Abstract
Abstract When water is dropped gradually to an acetone solution of concentrated polyurethane containing ionic functional groups, a phase inversion occurs and nanoparticles of waterborne polyurethane (WPU) are formed. Evaporation of water from such aqueous particle suspensions yields polyurethane sheets, but the properties of these sheets are different from those obtained by evaporation of solvent from WPU acetone solutions. In this study, ultrasonic spectroscopy analysis combined with an ultrasonic scattering theory was performed on the microparticle suspensions to evaluate the elastic modulus of the individual particles. The nanoparticles obtained by phase inversion are stabilized by ionic substituents, and the bulk modulus of the particles at 30 wt% was shown to be different from that of bulk sample obtained by drying acetone, in spite of the fact that they are the same polymer material. Even after 95% concentration beyond the maximum packing concentration, the film properties were observed to be particle size dependent, suggesting that the nanostructure plays an important role in film properties.
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