Polymerisation-Induced Self-Assembly in Non-Polar Solvents

This Thesis describes the reversible addition-fragmentation chain transfer (RAFT) dispersion polymerisation of benzyl methacrylate (BzMA) in non-polar solvents. Firstly, oil-soluble poly(lauryl methacrylate) (PLMA), poly(stearyl methacrylate) (PSMA) and poly(behenyl methacrylate) (PBhMA) macromolecular chain transfer agents (macro-CTAs) are synthesised via RAFT solution polymerisation in toluene. These macro-CTAs are then chain-extended in turn with varying amounts of BzMA in industrially-sourced mineral oil or a poly(α-olefin). Polymerisation-induced self-assembly (PISA) occurs under these conditions, where the soluble BzMA monomer polymerises to form an insoluble poly(benzyl methacrylate) (PBzMA) block, thus driving the in situ formation of spheres, worms or vesicles. Subtle differences in the phase diagrams constructed for PLMA-PBzMA diblock copolymer nano-objects are observed in different solvents. In such PISA formulations, the stabiliser block DP is an important parameter, because only kinetically-trapped spheres are accessible when sufficiently long stabilisers (e.g. PLMA39, PSMA18 or PBhMA37) are used. PLMA47-PBzMA100 spheres could be prepared at copolymer concentrations up to 50% w/w solids. Importantly, a highly convenient ‘one-pot’ synthetic protocol was developed, whereby 39 nm PLMA50-PBzMA100 spheres were prepared at 30% w/w solids within 9 h starting from LMA monomer. The phase diagram for PSMA13-PBzMAx diblock copolymer nanoparticles in mineral oil indicates that the final copolymer morphologies are only weakly dependent on copolymer concentration, which enables the synthesis of pure spheres, worms or vesicles at just 5.0% w/w solids. This facilitated in situ small-angle X-ray scattering (SAXS) studies during the PISA synthesis. When targeting PSMA31-PBzMA2000 spheres, the PBzMA core diameter and aggregation number per sphere (Ns) increased monotonically during the polymerisation. When targeting PSMA13-PBzMA150 vesicles, the full range of morphologies is observed, from soluble copolymer chains to the final vesicles via intermediate spheres and worms. Transmission electron microscopy (TEM) studies indicated that vesicles are formed from worms via transient octopi and jellyfish morphologies, which is consistent with observations previously reported for aqueous PISA formulations. A combination of dynamic light scattering (DLS), TEM and both in situ and post mortem SAXS analyses confirmed that the overall vesicle dimensions are conserved as the membrane thickens, which indicates an ‘inward growth’ mechanism. This is consistent with observations recently reported for an aqueous PISA formulation and hence suggests a universal vesicle growth mechanism for all PISA formulations. Dispersions of PSMA13-PBzMA65 worms form free-standing gels at 20 °C due to multiple inter-worm contacts, but heating leads to surface plasticisation. This induces a worm-to-sphere transition and concomitant degelation, since isotropic spheres cannot form inter-particle contacts at this copolymer concentration. The worm-to-sphere transition was characterised using TEM, DLS and rheology.