Enhancing sustainability in cross-linkable block copolymer formulations

Abstract

This research showcases the use of ionic liquid as a more sustainable and effective solvent for the synthesis of poly(2-hydroxyethyl methacrylate)-b-poly(n-butyl methacrylate) (PHEMA-b-PBuMA) diblock copolymers, developing the first emulsion polymerisation-induced self-assembly (PISA) formulation in ionic liquid in the process. Also, the viability of suitable candidates to introduce ultraviolet (UV) cross-linking functionality to poly(2-hydroxyethyl methacrylate) (PHEMA) was investigated, showcasing potential for use in applications such as inkjet printing. 1-Ethyl-3-methyl-imidazolium ethylsulfate, [EMIM][EtOSO3], N,N-dimethylformamide (DMF) and an ethanol/water mixture were used as reaction solvents for the chain extension of PHEMA macromolecular chain transfer agents (macro-CTAs) with n-butyl methacrylate (BuMA) via reversible addition-fragmentation chain transfer (RAFT) polymerisation to yield PHEMA-b-PBuMA diblock copolymers. Reactions conducted in [EMIM][EtOSO3] yielded high monomer conversions (> 86%) and diblock copolymers with molar mass dispersity values as low as 1.15 in shorter reaction times compared to other solvent systems. Subsequently, a series of diblock copolymers with PBuMA DPs varying from 50 to 1000 were synthesised in [EMIM][EtOSO3] via RAFT-mediated PISA under emulsion conditions. This yielded highly transparent dispersions, even at nanoparticle diameters >100 nm, due to the closely matched refractive index values of the [EMIM][EtOSO3] solvent and PBuMA nanoparticle core. The presence of spherical nanoparticles was confirmed by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). 4-Methylumbelliferone (4MU), cinnamic acid (CA), 9-anthracenecarboxylic acid (9ACA) and 3-(2-furyl)acrylic acid (FAA) were explored as UV cross-linkable monomer precursors. 1H nuclear magnetic resonance (NMR) and Fourier-transform infrared (FTIR) spectroscopy were used to confirm dimerization success, where CA was found to be the only precursor to not dimerize successfully. Subsequently, 4MU was converted into polymerisable 7-acryloyloxy-4-methylcoumarin (AOMC), whereas 9ACA and FAA were esterified with PHEMA, forming poly(9-anthrancenylethyl methacrylate) (P9AEMA) and poly(ethylfurfurylideneacetate methacrylate) (PEFMA), respectively. Overall, FAA was deemed the most suitable precursor due to easier purification and successful UV cross-linking before and after functionalisation.