Polyurethane networks based on hyperbranched polyesters: Synthesis and molecular relaxations

Several polyurethane networks based on hyperbranched polyesters (trade name Boltorn®H40) were synthesized. Molecular dynamics in these systems were investigated by means of broadband dielectric spectroscopy and comparatively by dynamic mechanical analysis. Glass transition temperatures were determined by differential scanning calorimetry. It was found that these techniques yield consistent results concerning molecular relaxation processes, however, the dielectric spectroscopy appears to be the most sensitive to show the secondary relaxation processes. The molecular relaxations are much more sensitive to the changes of the chemical character of polyurethane linear links between the hyperbranched centers, than to crosslinking density changed by the length of linear PU chains. The weak influence of the PU chain length on the molecular properties can be explained by the existence of hydrogen bonds forming a physical network, which is more dense and consequently, much stronger than the chemical network in the investigated temperature range.