Rheological cure characterization of an advanced functional polyurethane

•Rheological characterization for the formation of a functional polyurethane was studied.•The gelation was analyzed and its behavior can be predicted by percolation theory.•Three chemoviscosity models were examined in the pre-gel stage of the polyaddition reaction.•A deeper understanding of the chemorheology of PU systems has been achieved.•This metallo-PU finds its application in the chemistry of advanced energetic materials.

As part of our continuing study of the curing kinetic and chemorheological analysis of a functional polyurethane (PU) segmented block copolymer formation, in the current paper, we focus on the use the rheological measurements to monitor the entire curing process. The curing reaction was between a metallo-polyol derived of hydroxyl-terminated polybutadiene (HTPB), i.e., (ferrocenylbutyl) dimethylsilane grafted HTPB, and isophorone diisocyanate (IPDI). The evolution of viscoelastic properties, such as storage modulus (G′), loss modulus (G″) and complex viscosity (η*) was recorded in isothermal conditions, at four different temperatures in the range of 50–80 °C. The gel times (tgels) were determined by the loss tangent (tan δ) crossover at different frequencies, and the activation energy obtained from them was 69.8 kJ/mol. The rheological properties in the region of the gel point have shown that they follow the percolation theory, demonstrating a power law dependence for the shear modulus with a critical exponent n = 0.67 ± 0.01, for the higher curing temperatures, 70 and 80 °C, and a slightly lower one at lower temperatures, 50 and 60 °C. In addition, three different empirical models, among them the Arrhenius and Kiuna rheokinetic models, were used to predict the change in viscosity of this system with the time in the pre-gel region. These results illustrate the rheological curing behavior of this PU resin, its final application being found in the development of advanced energetic composite materials.

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