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Abstract
In recent years, self-assembled gels formed by aggregation of gelator molecules into entangled networks with entrapped solvent have found potential use in biomedical applications, due to their reversible formation and their ability to mimic the extracellular matrix (ECM). The general aim of this research is to examine the structure-property of various pyromellitamides to deepen understanding of gelation.
Two series of non-polar and polar tetraalkyl pyromellitamides were synthesised and investigated for their gelation abilities. The former were able to form organogels in non-polar solvents through intermolecular hydrogen bonding interactions, where the gelation efficiency was enhanced as the length of the alkyl tails deceases. Although no gelation was observed for the polar tetraalkyl pyromellitamides, a highly viscous solution was formed by tetra(ethyl-6-aminohexanoate) pyromellitamide in water, suggesting that gels can possibly form with modification on this compound.
Detailed AFM and TEM studies suggested that the aggregation of the tetrahexyl pyromellitamide evolved from anisotropic arrays through to hollow tubes and entangled networks. The evolution of aggregation of this compound was also explored by time-dependent viscosity studies, which revealed exponential growth of aggregates, possibly related to the rearrangement of the columnar stacks as indicated from X-ray diffraction studies. Viscosity measurements also showed that anion salts can trigger a gel-to-sol transition for the cyclohexane solution of tetrahexyl pyromelltiamide, in particular, stronger sensitivity was obtained for small anions.
An enantiomerically tetraalkyl pyromellitamide was found to form organogels in non-polar solvents more effectively than the corresponding diastereomeric mixture. Circular dichroism (CD), AFM and TEM studies revealed that the chirality of these compounds influenced their aggregation modes, where the enantiomeric and diastereomeric gelators were shown to aggregate into right-handed helical arrays and helical structures with opposite handedness, respectively.
Another series of pyromellitamides with various amino acid tails were also synthesised to attempt formation of biocompatible hydrogels. However, no gelation was obtained for these compounds in water. Instead, the
L-phenylalanine and L-alanine methyl ester pyromellitamides formed partial gels in acetone and in tetrahydrofuran, which exhibited lack of entanglement behaviour as shown by TEM studies.