RAFT synthesis of diblock copolymers for use as aqueous pigment dispersants

This Thesis focuses on the preparation of novel dispersants for aqueous pigments in the form of either diblock copolymer nanoparticles or molecularly-dissolved copolymer chains. The former are prepared via polymerisation-induced self-assembly, and both types of copolymer are synthesised using aqueous reversible addition-fragmentation chain transfer polymerisation.
Firstly, the synthesis and aqueous solution behaviour of novel poly(methacrylic acid)-poly(2-hydroxypropyl methacrylate) (PMAA-PHPMA) nanoparticles, via RAFT dispersion polymerisation of HPMA is reported. More specifically, PMAA50-PHPMA235 is investigated owing to its particularly interesting pH- and thermo-responsive behaviour. As synthesised at pH 5.5, relative large, polydisperse particles are obtained owing to the partially ionised nature of the PMAA block. At higher pH, this steric stabiliser block becomes highly anionic, leading to the formation of thermoresponsive nanoparticles. 1H NMR, dynamic light scattering (DLS), and transmission electron microscopy (TEM) studies indicate molecularly-dissolved chains are formed at 5 °C that aggregate to produce spheres at 10 – 35 °C and worm-like particles at 50 °C. The latter particles form a gel-like dispersion and shear-induced polarised light imaging yields a characteristic Maltese cross, as expected for anisotropic nanoparticles.
This PMAA50-PHPMA235 diblock copolymer is evaluated as a dispersant for commercial-grade carbon black. Its thermoresponsive behaviour leads to more effective pigment dispersant performance at lower temperatures. Moreover, the resulting carbon black dispersions exhibit greater long-term stability compared to that prepared with a commercial copolymer dispersant.
A novel synthetic route to zwitterionic diblock copolymers via an atom-efficient, wholly aqueous one-pot protocol is reported. Depending on the nature of the anionic and cationic comonomers and the solution pH that is selected, this involves RAFT aqueous solution polymerisation, RAFT aqueous dispersion polymerisation or RAFT aqueous emulsion polymerisation. Some of these zwitterionic diblock copolymers exhibit macroscopic precipitation at their isoelectric points (IEP), which enables efficient trithiocarbonate end-group removal using hydrazine followed by a wholly aqueous work-up.
Zwitterionic diblock copolymers are investigated as dispersants for two commercial inorganic pigments. Poly((2-dimethylamino)ethyl methacrylate)-poly(methacrylic acid) (PDMA-PMAA) is shown to be a particularly effective dispersant for transparent yellow iron oxide, which is known to be a typically problematic nano-sized pigment. DLS studies indicate a large reduction in the apparent pigment particle size compared to no dispersant, and rheology measurements confirm a substantial reduction in dispersion viscosity after milling using a high-energy mixer. Furthermore, these zwitterionic diblock copolymer dispersants require a lower concentration based on mass of pigment than commercially available dispersants.
Finally, the synthesis of zwitterionic poly((2-diethylamino)ethyl methacrylate)-poly(2-carboxyethyl acrylate) (PDEA-PCEA) diblock copolymer nanoparticles via RAFT aqueous dispersion polymerisation is investigated. Such copolymers can also be prepared using a convenient aqueous one-pot protocol and self-assemble to form either anionic micelles above or cationic micelles below the copolymer isoelectric point. This is the most convenient synthesis route yet reported for such schizophrenic diblock copolymers. Preliminary results for their effectiveness as nanoparticle dispersants for titanium dioxide is briefly reported; nanoparticle adsorption onto this white pigment appears to occur both below and above the IEP.