Core Cross-Linking of Dynamic Diblock Copolymer Micelles: Quantitative Study of Photopolymerization Efficiency and Micelle Structure

Asymmetric poly(ethylene oxide)-b-poly(ethyl acrylate) block copolymers bearing polymerizable (meth)acrylate groups at the end of the hydrophobic block were synthesized and self-assembled in water. The micellar structures were studied by static light scattering (SLS), dynamic light scattering (DLS) and small angle neutrons scattering (SANS). The micelles were checked to be dynamical and in a thermodynamic equilibrium. In order to stabilize the micelles, a photoinitiator was added and UV light was applied to initiate the cross-linking of micelles cores. Light scattering and SANS showed a very weak influence of the addition of the photoinitiator on the size and aggregation number of micelles. The photopolymerization takes place within a few seconds and the cross-linking process barely modifies the structural properties of the micelles. The efficiency of the core cross-linking was estimated in terms of (meth)acrylate double bonds conversion and rate of incorporation of unimers into the cross-linked structures. H-1 NMR showed a complete conversion of double bonds. Transmission electron microscopy (TEM), light and neutron scattering studies showed almost no modification of the structures, indicating that the exchange dynamics of unimers barely affects the structure during the cross-linking process. Size exclusion chromatography (SEC) analyses in DMF revealed a non-negligible amount of nonstabilized unimers. The crosslinked stars can be easily isolated from the remaining unimers by flash chromatography using hydrophobically modified silica gel.