Nanostructure, glass transition dynamics and elastic properties were studied in the 3D nanodiamond-containing composites based on polyurethane-poly(2-hydroxyethyl methacrylate) semi-interpenetrating polymer networks (PU-PHEMA semi-IPNs), neat PU or PHEMA matrices. Nanodiamond (ND) content in the nano-composites varied from 0.25 to 3 wt %. Combined differential scanning calorimetry/ laser-interferometric creep rate spectroscopy/atomic force microscopy approach was utilized. A large impact of small 3D ND additives on PU-PHEMA networks' dynamics and properties was revealed under conditions when an average inter-particle distance L exceeds by far gyration radius R-g. The pronounced heterogeneity of glass transitions' dynamics and two opposite effects were observed. The main effect was a strong suppression of PHEMA glass transition dynamics at 90-180 degrees C, with the enhancement of creep resistance and threefold to sixfold increasing modulus of elasticity. The peculiarly crosslinked structure of nanocomposites, due to double covalent hybridization, resulted in low rheological percolation threshold, and a synergistic effect in dynamics was observed. Less pronounced effect of accelerating dynamics in the temperature region between beta- and alpha-transitions in PHEMA was associated with dynamics in domains with loosened molecular packing. The distinct physical limit for "anomalous" decreasing T-g is predicted in terms of the notion of the common segmental nature of alpha- and beta-relaxations. (C) 2008 Wiley Periodicals, Inc.