Transparent diblock copolymer nanoparticle dispersions via efficient RAFT emulsion polymerisation in ionic liquid

Abstract

We report the first reversible addition-fragmentation chain transfer polymerisation-induced self assembly (RAFT-PISA) in ionic liquid (IL) that proceeds under emulsion conditions. Moreover, this formulation exploits refractive index contrast matching to generate highly transparent nanoparticle dispersions. Specifically, 1-ethyl-3-methyl-imidazolium ethylsulfate, [EMIM][EtOSO3], was used as the solvent for the chain extension of poly(2-hydroxyethyl methacrylate) (PHEMA) macromolecular chain transfer agents (macro-CTAs) using n-butyl methacrylate (BuMA) via RAFT emulsion polymerisation. Two series of PHEMA_x-b-PBuMA_y diblock copolymers with target PBuMA degrees of polymerisation (DPs) varying from 50 to 1000 were synthesised using either a PHEMA₂₁ or PHEMA₇₇ macro-CTA. All resulting nanoparticle dispersions yielded highly transparent dispersions, even when nanoparticle diameters exceeded 100 nm, due to the closely matched refractive index values of the [EMIM][EtOSO₃] solvent and PBuMA nanoparticle core. Detailed analysis using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) confirmed the presence of spherical nanoparticles. Furthermore, the synthesis of PHEMA-b-PBuMA via this new PISA formulation was directly compared to equivalent block copolymer syntheses conducted in N,N-dimethylformamide (DMF) or ethanol/water mixtures. It was found that syntheses conducted in [EMIM][EtOSO₃] resulted in the highest monomer conversions (up to >99%) and lowest dispersity (Đ_M) values (as low as 1.16) in the shortest reaction times (2 hours) compared to the other solvent systems. This work demonstrates the use of ILs as a more sustainable and effective solvent for RAFT PISA via the development of the first emulsion PISA formulation in IL.