Polymerization-induced self-assembly (PISA) has become a widely used technique for the rational design of diblock copolymer nano-objects in concentrated aqueous solution. Depending on the specific PISA formulation, reversible addition−fragmentation chain transfer (RAFT) aqueous dispersion polymerization typically provides straightforward access to either spheres, worms, or vesicles. In contrast, RAFT aqueous emulsion polymerization formulations often lead to just kinetically-trapped spheres. This limitation is currently not understood, and only a few empirical exceptions have been reported in the literature.
In the present work, the effect of monomer solubility on copolymer morphology is explored for an aqueous PISA formulation. More specifically a water-soluble poly(methacrylic acid) (PMAAx) stabilizer block is chain-extended with six methacrylic monomers exhibiting a range of water solubililties; benzyl methacrylate (BzMA), 2,2,2-trifluoroethyl methacrylate (TFEMA), n-butyl methacrylate (BMA), methyl methacrylate (MMA), 2-hydroxybutyl methacrylate (HBMA) and 2-hydroxypropyl methacrylate (HPMA). These studies demonstrated that non-spherical (anisotropic) nanoparticles were only obtained during polymerization of HBMA. Using HBMA (aqueous solubility = 20 g dm−3 at 70 °C) for the core-forming block allows access to an unusual “monkey nut” copolymer morphology over a relatively narrow range of target degrees of polymerization when using a poly(methacrylic acid) RAFT agent at pH 5. These new anisotropic nanoparticles have been characterized by transmission electron microscopy, dynamic light scattering, aqueous electrophoresis, shear-induced polarized light imaging (SIPLI), and small-angle X-ray scattering (SAXS). Polymerization of each of the other five monomers only lead to the formation of spherical nanoparticles, indicating that aqueous monomer solubility is indeed a key parameter for the synthesis of higher-order morphology nanoparticles via PISA in aqueous media. The PMAAx-PMMAy series of spherical block copolymer nanoparticles are characterized in more detail by SAXS and DSC and also evaluated as Pickering emulsifiers for the stabilization of oil-in-water emulsions.
The PISA formulations described above are sufficiently robust to enable high-throughput experiments to be performed using a commercial synthesis robot (Chemspeed Autoplant A100). More specifically, RAFT aqueous emulsion polymerization of either BMA and/or BzMA is used to prepare various examples of methacrylic multiblock copolymer nanoparticles using a PMAAx stabilizer block. Adequate stirring is essential to generate sufficiently small monomer droplets for such heterogeneous polymerizations to proceed efficiently. Good reproducibility can be achieved under such conditions, with well-defined spherical morphologies being obtained at up to 45% w/w solids. GPC studies indicated high blocking efficiencies but relatively broad molecular weight distributions (Ð = 1.36 – 1.85), suggesting the formation of well-defined (albeit rather polydisperse) block copolymer chains. These preliminary studies provide a sound basis for high-throughput screening of RAFT-mediated PISA formulations, which is likely to be required for commercialization of this technology. Our results indicate that PISA formulations enable the synthesis of methacrylic diblock and triblock copolymer nanoparticles in high overall yield (94–99%) within 1–3 h at 70 °C. However, tetrablocks suffer from incomplete conversions (87–96% within 5 h) and hence most likely represent the upper limit for this approach.
Finally, BMA is replaced with hexyl methacrylate (HxMA) in order to prepare a series of diblock, triblock and tetrablock copolymer nanoparticles that form films at room temperature. The resulting block copolymers are evaluated by NMR, DLS, TEM and GPC, and the corresponding films are analyzed by visible absorption spectroscopy, DSC and SAXS.
