Investigation of the polyurethane chain length influence on the molecular dynamics in networks crosslinked by hyperbranched polyester

Several non-conventional polyurethane (PU) networks crosslinked with hyperbranched polyester (Boltorn®H40) were synthesised with an aim to determine an influence of the PU chain length on molecular relaxations in such systems. The PU chain length was regulated by changing the macrodiol length or by changing the number of the repeating macrodiol/diisocyanate units n. Molecular dynamics were investigated by broadband dielectric spectroscopy and by dynamic mechanical analysis. It was found that the macrodiol length has a strong influence on the glass transition and the α-relaxation, and also on the crystallization. By contrast, the changes of n practically do not affect the molecular relaxations. This effect was explained by the formation of a physical network by hydrogen bonds between urethane groups, controlling the molecular mobility. The rheological measurements have shown, that at temperatures above 150 °C, when hydrogen bonds were thermally destroyed, not only macrodiol length but also n had strong influence on the flowing point.