A polymer blend based on thermoplastic elastomeric polyurethane and polymethylmethacrylate (TPU/PMMA) has been studied by positron annihilation lifetime spectroscopy (PALS) and thermomechanical analysis. Thermomechanical analysis allowed the determination of two glass transitions for the blends in the overall range of compositions. The first one (T-g(1)) showed a constant value of -45 degrees C for all blends, the same value of the pure TPU. The second glass transition (T-g(2)), which is associated with a PMMA rich phase, presented variations with composition. T-8(2) showed minimum values for the blends in the 20-40 wt % TPU range, which indicates increase of interaction in this composition region. PALS systematic investigation allowed the determination of relative mean free volume fractions, f(v)/C, and binary interchain interaction parameters, beta. These parameters exhibited a noticeable negative deviation from additivity in all range of composition and minima for the 20 wt % TPU blend. PALS results were interpreted as associated to a strong attractive interchain interaction between TPU and PMMA in the PMMA rich phase which contracts the free volume fraction of the blends. Moreover, the miscibility achieved in the PMMA rich phase would allow a good adhesion between this phase and the TPU phase, which was corroborated by scanning electron microscopy images. (c) 2007 Wiley Periodicals, tnc.