Polyurethane Networks Bearing Dendritic Wedges:  Synthesis and Some Properties

Polyurethane networks bearing dendritic wedges of the second, third and fourth generation were prepared by polymerization of dendritic diols of the corresponding generation with a commercially available triisocyanate (Desmodur RFE). The formation of a three-dimensional network was greatly influenced by the shielding effect especially of dendritic wedges of generation four. This phenomenon led to an increase of the sol fraction with rising generation number. The swelling behavior of polyurethane networks was studied by means of molecular dynamics simulations and the results were compared, in a qualitative sense, with experimental data. As opposed to the theoretical data for perfect networks, the experimental swelling ratio increased as a function of generation number. The discrepancy between molecular simulations and experimental data was attributed to the imperfect character, i.e., the presence of defects, of synthesized networks comprising dendritic wedges of higher generations. Since the cross-linker Desmodur RFE contains phosphorus, 31P NMR was applied to reveal the effect of dendritic wedges on the mobility of junction points. Both the line widths and T 1 relaxation data proved to be sensitive to the effect of the generation number on the mobility of network junctions.