The aim of this proposal is to collect SAXS/WAXD based information on the semicrystalline structure of double crystallizable poly(butylene terephthalate)/poly(ethylene glycol) (PBT/PEG) multiblock copolymer obtained by very rapid isothermal crystallization at high supercooling. Varying the isothermal crystallization temperature allows not only to crystallize each block (PBT or PEG) separately at selected temperatures, but also to govern the blocks’ crystallization sequence in double crystallizable block copolymers. Furthermore, as the first block crystallizes a certain (intermediate) morphology is generated that strongly affects the crystallization kinetics of the second one as well as the final morphology. The intermediate and final structures remain unknown. To address this issue, SAXS/WAXS combined with Fast Scanning Calorimetry (FSC) will be applied.