Self-aggregates of hydrophobically modified poly(2-hydroxyethyl aspartamide) in aqueous solution

Abstract

Amphiphilic graft copolymers composed of poly(2-hydroxyethyl aspartamide)(PHEA) as the hydrophilic backbone and dehydrocholic acid(DHA) as the hydrophobic segment were successfully synthesized and the nano-size self-aggregates in aqueous solution was prepared. PHEA was obtained by a simple reaction of ethanolamine with synthesized poly(succinimide) and PHEA-g-DHA was synthesized through ester linkage, in the presence of dicyclohexylcarbodiimide (DCC) as coupling reagent. The synthesis was verified by NMR analysis and the degree of substitution of DHA groups was determined from $1^H$ NMR and elemental analysis. To study the solution property of the amphiphilic polymer in aqueous solution, the only four copolymers soluble in water were employed in this study. Nano-size self-aggregates in aqueous solution were prepared by sonication using a bath type sonifier. The self-aggregation behavior in aqueous solution was characterized by light scattering method and fluorescence spectroscopy. The self-aggregates were formed by the intramolecular and/or intermolecular interactions of grafted DHA groups. The degree of substitution(DS) of DHA groups influenced the association forms and mechanism between the hydrophobes of copolymers. The copolymer of low DS formed the self-aggregates mainly induced by intermolecular interactions of DHA groups while that of high DS formed the self-aggregates induced by intramolecular interactions at low polymer concentration. The more grafting amount of DHA groups induced the intramolecular interactions of DHA groups and compact self-aggregates in the concentration range studied. The polymer association process followed a closed association mechanism in the case of the copolymers of low DS, but the copolymers became to follow an open association mechanism with increasing the degree of substitution of DHA groups. It is believed, when the self-aggregates of DHA conjugated PHEA are formed, the grafted DHA groups are not sufficiently shielded by t...