Self-assembled nanostructures from poly(aspartic acid) derivatives and their rheological behaviors

Abstract

A series of amphiphilic graft copolymers, poly(aspartic acid) grafted with dodecyl chains with different degree of substitution (DS) were synthesized, and their self-assembled nanoparticles in aqueous solution were prepared by a direct dissolution method. In this study, the morphological changes of self-assembled nanoparticles were induced as a function of DS of dodecyl chains, which are related to hydrophilic/hydrophobic ratio. At the same polymer concentration condition, the morphologies of self-aggregates are changed from spherical micelles to wormlike micelles, disklike micelles, and finally to tubular networks and vesicles in orders of DS of dodecyl chains. Various morphologies were characterized by TEM, DLS and SANS measurement, and their structural transitions could be explained in terms of packing parameter and interfacial energy. Charge interaction is also an important factor affecting self-aggregates behaviors. The most important aspect of this study is that a novel complex morphology was observed just by mixing two different charged polymers. The complexes of negatively charged $PAsp-g-C_{12}$ and positively charged PLL showed the structural transition of morphology from spherical micelles to vesicles, and it would be due to the coassembly of molecules induced by the electrostatic charge interaction. On the other hand, in case of tubular network (A60) and wormlike micelle (A35) aqueous solutions, there was phenomenal increase of viscosity according to the increase of polymer concentration. Their rheological behaviors were confirmed by both steady and dynamic shear measurement using ARES rheometer. And their rheological mechanisms could be explained in terms of the formation of three dimensional tubular networks and the entanglement of wormlike micelles. Consequently, self-aggregates behaviors and their structures are affected profoundly by the hydrophilic/hydrophobic ratio and the electrostatic charge interaction at molecular level. And rheol...