Controlling sandwich-structure of poly(ethylene terephthalate) (PET) microcellular foams using coupling of CO2 diffusion and CO2-induced crystallization is presented in this article. The intrinsic kinetics of CO2-induced crystallization of amorphous PET at 25 degrees C and different CO2 pressures were detected using in situ high-pressure Fourier transform infrared spectroscopy and correlated by Avrami equation. Sorption of CO2 in PET was measured using magnetic suspension balance and the diffusivity determined by Fick's second law. A model coupling CO2 diffusion in and CO2-induced crystallization of PET was proposed to calculate the CO2 concentration as well as crystallinity distributions in PET sheet at different saturation times. It was revealed that a sandwich crystallization structure could be built in PET sheet, based on which a solid-state foaming process was used to manipulate the sandwich-structure of PET microcellular foams with two microcellular or even ultra-microcellular foamed crystalline layers outside and a microcellular foamed amorphous layer inside. (C) 2011 American Institute of Chemical Engineers AIChE J, 58: 2512-2523, 2012