Thermal-induced simultaneous liquid–liquid phase separation and liquid–solid transition in aqueous polyurethane dispersions

Thermal-induced simultaneous phase separation and liquid–solid transition (gelation) in waterborne polyurethane dispersions has been detected morphologically and rheologically. The viscoelastic material functions, such as dynamic shear moduli, G′ and G″ complex shear viscosity, η* and loss tangent, tan δ were found to be very sensitive to the structure evolution during the gelation process and the subsequent formation of a fractal polymer gel. At the onset temperature of the gelation process, an abrupt increase in G′, G″ and η* (several orders of magnitude) was observed during the dynamic temperature ramps (2 °C/min heating-rate) over a wide range of angular frequency. The temperature dependencies of G′, G″ and tan δ were found to be frequency independent at the gel-point, Tgel, providing a fingerprint for determining Tgel of the dispersions. Furthermore, a dramatic increase in zero-shear viscosity, η0 (v-shape) was observed at T=Tgel and found to be in good agreement with the value obtained from the tan δ versus T data. As expected, the time–temperature–superposition principle was found to be only valid for temperatures lower than the Tgel; the principle failed at T≥70 °C. The morphology of the dispersions at 70 °C for 2 h showed for 36, 38 and 40 wt% formation of a network structure having a unique periodicity and phase connectivity. A lower critical solution temperature (LCST) phase diagram was estimated based on the different morphologies of the dispersions. The coexistence of liquid–liquid and liquid–solid transitions at the same temperature range confirmed the complex behavior of the polyurethane dispersions, pointing to the need for a new theory that explicitly takes this special behavior into account.