Polyhedral Oligomeric Silsesquioxanne particles were tethered by urethane or urea linkages on the hard segments of polyurethane chains with varying segment M-w. Evidence by imaging, thermal and dielectric techniques suggests that tethering of POSS by urethane bond leads to increase of glass transition temperature by a few degrees due to two mechanisms: the direct POSS-chain interactions and the indirect slowing of dynamics by disruption of microphase separation. The direct mechanism is dominant for the materials with long (high M-w) segments and the indirect becomes more important as the M-w of the segments decreases. High polarity of the urea bond counteracts the effect of disruption of microphase separation and enhances an interfacial glass transition mechanism alpha', slower than alpha, leading to significantly higher apparent glass transition temperatures, despite its small effect on time scales of the individual relaxations. (C) 2013 Elsevier Ltd. All rights reserved.