Semi-interpenetrating networks of poly(ethylene oxide) (PEO) and highly crosslinked poly(methacrylate)s were generated from solutions of PEO in mixtures of methacrylate monomers and dimethacrylate crosslinkers. The deep quenching of the solutions into the unstable region resulted in microphase separation via a spinodal decomposition mechanism. Through the crystallization of the PEO inside the polymer-rich phase, the domain size was reduced below the Cahn-Hilliard limit. The microstructure was permanently preserved by subsequent. UV-initiated polymerization of the monomers well below the PEO melting temperature. The semi-interpenetrating networks were characterized by differential scanning calorimetry, small-angle X-ray scattering, NMR spin-diffusion measurements, and electron microscopy. Morphologies based on networks of cylindrical PEO aggregates with diameters of 10 +/- 2 nm were observed, nearly independent of the molecular weight of the used PEG.