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Aqueous polyurethane dispersions were prepared with 4−10 wt % of functionalized polyhedral oligomeric silsesquioxanes (POSS) via homogeneous solution polymerization in acetone followed by solvent exchange with water. The use of acetone as the initial polymerization solvent allowed for the facile incorporation of both diamino and dihydroxy functional POSS monomers in a homogeneous reaction environment. After addition of water and removal of the acetone, stable dispersions with unimodal particle sizes were obtained. The incorporation of the POSS monomers did not have a significant effect on the dispersion's properties; however, the physical properties of the isolated polymers did display significant changes, with notable increases in storage modulus, T g, complex viscosity, and surface hydrophobicity. These changes were attributed to the incorporation of the POSS residues into the polyurethane hard segment domains found. Though no sign of any gross phase heterogeneity due to the inclusion of POSS moieties was detected by either thermal characterization or wide-angle X-ray diffraction (WAXD), a significant change was observed by atomic force microscopy (AFM) when the samples were recast from organic solvent.

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, t gel, 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 t gel 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 ‘ ‘ ∼ ω n with critical exponents n ‘ and n ‘ ‘ for G ‘ and G ‘ ‘, respectively. The values of n ‘ and n ‘ ‘ are identical at t gel (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 T gel = 67 °C. The value of T gel = 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, G eq, conformed well with power law scaling functions of the relative distance from the gel point, ε, i.e., η0 ∼ ε- k and G eq ∼ ε z (where k and z are scaling parameters).

There are many variables in the preparation of aqueous polyurethane (PU) dispersions. Carboxylic acid content, solid content, degree of pre/postneutralization of the carboxylic acids, and chain extension all impact dispersion particle size, viscosity, pH, molecular weights, and glass transition temperature. This study evaluated the impact of these variables on a given PU dispersion formulation prepared from isophorone diisocyanate, an aliphatic polyester polyol, dimethylol propionic acid, and hexamethylene diamine with triethyl amine as the neutralizing base and N-methyl pyrrolidone as the cosolvent. Changes in carboxylic acid content, degree of preneutralization, and chain extension were found to have the expected impacts on dispersions properties. Increased ionic content in the dispersion step led to lower particle size and higher viscosity, increased chain extension with its concurrent increase in molecular improved subsequent film properties. Surprising results were obtained by varying the amount of postneutralization and from increased solids content at the time of dispersion. Unexpectedly, both of these variations led to much higher dispersion viscosities and particle size in solution. To have these changes take place, it is theorized that there is a major change in solution morphology caused by these modifications.

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Isothermal and nonisothermal kinetics studies of thermal-induced gelation for waterborne polyurethane dispersions have been investigated rheologically. The change in the viscoelastic material functions such as elastic storage modulus, G ‘, viscous loss modulus, G ‘ ‘ and complex dynamic viscosity, η* during the gelation process was evaluated accurately for the first time. The isothermal kinetics reaction was described using a phenomenological equation based on the Malkin and Kulichikhin model that was originally developed for predicting isothermal curing kinetics of thermosetting polymers from differential scanning calorimetery (DSC) data. The Malkin and Kulichikhin model was found to conform excellently well for the rheokinetics data presented here. The rate of the gelation process was found to be a second-order reaction regardless of the temperature and shear frequency, and to be in good agreement with literature data. The isothermal gelation kinetics was also analyzed using a standard isoconversional method that is based on replicated experimental data and model-free kinetics calculations. This isoconversional method evaluates an effective activation energy that is independent of the degree of conversion, indicating that the rate of gelation is controlled by a single step (homogeneous) process with no change in the fractal gel formation mechanism at different degree of conversions. The temperature dependence of the gelation rate constant was well described by an Arrhenius plot with an average apparent activation energy equal to 127 ± 2 kJ/mol, in reasonable agreement with the value obtained from the temperature dependence of gel time, t gel. The nonisothermal kinetics reaction rate was interpreted using the classical rate equation, the Arrhenius equation and the time−temperature relationships. A frequency-independent apparent activation energy was evaluated nonisothermally and found to be similar to that obtained from isothermal kinetics data. The high value of activation energy is thought to be due to the strong interaction between the PU-dispersed particles during the gelation process, making a significant contribution to the rate of structure formation. It is noteworthy that, in some respects, these results resemble those from other cross-linking polymer networks and gels measured by DSC, yet in very important ways the aqueous PUDs of the present study is quite unique.

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.