Structure formation and properties of polyurethane-unsaturated polyester interpenetrating polymer networks

Interpenetrating polymer networks (IPNs) based on a polyurethane (PU) and three unsaturated polyester (UPEs) resins with different functional end groups were prepared. The interactions between components of the polyurethane and unsaturated polyester phases were examined by comparing rheological and reaction kinetic changes. It was found that the reaction between end groups of unsaturated polyester molecules and isocyanates led to more reactive vinyl polymers. The formation of polyurethane network was also accelerated by the presence of carboxyl groups on the unsaturated polyester molecules. The dynamics of phase separation in the IPN system were further studied by using phase contrast optical microscopy and transmission electron microscopy during the course of polymerization. Except at low temperatures, the phase separation patterns were found to follow the spinodal decomposition mechanism. An interconnected structure developed quickly and was followed by coalescence of the periodic phase to form droplet/matrix type of morphology. The size of the dispersed domain gradually increased until the structure was locked by chemical gelation. The mechanical properties such as tensile strength, elongation at break, impact strength, and dynamic mechanical properties of IPNs, were studied by changing reaction temperature, PU reaction rate, and UPE reaction rate. Due to the microgel formation of UPE, the first formed network tends to be the dispersed phase. At room temperature, when PU reaction was faster, the PU phase became the dispersed domain. On the other hand, when the UPE reaction became faster, a dispersed UPE microgel domain was obtained. When both reactions occurred at the same time, the matrix tends to be co-continuous. This reaction sequence also strongly affected the extent of phase mixing. The tensile properties and impact strength showed a strong correlation with the phase morphology. It was found that the phase morphology with a co-continuous matrix had a synergistic behavior both in tensile properties and impact strength.