Rheokinetics of thermal-induced gelation of waterborne polyurethane dispersions

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 degrees 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, eta*. The gel point, t(gel), was determined from the point of intersection in tan delta vs gelation time for different constant shear frequencies, where tan delta 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 delta vs t. At the gel point, G' and G showed a power law with shear frequency, i.e., G' similar to G similar to omega(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 similar to 0.58), and both decreased exponentially with gelation time at 70 degrees 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 degrees C. The value of T-gel = 67 degrees C was in good agreement with that obtained previously from the temperature at which tan delta is frequency independent and also from the temperature at which G' and G coincided. The zero shear viscosity, eta(0), and the equilibrium shear modulus, G(eq), conformed well with power law scaling functions of the relative distance from the gel point, 6, i.e., eta(0) similar to epsilon(-k) and G(eq) similar to epsilon(z) (where k and z are scaling parameters).