Molecular dynamics in polyester- or polyether-urethane networks based on different diisocyanates

Different non-conventional polyurethane networks crosslinked with a hyperbranched polyester (Boltorn®H40) were synthesised with an aim to determine the influence of the polyurethane chemical structure as well as of the length of the linear chains between crosslinking centres on molecular relaxations in such systems. For that purpose, both polyether- and polyester-type macrodiols as well as two different diisocyanates were used to synthesise the connecting polyurethane chains between crosslinks. Molecular dynamics were investigated by dielectric spectroscopy and by dynamic mechanical analysis. It was found that the changes of the repeating macrodiol-diisocyanate unit number (i.e. length of the polyurethane linear chains) practically did not affect the molecular relaxations. This effect was explained by the formation by hydrogen bonds between urethane groups of similar, independent of the polyurethane linear chain length, physical networks, which control the molecular mobility. By contrast, the chemical nature of the precursors strongly influences the molecular relaxation associated with glass transition, and to some extent also the sub-glass secondary relaxation processes occurring in the investigated networks.