Studies of the molecular dynamics in polyurethane networks with hyperbranched crosslinkers of different coordination numbers

摘要

Several new polyurethane networks based on hyperbranched polyesters (trade name Boltorn) were synthesized to investigate the influence of the hyperbranched crosslinking agent on the molecular dynamics of the linear segments containing urethane groups. For comparison, linear polyurethanes as well as polyurethanes crosslinked with the classical crosslinker trimethylolpropane were prepared. Broadband dielectric spectroscopy, dynamic mechanical analysis, and differential scanning calorimetry yielded consistent results concerning the molecular relaxation processes; however, dielectric spectroscopy appeared to be more sensitive concerning the secondary relaxation processes. In the temperature range up to 150 degrees C, the molecular relaxations were very similar in all the investigated samples, despite considerable structural differences. The weak influence of the crosslinking on the molecular properties could be explained by the existence of hydrogen bonds forming a physical network, which was very dense in this temperature range in comparison with the chemical crosslinks and therefore dominated the molecular mobility in all the investigated systems. This hypothesis was confirmed by rheological measurements performed at temperatures above 150 degrees C, when the hydrogen bonds should be thermally destroyed. At these temperatures, the effect of crosslinking was manifested by a strong shift of the flowing point: in the linear polyurethanes, this point occurred at much higher frequencies (and lower temperatures) than in the crosslinked analogues. (C) 2007 Wiley Periodicals, Inc.