Rheological Behavior of POSS/Polyurethane-urea Nanocomposite Films Prepared by Homogeneous Solution Polymerization In Aqueous Dispersions

Reinforced polyurethane-urea nanocomposite with reactive polyhedral oligomeric silsesquioxanes (POSS) has been prepared via environmentally friendly aqueous dispersion with no organic solvent. Rheological behavior of this important class of materials has been investigated as a function of POSS concentration over a wide range of shear frequency and temperature (-100 to 230 degrees C). The functionalized diamino-POSS was reacted initially with isophorone diisocyanate (the urethane hard segments) before adding the polyester diol (the soft segments). The complete reaction of diamino-POSS with urethane segments was confirmed rheologically and morphologically (TEM). The molecular relaxations of the hard and soft segments of PU/POSS nanocomposites were investigated using the rectangular torsional mode in the glassy and rubbery states. It was found that the storage elastic modulus increased systematically only in the high-temperature range (i.e., the range of the T-g of the urethane segments) while the modulus did not significantly change at low temperatures corresponding to the range of the T-g of the polyester soft segments. In addition, the rheological behavior of the pure PU in the melt confirmed the existence of microphase separation of the hard and soft segments at 140 degrees C. The value of the microphase separation temperature (T-MPS) was found to be concentration independent when the POSS concentration is \u3c= 6 wt %. For 10 wt % POSS the T-MPS shifted by 20 degrees C to a higher temperature. The viscoelastic material functions (G\u27, G \u27\u27, and eta* ) for pure PU film and samples with POSS \u3c= 6 wt % were found to be well described by the time-temperature superposition (or WLF) principle in the low-temperature range studied (i.e., T \u3c T-MPS (140 degrees C)); at higher temperatures the superposition principle failed to describe the experimental data. For 10 wt % POSS film, the validity of WLF principle was extended up to 160 degrees C due to the shift of T-MPS to higher temperature. The incorporation of POSS to the hard segments of PU produced a more homogeneous structure for the 10 wt % POSS as confirmed by TEM. In addition, the viscosity and activation energy of flow were increased dramatically by adding POSS to PU. The thermal stability of PU under a nitrogen atmosphere did not improve by adding POSS, but it improved slightly when tested in an oxygen atmosphere