Synthesis of high molecular weight polymers as low-viscosity latex particles by RAFT aqueous dispersion polymerisation

This Thesis uses reversible addition-fragmentation chain transfer (RAFT) polymerisation to develop new aqueous dispersion polymerisation formulations in either salty or acidic aqueous media. A suitable steric stabiliser is chain-extended with a second block, which becomes insoluble at a critical degree of polymerisation (DP). Hence the resulting amphiphilic diblock copolymer chains self-assemble to form spherical nanoparticles via polymerisation-induced self-assembly (PISA), which ensures a low-viscosity colloidal dispersion. To identify suitable steric stabilisers and core-forming blocks for the high (or low) salt formulations studied in Chapters 2 to 5, various hydrophilic homopolymers were subjected to aqueous solubility tests. This involved attempted dissolution of each homopolymer in a series of known concentrations of ammonium sulfate, thus identifying the minimum salt concentration required for their insolubility. Various cationic, anionic, zwitterionic and non-ionic stabilisers proved to be suitable water-soluble precursors for chain extension with acrylamide-based core-forming blocks. Formulations include poly(2-(acryloyloxy)ethyl trimethylammonium chloride)–poly(N,N-dimethylacrylamide) (PATAC− PDMAC), poly(2-(methacryloyloxy)ethyl phosphorylcholine−poly(N,N-dimethylacrylamide) (PMPC−PDMAC), poly(sodium acrylate)–polyacrylamide (PNaAc−PAM), poly(2-hydroxyethyl acrylamide)−poly(N-acryloylmorpholine) (PHEAC−PNAM) in the presence of added salt and poly(2-acrylamido-2-methylpropanesulfonic acid, sodium salt)−poly(2-carboxyethyl acrylate) (PAMPS−PCEA) at pH 2.0. In this latter case, the PCEA core block is insoluble at this pH owing to protonation of its pendent carboxylic acid groups.
Dilution of the first four colloidal dispersions with deionised water or increasing the pH of the latter formulation leads to molecular dissolution to produce a viscous solution. If the core-forming block is sufficiently long, transparent free-standing gels can be obtained. Such dilution- or pH-triggered viscosity modification has potential applications for home and personal care products or as a dewatering flocculant in oilfield applications. In this Thesis, various synthesis parameters are optimised with the aim of maximising this thickening effect, including the upper limits solids content, final monomer conversion, chain extension efficiency and core-forming block DP. Furthermore, experimental reproducibility was optimised by judicious selection of the initiator type, reaction temperature and solution pH. For example, PMPC139-PDMAC6000 particles were prepared at 20% w/w solids using a potassium persulfate/ascorbic acid redox initiator at 30°C in the presence of 2.0 M ammonium sulfate.
These diblock copolymer particles were characterised using dynamic light scattering (DLS), laser diffraction, electrophoretic mobility and optical microscopy. Their mean diameter is significantly larger than those typically reported in the PISA literature. This is partly because of the relatively high DP targeted for the core-forming block but also because the particle cores are not fully dehydrated. 1H NMR spectroscopy was used to calculate monomer conversions and to assess the relative degree of hydration of the core-forming block at various salt concentrations. The diblock copolymer chains were also analysed by gel permeation chromatography (GPC) using a pH 7.0 TRIZMA buffer eluent combined with a refractive index detector. This technique indicated high chain extension efficiencies but broad molecular weight distributions (Mw/Mn ~ 1.5-2.5). The latter is consistent with the relatively low [RAFT agent]/[initiator] molar ratios required for such formulations. In addition, static light scattering in aqueous eluent was used to determine absolute Mw values of up to 2.5 MDa for the ‘low salt’ PHEAC−PNAM formulation. Finally, rotational rheometry was used to determine the solution viscosity of the initial low-viscosity dispersions and highly viscous solutions obtained after either lowering the salt concentration or raising the solution pH. In this context, it is worth emphasising that relatively high dispersities are beneficial for maximising the solution viscosity, because this parameter is much more sensitive to Mw than Mn.
Ultimately, the scope for potential commercial applications will depend on the relative cost and reproducibility of these aqueous PISA syntheses. Thus, cost-effective monomers such as acrylic acid, acrylamide and acrylonitrile were investigated in this work.