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Synthesis of amphiphilic acrylic copolymers and investigation of their self-assembly using X-ray scattering techniques

Neal, Thomas (2019) Synthesis of amphiphilic acrylic copolymers and investigation of their self-assembly using X-ray scattering techniques. PhD thesis, University of Sheffield.

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Abstract

In this thesis, the effect of copolymer composition, molecular weight, and monomer hydrophobicity on the self-assembly of anionic amphiphilic statistical copolymers in aqueous solutions was investigated using structural characterisation techniques such as small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). It was found that they preferably form spherical particles. The size of the particles formed by the aggregation of amphiphilic statistical copolymers is heavily dependent on the copolymer composition but is largely independent of molecular weight, where particles with larger ratios of the hydrophilic monomer produced smaller particles. Particle size was only found to be dependent of molecular weight when unimolecular micelles (single chain nanoparticles) were formed. Additionally, the hydrophobicity (logP) of the hydrophobic comonomer has a great influence on particle size, where increasing the hydrophobicity increases the copolymer particle size. The relationship between copolymer composition and particle size was rationalised in terms of particle surface charge density, where the copolymers aggregate together to induce a critical surface charge density that allows colloidal stability to be achieved. The critical surface charge density was quantified as the fraction of the particle surface covered by the charged monomer (SAfrac.MAA), leading to the development of the particle surface charge density (PSC) model. It was found that the PSC model fit well to the experimental data collected for all the copolymer series and a SAfrac.MAA was calculated for each series where the hydrophobic monomer was varied. Furthermore, a linear relationship between the logP of the hydrophobic component and the SAfrac.MAA required to induce stabilisation of the particle dispersion was observed. Additionally, the effect of hydrophile-hydrophobe distribution along polymer backbones has been examined. It was found that there was a clear correlation between the monomer distribution and the particle size, where blockier structures, in comparison with evenly distributed monomers, produce larger copolymer particles. This result was rationalised as a reduction in the efficiency by which the charged hydrophile, when in a blocky arrangement, can stabilise the hydrophobic component leading to an increase in particle size. Finally, a range of amphiphilic triblock copolymers were synthesised, where the DPs of the individual hard and soft blocks were varied. The amphiphilic triblocks were found to assemble into spherical particles in water but remained dissolved in methyl ethyl ketone (MEK). Films were cast of the triblock copolymers from both solvent environments, and the structural phase separation in the films was studied using a combination of SAXS and AFM. It was found that the copolymer films cast from MEK were able to phase separate into well-defined structures, where the size and morphology of the phase separation was dependent on the block length and hard-to-soft block ratio, respectively. These morphologies were consistent with the predicted self-assembly of diblock copolymers in the bulk. Conversely, the water-cast films partially retained the particulate structure present in solution, produced kinetically trapped phase separated structures. However, by annealing the water-cast film at 150 °C the lower energy phase-separated structure observed in the MEK films can be achieved.

Item Type: Thesis (PhD)
Keywords: Amphiphilic, Statistical Copolymer, Self-assembly, SAXS, X-ray scattering
Academic Units: The University of Sheffield > Faculty of Science (Sheffield) > Chemistry (Sheffield)
Depositing User: Mr Thomas Neal
Date Deposited: 17 Feb 2020 09:59
Last Modified: 17 Feb 2020 09:59
URI: http://etheses.whiterose.ac.uk/id/eprint/25973

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