions and intramolecular steric repulsions between the electronic clouds of the two ester groups, which lead to degraded packing and polymorphism, prevent these molecules from achieving optimal functionality as phase change materials (PCM). The present work demonstrates that the incorporation of a planar benzene ring between the two ester groups restricts intramolecular steric repulsion, increases the intermolecular attractions and effectively limits molecular flexibility, leading to the formation of stable monomorphs and enhancement of the thermal properties. The terephthalate diesters prepared in this work crystallized in the most stable triclinic β -polymorph regardless of cooling rate, demonstrated melting temperatures up to 86 °C and enthalpies up to 260J/g as well as higher thermal stabilities than corresponding aliphatic diol and diacid diesters. The enhanced thermal properties are mainly attributed to the π-π intermolecular attractions established between planar benzene rings inherent quadrupole moment and resonance effect from the delocalization of π-electrons within the extended conjugated π-system formed between the benzene ring and ester groups. The π-π intermolecular attractions and resonance stabilized ester group contributed to a higher energy barrier for rotation to occur at the center molecules and resulted in the stiffening of the fatty chains, facilitating periodic crystal packing. • Terephthalates are excellent PCMs for medium temperature thermal energy storage. • High melting point (86 °C) and thermal energy storage density (260J/g) achieved. • Rapidly store/release heat via sharp phase transitions with low supercooling. • Planar ring, π-π interaction and resonance effect instrumental performance. • Molecular packing constrained into the most stable monomorph regardless of kinetics.