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Thermal-induced gelation for waterborne polyurethane dispersion has been studied rheologically under isothermal condition over a wide range of frequencies at different constant temperatures (55, 60, 65, and 70 °C). The elastic storage modulus, G‘, at a constant temperature in the vicinity of the gel point increases abruptly, and the magnitude of the elevation in G‘ was found to be temperature dependent. Similar behavior has been observed for both the viscous loss modulus, G‘‘, and the complex dynamic viscosity, η*. The gel point, tgel, was determined from the point of intersection in tan δ vs gelation time for different constant shear frequencies, where tan δ is frequency independent and all curves cross over, indicating the validity of the Winter−Chambon criterion. The value of tgel obtained from the coincidence of G‘ and G‘‘ was in excellent agreement with that obtained from tan δ vs t. At the gel point, G‘ and G‘‘ showed a power law with shear frequency, i.e., G‘ ~ G‘‘ ~ ωⁿ with critical exponents n‘ and n‘‘ for G‘ and G‘‘, respectively. The values of n‘ and n‘‘ are identical at tgel (n‘ and n‘‘ ~ 0.58), and both decreased exponentially with gelation time at 70 °C. The exponent values n‘ and n‘‘ are in good agreement with that predicted from the percolation theory (i.e., n = 2/3). In addition, the temperature dependence of n‘ and n‘‘ was investigated in the vicinity of the gel point. Both n‘ and n‘‘ decreased with temperature and intersected at the gel temperature, i.e., n‘ = n‘‘ at Tgel = 67 °C. The value of Tgel = 67 °C was in good agreement with that obtained previously from the temperature at which tan δ is frequency independent and also from the temperature at which G‘ and G‘‘ coincided. The zero shear viscosity, η0, and the equilibrium shear modulus, Geq, conformed well with power law scaling functions of the relative distance from the gel point, ε, i.e., η0 ~ ε^-k and Geq ~ ε^z (where k and z are scaling parameters).

Rheological behavior of waterborne polyurethane dispersions was investigated with small-amplitude oscillatory shear flow experiments over wide ranges of concentration, degree of neutralization, chain extension, and temperatures to accelerate efforts to understand their film formation characteristics. The rheological properties of these environmentally friendly dispersions were found to be dependent on composition and degree of postneutralization. But the chain extension and degree of preneutralization were observed to have little effect on the rheological behavior of the dispersions at a constant polyurethane (PU) concentration. The complex viscosity of the polyurethane dispersions increased dramatically at a critical concentration of polyurethane (φ = 0.43), below which the viscosity increased slightly with composition. At this critical concentration the particles are crowded, and the observed viscosity increase is ascribed to the hydrodynamic interaction between the different particles. Furthermore, both G‘ ‘ and G‘ ‘ are strongly increased with increasing PU wt % in the dispersions (i.e., the higher the concentration of PU, the higher the values of G‘ and G‘ ‘). At 46 wt % PU the values of G‘ and G‘ ‘ are no longer frequency dependent, and G‘ is almost 1 order of magnitude higher than G‘ ‘, indicating formation of a fractal-type gel. The viscoelastic material functions were well described by simple power law equations and a Maxwellian (Hookean) model with 2−3 relaxation modes based on PU concentration and degrees of postneutralization at 30 °C. Time−temperature superposition of the dynamic moduli was good at temperatures and PU concentrations below that of the critical gel, and the temperature dependence of the shift factors conformed well to predictions from an Arrhenius-type relation, enabling calculation of the flow activation energy of 45 kJ mol^-1 for the PUDs. As expected, time−temperature superposition failed to represent the behavior of the PUDs near the critical gel point. While the results of this study indicate a number of similarities to critical gelling systems, observed deviations from the viscoelastic behavior of Brownian suspensions of hard spheres were obtained, indicating that a more complicated theory that explicitly takes the intrinsic interactions, concentrations, and size distributions of the PU particles into account may be necessary for a more accurate quantitative description of these special model PUDs with enhanced benefits.

The variations in morphology and local structure in a series of tin fluorophosphate glass-polyethylene (TFPPE) hybrids were investigated using solid-state ¹H, ¹³C, and ³¹P MAS NMR. The combination of direct polarization ¹³C MAS NMR experiments with various delay times and carbon spin−lattice relaxation filtered cross-polarized MAS NMR experiments allowed quantification of the monoclinic, orthorhombic, amorphous, and intermediate phases in the low-density polyethylene (LDPE) component of the TFPPE hybrids. These ¹³C MAS NMR results showed an increase in the crystallinity with the addition of tin fluorophosphate (TFP) glass into the hybrids. The ³¹P MAS NMR spectra reveal major changes in the local structure of the TFP glass component, with several new phosphorus species being formed during the production of the hybrid. The results of the ¹H-¹³C and ¹H-³¹P 2D wide line spectroscopy experiments demonstrate that all of these new phosphorus species are spatially correlated with the two new proton species that appear in the ¹H MAS NMR of the TFPPE hybrids. There is no evidence of correlation between these new phosphorus species and the methylene protons of the LDPE polymer component.