Synthesis and Properties of Stimuli-Responsive Ionic Nanogels Prepared via RAFT Dispersion Polymerisation

Stimuli-responsive nanogels are of increasing interest in a variety of biomedical applications and therefore various synthetic approaches have been established to prepare these nanomaterials. However, some of these techniques can include the use of surfactants or highly toxic organic solvents whose presence could be detrimental for intended biomaterials. Previous work has established the use of reversible addition-fragmentation chain-transfer (RAFT) dispersion aqueous polymerisation as a versatile approach to preparing nanogels with controlled composition, architecture, and functionality under environmentally-friendly conditions. Although the nanogel formation via this approach is well-understood, only a few studies have developed temperature-responsive nanogels and no thorough studies are available on the synthesis of nanogels from only weak polyelectrolytes, which are relevant pH-sensitive nanomaterials. This thesis comprises fundamental studies on the synthesis and properties of two classes of stimuli-responsive ionic nanogels prepared by RAFT dispersion polymerisation. Studies were developed to prepare ionic nanogels from an anionic polyelectrolyte poly(acrylic acid) (PAA) or a cationic polyelectrolyte poly(2-(dimethylamino)ethyl methacrylate)) (PDMAEMA)-based macromolecular chain-transfer agents (macro-CTAs). The chain extension of the macro-CTAs with N-isopropylacrylamide (NIPAM) and N,N´-methylenebisacrylamide (BIS) produced thermo-responsive ionic nanogels in a one-pot synthesis. In both cases, the effects of the systematic variation of the core-forming PNIPAM/BIS block and the shell-stabiliser polyelectrolyte block on the physicochemical properties of the nanogels were determined mainly by DLS and TEM analyses. Irrespective of their composition, the two classes of nanogels were capable to respond to varying degrees of temperature, pH, and salt addition. Additionally, the effects of synthesis cononsolvent composition (water-ethanol) on the properties of the anionic nanogels was particularly investigated for nanogels with the same PNIPAM/BIS composition. Studies suggest that the addition of ethanol for the copolymerisation not only affects the nanogel particle formation but also their size, tacticity and thus the volume phase transition temperature (VPTT) of nanogels with similar composition. In the case of the cationic nanogels, the use of acetate buffer for the synthesis of a PDMAEMA macro-CTA caused the simultaneous polymerisation and hydrolysis of the monomer, thus forming a polyampholyte composed mainly of PDMAEMA and minorly of poly(methacrylic acid) (PMAA). Regardless of the polyampholyte nature of the macro-CTA, results showed that the use of an acetate buffer allowed the preparation of pH- and temperature-sensitive nanogels with controlled size.