Full sequential interpenetrating polymer networks (seq-IPN) of cross-linked polyurethane (CPU) and heterocyclic polymer networks (HPN) based on thermally cured dicyanic ether of Bisphenol A (DCE) were characterized by small-angle X-ray diffraction, dynamic mechanical analysis, stretching calorimetry and microhardness measurements. Neat CPU was shown to be a microphase-separated system characterized by a regular, three-dimensional macrolattice of network junctions, embedded in uniform-size microdomains of stiff chain fragments which spanned the continuous matrix of soft chain fragments. In contrast, no large-scale structural heterogeneities were detected in the HPN. The X-ray long spacing (L), the degree of microphase segregation (DMS), the cu-relaxation temperature and the mechanical properties (elastic modulus and microhardness) were studied as a function of HPN content. Results are explained in the light of a model that discusses the maximum degree of CPV swelling by molten DCE as a function of composition. It is suggested that predominantly chemical interactions between the molten DCE and the stiff chain fragment microdomains, reinforcing primary physical interactions, are responsible for the observed transition at 40% HPN content to a more homogeneous phase morphology of seq-IPNS. (C) 2000