In this work, we report on the thermal and thermomechanical properties of rigid PU foams modified with POSS moieties, paying special attention to molecular dynamics of these systems. 1,2-Propanediolisobutyl POSS (PHI-POSS) are used as pendent groups and octa(3-hydroxy-3-methylbutyldimethylsiloxy) POSS (OCTA-POSS) serve as chemical crosslinks. DSC reveals an initial slowing down of the segmental dynamics for small loadings (5 wt%) of both POSS moieties, followed by an acceleration, probably due to changes in the topology of the PU network and the morphology of the foam. A low-temperature mechanical relaxation is observed by DMA and is attributed tentatively to a local relaxation, much similar to that observed in hyperbranched polyurethanes.