Open microcellular foams of polyethylene terephthalate (PET)/polycarbonate (PC) blends were prepared by controlling their foaming behavior at the interface between these two polymers. Interface modification was a crucial factor in governing the foaming behavior and cell morphology of the blend foams: annealing at 280 degrees C, i.e., conducting the transesterification reaction, generates a PET-b-PC copolymer, which lowers the interfacial tension, increases the affinity between PET and PC, and decreases the crystallinity of the PET domains. When CO2 foaming was performed at the interface modified with the copolymer, an interesting fibril-like structure was formed. The cell density of the PET/PC blend then increased, and its cell size reduced to the microscale while maintaining a high open-cell ratio. The effect of heat annealing (transesterification reaction) on CO2-foaming was studied to reveal the relationship among the interface affinity, crystallinity, and degree of fibrillation. The optimal heat-annealing procedure generated a fibril-like structure in the PET/PC blend foams with a high cell density (7 x 10(11) cm(-3)), small cell size (less than 2 m), and 100% open-cell ratio. POLYM. ENG. SCI., 55:375-385, 2015. (c) 2014 Society of Plastics Engineers