Synthesis of well-defined diblock copolymer nano-objects by RAFT non-aqueous emulsion polymerization of N-(2-acryloyloxy)ethyl pyrrolidone in non-polar media

Abstract

Polymerization-induced self-assembly (PISA) is widely recognized to be a powerful technique for the preparation of diblock copolymer nano-objects in various solvents. Herein a highly unusual non-aqueous emulsion polymerization formulation is reported. More specifically, the reversible addition–fragmentation chain transfer (RAFT) polymerization of N-(2-acryloyloxy)ethyl pyrrolidone (NAEP) is conducted in n-dodecane using a poly(stearyl methacrylate) (PSMA) precursor to produce sterically-stabilized spherical nanoparticles at 90 °C. This relatively high polymerization temperature was required to ensure sufficient background solubility for the highly polar NAEP monomer, which is immiscible with the non-polar continuous phase. A relatively long PSMA precursor (mean degree of polymerization, DP = 36) was required to ensure colloidal stability, which meant that only kinetically-trapped spheres could be obtained. Dynamic light scattering (DLS) studies indicated that the resulting PSMA₃₆–PNAEP_x (x = 60 to 500) spheres were relatively well-defined (DLS polydispersity <0.10) and the z-average diameter increased linearly with PNAEP DP up to 261 nm. Differential scanning calorimetry studies confirmed a relatively low glass transition temperature (T_g) for the core-forming PNAEP block, which hindered accurate sizing of the nanoparticles by TEM. However, introducing ethylene glycol diacrylate (EGDA) as a third block to covalently crosslink the nanoparticle cores enabled a spherical morphology to be identified by transmission electron microscopy studies. This assignment was confirmed by small angle X-ray scattering studies of the linear diblock copolymer nanoparticles. Finally, hydrophobic linear PSMA₃₆–PNAEP₇₀ spheres were evaluated as a putative Pickering emulsifier for n-dodecane–water mixtures. Unexpectedly, addition of an equal volume of water followed by high-shear homogenization always produced oil-in-water (o/w) emulsions, rather than water-in-oil (w/o) emulsions. Moreover, core-crosslinked PSMA₃₆–PNAEP₆₀–PEGDA₁₀ spheres also produced o/w Pickering emulsions, suggesting that such Pickering emulsions must be formed by nanoparticle adsorption at the inner surface of the oil droplets. DLS studies of the continuous phase obtained after either creaming (o/w emulsion) or sedimentation (w/o emulsion) of the droplet phase were consistent with this interpretation. Furthermore, certain experimental conditions (e.g. ≥0.5% w/w copolymer concentration for linear PSMA₃₆–PNAEP_x nanoparticles, ≥0.1% w/w for core-crosslinked nanoparticles, or n-dodecane volume fractions ≤0.60) produced w/o/w double emulsions in a single step, as confirmed by fluorescence microscopy studies.