Catastrophic phase inversion in region II of an ionomeric polymer-water system.

Previous work has identified distinct regions, on a phase inversion map, for dispersions of polyurethane ionomer (PUI) and water. In this study, events that occur, before, during, and after catastrophic phase inversion (provoked by adding water to polyurethane ionomer (PUI) in the RII regions of the phase inversion map) have been studied in order to characterise the inversion mechanism. Before phase inversion, initial water addition leads to the hydration of ionic groups and eventually water drops start to form in the hydrophobic portions of the polymer matrix. At the phase inversion point, the PUI-water interface restructures and the ionomer disintegrates into a dispersion of spherical particles enclosed by a continuous aqueous phase. It is suggested that pseudo-drop structures are formed simultaneously during the production of the small polymer-in-water drops. After phase inversion, water addition dilutes the emulsion and destroys the apparent ionic-centre-rich environment surrounding any isolated ionic groups on a particle surface. The larger water-in-polymer drops are likely to have participated in the phase inversion and the smaller water drops form the primary water drops in the multiple emulsions. The resultant emulsions are stable over a period of a few months but very few multiple drops remain after 1(1/4) years.

Different dispersion regions during the phase inversion of an ionomeric polymer-water system.

Catastrophic phase inversion is induced by changing the phase ratio in a liquid-liquid dispersion and is widely used during the dispersion stage in the production of aqueous polyurethane ionomer (PUI) colloids. In the work reported here, water was added to polyurethane ionomer prepolymer (PUIp) until the water became the continuous phase. Three different dispersion regions have been discovered by changing the ionic group content. Stable emulsions containing small polymer drops were produced in Region I. Stable coarse emulsions containing a mixture of drop structures were produced in Region II, but only temporary dispersions could be produced in Region III. Conductivity measurements could not always be used to detect the phase inversion points effectively because the PUIp was swollen by water. Therefore, torque change measurements have been used in conjunction with the conductivity measurements to detect the phase inversion points for all three dispersion regions. Scanning electron microscopy (SEM) and optical microscopy were used to obtain images of these dispersions in the different regions. A catastrophic phase inversion map is used to represent the changes that occur in the PUIp-W dispersions. This map is plotted using the ionic group content as the ordinate and water content (at the phase inversion points) as the abscissa.