One-step preparation of anisotropic snowman-like poly(ionic liquid) microspheres via microwave-assisted dispersion polymerization†

Anisotropic polymer microspheres (e.g. those with a snowman-like shape) have different properties from isotropic microspheres and they are widely used in photonic crystals, bioengineering, rheology, and so on. These anisotropic polymer microspheres are usually prepared by seed emulsion polymerization, dynamic swelling, microfluidic methods, etc. However, these methods are multi-step and lengthy and they are mainly used to prepare neutral polymer microspheres. Dispersion polymerization is a simple method to prepare polymer microspheres, whereas conventional dispersion polymerization can only produce spherical polymer microspheres due to the limitations of the nucleation growth mechanism and minimum surface energy theory. In this paper, we report a one-step microwave-assisted dispersion polymerization method that can prepare anisotropic poly(ionic liquid) microspheres with a controlled snowman-like morphology. This method is simple and only involves the one-step polymerization reaction of a solution containing an ionic liquid monomer, cross-linking agent, initiator and stabilizer in anhydrous ethanol at a specific temperature under microwave irradiation for 90 min. The morphology of the poly(ionic liquid) microspheres formed at different stages of the reaction is examined using optical microscopy and scanning electron microscopy. This shows that the rapid formation of the cross-linked poly(ionic liquid) microspheres with divinylbenzene as the cross-linking agent under microwave irradiation is the key to the formation of the anisotropic snowman-like poly(ionic liquid) microspheres. Through varying the amounts of monomer, cross-linking agent, initiator, solvent and the reaction temperature, the morphology of the anisotropic snowman-like poly(ionic liquid) microspheres can be modulated. In addition, using discontinuous microwave irradiation can promote the phase separation and increase the anisotropy of the snowman-like poly(ionic liquid) microspheres.