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Structure–property relationships in thermoplastic-apparent interpenetrating polymer networks (t-AIPNs), prepared by mechanical blending in a common solvent of crystallizable polyurethane (CPU) and styrene/acrylic acid random copolymer (S/AA), were investigated by means of wide-angle and small-angle X-ray scattering (WAXS and SAXS), dynamic mechanical analysis (DMA), thermally stimulated depolarization currents (TSDC) techniques, dielectric relaxation spectroscopy (DRS), and density, water uptake, deformation, and strength characteristics measurements. Several mechanical and dielectric relaxations of the pure components were characterized, and the effects thereupon induced by blending were followed. The two components show weak affinity to each other. The t-AIPNs can be classified into two groups with high and low contents of CPU, showing essentially the behavior of CPU and of S/AA, respectively. On the other hand, deviations from additivity in several properties indicate interactions between the two components, caused by the formation of H-bonds between their functional groups, and resulting in partial miscibility. In addition, significant changes are observed on some properties of the t-AIPNs on addition of small amounts of either of the components. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 385–397, 1999