Controlling the interfacial behaviour of colloidal microgel systems

Thorne, Joanna Bronwen (2012) Controlling the interfacial behaviour of colloidal microgel systems. PhD thesis, University of Greenwich.

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Abstract

Hydrophobically modified colloidal microgel particles were prepared by a surfactant-free emulsion polymerization of N-isopropylacrylamide (NIPAM) with more hydrophobic vinyl ether/ester/silane co-monomers (10 % total mass monomer). Most of the resultant dispersions were novel co-polymer microgels and all exhibited a thermo-sensitive volume phase transition. Key properties of the microgels, i.e. size, electrophoretic mobility and volume phase transition temperature (VPTT), were determined by dynamic light scattering (DLS), laser Doppler electrophoresis, UV-visible spectrophotometry and scanning electron microscopy. The influence of co-monomer incorporation, e.g. structure and relative hydrophobicity, upon the physicochemical properties of the microgel was examined. Hydrophobic modification strongly influenced size, slightly altered electrophoretic mobility and left the VPTT relatively unaffected.

The interfacial properties of the microgels were studied by tensiometry. Substantial reductions in the surface tension of water were observed for all microgels, close in magnitude to that achieved by the surfactant sodium dodecyl sulphate, but at a far lower concentration. The effect was influenced by a complex combination of parameters including size, charge, conformation, co-monomer type, temperature and solvent quality.

A DLS study of the swelling response of microgels in the presence of less polar co-solvents (shortchain alcohols) found that hydrophobic modification altered alcohol-induced de-swelling/
re-entrant swelling behaviour and particle-dispersant interactions. Furthermore, the characteristic temperature-driven volume collapse above the VPTT was overcome by alcohol addition.

Finally, the stability and heteroflocculation of anionic/cationic mixtures of poly(NIPAM) microgels was studied by DLS and UV-visible spectrophotometry as a function of dispersion temperature, pH and electrolyte concentration. Electrolytes of increasing cation valency (NaCl, MgCl2, LaCl3) were used to reduce the particle Debye length. Conditions conducive to preserving stability or encouraging flocculation were identified and related to the complex balance of particle interactions.

It is anticipated that the results of these investigations may support future development of microgels with applications relating to more hydrophobic environments and materials.

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