Polyurea Networks from Moisture-Cure, Reaction-Setting, Aliphatic Polyisocyanates with Tunable Mechanical and Thermal Properties

A series of one-component aliphatic polyisocyanates has been synthesized and analyzed before and after the moisture-curing process. The resulting reactive polymers are monodisperse tetrafunctionalized reactive molecules that become aliphatic polyurea networks with enhanced tunability of mechanical, chemical, and thermal properties compared to polyurethane and polyamide counterparts. This tunability responds to the insertion of a soft polypropylene oxide polymer segment between two triisocyanate molecules, as demonstrated by two-dimensional diffusion ordered spectroscopy (2D DOSY) 1H NMR experiments and confirmed by Fourier transform infrared (FT-IR) spectroscopy. After curing, the polyurea networks show two glass transition temperatures: a characteristic of microphase-segregated block copolymers and a transient network due to the presence of the hydrogen bonds between the urea motives, as concluded from the evaluation of dynamic mechanical analysis (DMA), wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC) experiments. Therefore, controlling the volume fraction, i.e., the degree of polymerization of the polypropylene oxide soft domains, the cured materials exhibit viscoplastic or viscoelastic properties, making these polyurea networks very attractive as adhesives or sealants.