Effects of Cyclodextrins on the Kinetics of Emulsion Polymerisation
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Cyclodextrins (CD) are semi-natural oligosaccharides composed of a number of D-glucose units. They are produced from renewable resources, and have been found to be of catalytic effect for the emulsion polymerization of many monomers. Using monomers whose emulsion polymerization kinetics have been thoroughly studied, this research analyses the effect of CD on the entry and exit rate coefficients for the emulsion polymerization of styrene, and the entry and termination rate coefficients for the emulsion polymerization of MMA.
Throughout the course of the work, CD was found to have a positive impact on the polymerization rate of styrene in a polystyrene latex stabilized with a cationic surfactant. Furthermore, the exit rate coefficient for this latex was found, via γ-relaxation experiments, to increase in proportion to the styrene solubility in water, exactly as predicted by theory. Of itself this would lead to a decrease in reaction rate. That there is still an overall increase in the reaction rate in the presence of CD is because of a quite strong effect on entry rate coefficients. Again, this is consistent with the prevailing theory for entry, that of Maxwell and Morrison, which says that increased aqueous phase solubility of monomer will lead to faster entry.
Intriguingly, experiments done on a polystyrene latex stabilized with an anionic surfactant showed a different effect for CD: γ-relaxation experiments found very little effect of CD on exit rate, and chemically initiated experiments found the same for overall rate. This is consistent with CD having little effect on aqueous phase styrene solubility, which in fact is what direct measurements via UV-visible spectroscopy indicated. It is speculated that the anionic surfactant was successfully competing with styrene to occupy the CD cavities. On the other hand, measurements suggested that styrene successfully competes with cationic surfactant, which is consistent with kinetic results.
Experiments of the above nature were then carried out with methyl methacrylate (MMA), a more water soluble monomer than styrene and one with emulsion polymerisation kinetics of a different nature (so-called pseudo-bulk). γ-relaxation experiments found no effect of CD on termination rate coefficients, exactly as one would expect given that termination is an intra-particle reaction whereas CD exists in the aqueous phase. However the same experiments also revealed an unexpected effect of CD on entry: the thermal entry rate coefficient was found to increase markedly in the presence of CD. It seems likely that this unusual effect stems from interaction of products of γ radiolysis with CD.
Results for chemically-initiated polymerization of MMA were inconclusive. Under some conditions there was actually retardation in the presence of CD, which is actually consistent with measurements of MMA solubility in water, which suggested a slightly negative effect of CD. However it is hard to explain such a phenomenon. Further, under other conditions it was found that CD either had no effect on chemically-initiated rate or could even increase it slightly. The only safe conclusion at this stage is that CD has no major effect on MMA kinetics, which arguably is consistent with MMA being relatively water soluble: intuitively one would expect that CD is most useful (‘catalytic’) for the EP of monomers of low solubility.