In this study, the influence of rheological and morphological properties on the foaming behaviour of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers and their composites with PMMA modified silica nanoparticles is discussed. The blowing agent was carbon dioxide. Cylindrical and lamellar types of PS-b-PMMA diblock copolymers with different molecular weights were chosen in order to elucidate the influence of morphology and molecular weight on the foaming behaviour. The microphase-separated morphology of the diblock copolymers was studied by small-angle x-ray scattering and microscopic investigations. The rheological behaviour of the materials under shear was analysed using linear viscoelastic shear oscillations and creep experiments in order to probe the microstructure at different time scales. High pressure differential scanning calorimetry measurements indicate that the blowing agent carbon dioxide was dissolved in the PS and in the PMMA domains. Generally, the diblock copolymers of our study with a cylindrical morphology led to foams with a lower density than the ones with a lamellar morphology. This effect indicates that large stresses are necessary to deform lamellar structures during nucleation and expansion of foam cells. The diblock copolymer with a cylindrical morphology and polystyrene as the matrix was associated with an omega-independent plateau at low frequencies of the shear oscillations. After foaming, the cell walls and the surface of the foamed PS-b-PMMA diblock copolymer with the polystyrene matrix depicted the cylindrical morphology of the microphase-separated structure. (C) 2013 Elsevier Ltd. All rights reserved.