Thermally expandable microcapsules, often called microspheres or microballoons, are utilized in compression, injection molding and extrusion processes to foam different types of polymers. Microballoons consist of a polymer shell and a liquid hydrocarbon core. Hydrocarbons are used as a physical blowing agent. In this study, a mathematical model was developed to describe the expansion behavior of a microballoon in air and in a polymer matrix. The model was used to determine the key factors in improving the expandability of the balloon at designated temperatures. The viscoelastic properties of the polymer shell, evaporation of hydrocarbons in the balloon and diffusion behavior of the blowing agent through the polymer shell were taken into account in the model. The results of the developed model showed quite good agreement with the experimentally observed thermal expansion behavior of a microballoon. A sensitivity analysis of the expansion behavior with respect to the properties of the microballoon was also conducted to devise an optimal design strategy for high-expansion microballoons. (C) 2013 Elsevier Ltd. All rights reserved.