Polyurethanes with POSS pendent on flexible hard segments: Morphology and glass transition

Polyhedral Oligomeric Silsesquioxanne (POSS) moieties are tethered on a polyurethane chain, as side groups on the hard domains, which are based on the flexible hexamethylene diisocyanate (HDI). Their effect on morphology, and consequently on molecular dynamics, is studied in detail by microscopy, scattering, spectroscopic, thermal, and dielectric techniques. Albeit the PU micromorphology does not change radically, POSS-rich domains are formed. The segmental dynamics is severely slowed down in a continuous manner, i.e. without the emergence of a new, distinct relaxation, as was the case in a similar system based on the aromatic methylene diisocyanate (MDI). Strikingly, the slowed down components do not seem to have any calorimetric footprint, and hence the calorimetric Tg remains practically unaffected. The linear HDI systems have faster dynamics in the soft phase, as compared to the aromatic MDI ones, presumably owing to higher flexibility and better microphase separation.