Self-Aggregation of Gel Forming PEG-PLA Star Block Copolymers in Water

The aggregation behavior and dynamics of poly(ethylene glycol) (PEG) and poly(lactide) (PLA) chains in a homologous series of eight-armed PEG-PLA star block copolymers ((PEG(65)-NHCO-PLA(n))(8) with n = 11, 13, and 15) in water at different concentrations and temperatures were studied by means of H-1 and C-13 NMR spectroscopy and H-1 longitudinal relaxation time analysis. The state of water in these systems was also investigated through the combined use of H-1 and H-2 longitudinal relaxation time measurement. On the basis of the NMR experimental findings and of dynamic light scattering measurements, (PEG(65)-NHCO-PLA(n))(8) in water can be described as self-aggregated systems with quite rigid hydrophobic domains made of PLA chains and aqueous domains where both PEG chains and water molecules undergo fast dynamics. A smaller number of rigid domains was found for (PEG(65)-NHCO-PLA(11))(8) with respect to the homologous copolymers with longer PLA chains. At low concentrations, the PLA domains are mainly formed by chains belonging to the same molecule, thus giving rise to unimolecular micelles. At intermediate concentrations, that is, above the critical association concentration (CAC) but below the critical gel concentration (CGC), nanogels are formed by interconnection of several PLA domains through shared unimers. Above the CGC, the network is extended to the entire system, giving rise to macroscopic gels. In all cases, a fraction of PLA chains remains quite mobile and exposed to water due to topological constraints of the star architecture.