RAFT aqueous dispersion polymerization of 4-hydroxybutyl acrylate: effect of end-group ionization on the formation and colloidal stability of sterically-stabilized diblock copolymer nanoparticles

Abstract

A series of all-acrylic poly(2-hydroxyethyl acrylate)_x-poly(4-hydroxybutyl acrylate)_y (PHEA_x–PHBA_y) diblock copolymer nanoparticles were prepared via an efficient one-pot RAFT aqueous dispersion polymerization protocol using either a carboxylic acid-functionalized RAFT agent (HOOC–PHEA_x–PHBA_y) or a morpholine-functionalized RAFT agent (Mo–PHEA_x–PHBA_y). The pH-dependent colloidal stability of the resulting sterically-stabilized nanoparticles was assessed by dynamic light scattering (DLS) and aqueous electrophoresis. The HOOC–PHEA₇₃–PHBA₂₁₇ nanoparticles exhibited reversible flocculation below pH 5.1, whereas the Mo–PHEA₇₆–PHBA₁₆₀ nanoparticles flocculated above pH 5. Moreover, the HOOC–PHEA₇₃–PHBA₂₁₇ nanoparticles proved to be sensitive to added salt, with incipient flocculation occurring in the presence of 20–60 mM KCl owing to charge screening. Thus, such nanoparticles require end-group ionization to confer colloidal stability via electrosteric stabilization. However, reducing the PHBA/PHEA molar ratio and/or increasing the PHEA_x stabilizer DP, leads to more efficient steric stabilization and hence enhanced colloidal stability. A series of HOOC–PHEA₇₃–PHBA_104–421 nano-objects prepared at pH 7 were characterized by visual inspection, DLS studies and shear-induced polarized light imaging. Discrepancies between these characterization techniques indicated that the worms and vesicles were unstable with respect to dilution. TEM studies were conducted after covalent stabilization of the nano-objects using glutaraldehyde (GA). More specifically, TEM studies of GA-crosslinked HOOC–PHEA₇₃–PHBA₂₄₃ and HOOC–PHEA₇₃–PHBA₄₂₁ nano-objects indicated the presence of spheres in both cases when crosslinked at 0.1% w/w and either worms or vesicles respectively when crosslinked at 10–20% w/w. Finally, HOOC–PHEA₇₃–PHBA₂₆₅ nano-objects were examined by variable temperature oscillatory rheology: thermoreversible sphere/worm and worm/vesicle transitions were observed between 2 and 50 °C.