Morphology development from a semi-interpenetrating polymer network structure (semi-IPN structure), induced by first-stage photocuring and aided by annealing, is investigated in a binary blend of a photocurable polymer (2,2-bis(4-(acryloxydiethoxy)phenyl)propane; BPE4), and a linear polymer (polysulfone; PSU). A blend with a miscible semi-IPN structure is produced by photocuring below the glass transition temperature (Tg) of the homogenous mixture of BPE4 and PSU, and is then annealed and post-cured at a higher temperature. The investigation reveals that the phase structures, which can be controlled by appropriately tuning the annealing temperature, range from BPE4-rich domain structures, through interconnected structures, to PSU-rich domain structures. The BPE4-rich domains are connected linearly to form an interconnected structure. The BPE4-rich and PSU-rich domains are restricted to the nanometre scale, with sizes of less than 100 nm over almost the entire operating range of annealing temperatures and compositions. By adjusting the degree of the crosslinking in the semi-IPN structures, the characteristic length scale of the phase-separated structures can be varied. The mechanical properties (tensile strength and modulus) are at a maximum in the blend with the interconnected structure, due to the high conversion of BPE4 and the strong interfacial adhesion between the two phases, supported by the stability of the morphology resulting from the annealing.