Aggregates and hydrogels prepared by self-assembly of amphiphilic copolymers with surfactants

A series of water-insoluble polylactide/poly(ethylene glycol) (PLA/PEG) block copolymers were synthesized by ring-opening polymerization of lactide in the presence of mono- or dihydroxyl PEG, using nontoxic zinc lactate as catalyst. Interactions between the resulting copolymers and sodium dodecyl sulfate (SDS) in water were studied by varying SDS fraction and copolymer concentration, using ultraviolet-visible spectrometer. Light transmission results show that all the insoluble copolymers strongly interact with SDS, and the solubility of the copolymers is improved with increasing SDS fraction. Copolymers with triblock structures or higher molar masses present larger variation of solubility as compared to those with diblock structures or lower molar masses. Transmission electron microscopy and dynamic light scattering were then employed to examine the microstructure of aggregates in the mixture solutions. Various aggregates such as vesicles, branch-like micelles, spherical micelles, or nanogels were observed, depending on the SDS fraction and copolymer concentration. It is assumed that at low SDS fractions, surfactant molecules attach to PLA segments and make the copolymers more soluble to form various aggregates. At high SDS fractions, junctions composed of SDS aggregates with PLA segments involved inside are formed in the case of triblock copolymers and diblock ones with high molar masses. These junctions lead to cross-linking of copolymer chains to yield a nanogel. Hydrogels can be obtained at high concentrations as confirmed by rheological measurements.