Polymerisation-induced self-assembly using industrially relevant monomers

This Thesis focuses on preparation of block copolymer nano-objects prepared by polymerisation-induced self-assembly (PISA) using reversible addition-fragmentation chain transfer (RAFT) polymerisation in aqueous media, alcoholic solution or various n-alkanes. First, a poly(N-(2-methacryloyloxy)ethyl pyrrolidone) (PNMEP) precursor was prepared in ethanol and used as a steric stabiliser block for the preparation of spherical nanoparticles via either RAFT aqueous dispersion polymerisation of 2-hydroxypropyl methacrylate (HPMA) or RAFT aqueous emulsion polymerisation of 2-ethoxyethyl methacrylate (EEMA). PNMEP homopolymer exhibits lower critical solution temperature (LCST) behaviour in water so an ionisable carboxylic acid end-group was required to ensure sufficiently hydrophilic character for its use as an electrosteric stabiliser at pH 7. However, such anionic carboxylate end-groups only enable the synthesis of kinetically-trapped spherical nanoparticles, regardless of the mean degree of polymerisation (DP) of the PNMEP block or the solids concentration. Flocculation occurred either on addition of salt to screen the electrostatic charge or by adjusting the dispersion pH below the pKa of the carboxylic acid end-groups.
A PNMEP precursor was then used for the RAFT alcoholic dispersion polymerisation of lauryl methacrylate (LMA) to produce PNMEP-PLMA diblock copolymer nano-objects. Water was added as a co-solvent to enhance the relatively slow rate of polymerisation for such PISA formulations. The addition of 20% w/w water yielded the fastest polymerisations, which enabled the highest final LMA conversions to be achieved. Transmission electron microscopy (TEM) studies indicated the apparent formation of spherical nanoparticles for all target PLMA DPs, albeit with contamination by a minor population of lamellae. However, small-angle X-ray scattering (SAXS) studies confirmed that these diblock copolymers formed mainly vesicles rather than spheres, with thicker vesicle membranes being obtained when targeting higher PLMA DPs. The Tg of the PLMA block was around -48 °C for all target PLMA DPs, indicating that the vesicle membranes were highly deformable. Finally, the dithiobenzoate RAFT end-groups were readily removed from an aqueous dispersion of PNMEP28-PLMA87 vesicles at 50 °C by prolonged irradiation using visible LED light (λ = 405 nm).
Poly(tert-octyl acrylamide) (POAA) was used as a steric stabiliser for the RAFT dispersion polymerisation of N,N-dimethylacrylamide (DMAC) in n-heptane. This PISA formulation produced spherical nanoparticles but their tendency to film-form on drying meant that crosslinking was required for TEM studies. The upper critical solution temperature (UCST) behaviour of POAA85-PDMAC150 spherical nanoparticles in various n-alkanes was investigated by turbidimetry. Little or no UCST behaviour was observed for POAA85-PDMAC150 spheres prepared in either n-heptane or n-octane. However, progressively higher cloud points were observed for longer n-alkanes (2 °C for n-decane, 27 °C for n-dodecane, 35 °C for n-tetradecane and 55 °C for n-hexadecane). Dynamic light scattering (DLS) studies indicated that such nanoparticle aggregation was thermoreversible. POAA85-PDMACx spheres were evaluated as putative Pickering emulsifiers. Unexpectedly, these highly hydrophobic nanoparticles produced oil-in-water (o/w) rather than water-in-oil (w/o) emulsions. Further experiments suggest that these nanoparticles adsorb at the inner surface of the oil droplets, rather than undergoing in situ micellar inversion during high-shear homogenisation.
Finally, a poly(stearyl methacrylate) precursor was evaluated for the RAFT polymerisation of N-(2-acryloyloxy)ethyl pyrrolidone) (NAEP) in n-dodecane. A PSMA DP of 36 was required to ensure colloidal stability of the nano-objects, which meant that only kinetically-trapped spheres could be obtained. Owing to the immiscibility of NAEP with n-dodecane, elevated temperatures were required for chain extension of PSMA36 when using this highly polar monomer. Interestingly, this formulation proved to be a rare example of a non-aqueous emulsion polymerisation even when the NAEP polymerisation was conducted at 90 °C. Finally, PSMA36-PNAEP70 spheres were briefly evaluated for their performance as a putative Pickering emulsifier to produce w/o/w double emulsions by high shear homogenisation of various n-dodecane/water mixtures.