Preparation of diblock copolymer nano-objects via polymerization-induced self-assembly in non-polar media

This Thesis focuses on the synthesis, characterization and potential use of sterically-stabilized diblock copolymer nanoparticles prepared in non-polar solvents via polymerization-induced self-assembly (PISA). This involved chain extension of an oil-soluble poly(n-alkyl methacrylate) precursor via reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization of a carefully selected methacrylic monomer. The growing second block becomes insoluble at a critical degree of polymerization (DP), which leads to in situ self-assembly to form spherical, worm-like or vesicular nanoparticles. Firstly, a poly(stearyl methacrylate)-poly(2-hydroxypropyl methacrylate) [PSMA-PHPMA] formulation was examined using mineral oil as the solvent. 1H NMR kinetic studies conducted during the synthesis of PSMA9-PHPMA150 vesicles confirmed that the polar nature of the HPMA monomer leads to a relatively fast polymerization (94% conversion within 40 min) compared to the corresponding poly(stearyl methacrylate)-poly(benzyl methacrylate) PSMA9-PBzMA150 vesicles, for which only 37% BzMA conversion was achieved within the same timescale. PSMA9-PHPMA70 worms underwent degelation on heating, with transmission electron microscopy (TEM) analysis indicating an unexpected partial worm-to-vesicle transition. Replacing HPMA with 2,2,2-trifluoroethyl methacrylate (TFEMA) enabled ~240 nm diameter PSMA9-PTFEMA300 vesicles to be obtained at 25% w/w solids in n-dodecane as highly transparent dispersion (97% transmittance at 600 nm). This was attributed to the relatively low refractive index of PTFEMA, which matches that of the n-alkane at 25 °C. By varying the type of n-alkane, highly transparent vesicles could also be obtained at either 50 or 90 °C. Examining the synthesis of highly transparent PSMA16-PTFEMA86 spheres via in situ visible spectroscopy in n-hexadecane at 90 °C indicated the premature loss of dithiobenzoate end-groups under such conditions. A more industrially-relevant PISA formulation utilized a poly(lauryl methacrylate) PLMA precursor for the RAFT dispersion polymerization of methyl methacrylate (MMA) in mineral oil at 90 °C. However, only spheres and short worm-like particles could be accessed when using this commodity monomer: targeting higher PMMA DPs unexpectedly produced colloidally unstable spherical aggregates. This morphological constraint was attributed to the high glass transition temperature (Tg) of the PMMA core-forming block and could not be overcome by conducting the synthesis above the Tg of PMMA (115 °C). According to TEM and dynamic light scattering (DLS) analysis, PLMA22-PMMA69 short worms underwent a partially reversible worm-to-sphere transition on heating. Either long worms or vesicles could be accessed by statistically copolymerizing just 10 mol% lauryl methacrylate (LMA) with MMA at 115 °C. This LMA comonomer enhances solvent plasticization of the core-forming copolymer chains. Moreover, differential scanning calorimetry (DSC) studies indicate a significant reduction in the effective Tg to well below the synthesis temperature. The resulting worms and vesicles exhibited thermoreversible worm-to-sphere and vesicle-to-worm transitions on heating. Epoxy-functional spheres were prepared in mineral oil by using glycidyl methacrylate (GlyMA) to grow the core-forming block from a PLMA precursor. Alternatively, a P(LMA-stat-GlyMA) precursor prepared via statistical copolymerization of LMA with GlyMA was used for the RAFT dispersion polymerization of either MMA or BzMA. The potential post-polymerization modification of such spheres was assessed using benzyamine, water or 50% v/v aqueous acetic acid using 1H NMR or Fourier transform infrared spectroscopy (FT-IR) spectroscopy. The surface adsorption of such epoxy-functional spheres onto stainless steel from n-dodecane was compared to non-functional PLMA-PMMA or PLMA-PBzMA spheres using quartz crystal microbalance with dissipation (QCM-D) at 20 °C. Placing epoxy groups within the steric stabilizer chains enhances the extent of adsorption significantly. For example, the adsorbed mass (Γ) obtained for ~50 nm P(LMA50-stat-GlyMA9)-PBzMA245 nanoparticles is more than five-fold higher than that achieved when using the corresponding non-functional PLMA63-PBzMA245 nanoparticles (Γ = 31.3 vs. 6.4 mg m-2). SEM analysis confirmed a comparable enhancement in surface coverage for the epoxy-functional spheres. Furthermore, QCM-D studies performed at 40 °C led to a higher adsorbed mass for the former type of nanoparticles, which suggests that the epoxy groups react with the hydroxyl groups present at the surface of the stainless steel to form covalent bonds. Mini-traction machine (MTM) tribological experiments confirmed that stronger nanoparticle adsorption led to a significantly lower frictional coefficient.