Supramolecular structures such as worm-like and spherical micelles and bilayers (polymersomes) constructed from the amphiphilic di-block copolymer, polyethyleneoxide-b-polycaprolactone (PEO-b-PCL) have already shown potential for applications in drug delivery. The utility of these structures stems from their favorable physico-chemical attributes such as biocompatibility of the two blocks and in particular the hydrolytic degradability of polycaprolactone. Although the molecular principles that dictate the evolution of a distinct supra-molecular structure from amphiphilic diblock copolymers is well understood, no systematic study has been done for PEO-b-PCL diblock copolymer systems. Herein, we report on the morphological phase diagram obtained by studying various PEO-b-PCL diblock copolymers that differ in their molecular weights and block ratios. The results demonstrate that distinct morphologies can be predictably obtained by controlling the ratios of the two blocks and binary blends of different PEO-b-PCL diblock copolymers can be used to control the distribution of morphologies. Further, characterization of PEO-b-PCL worm-like micelles shows that the persistence length (lp) and the average contour length () increase with the size of the PCL block. Importantly, although PCL is a semi-crystalline polymer, its ability to crystallize within the hydrophobic core is seemingly dependent on the supra-molecular morphology. Time-lapse fluorescence microscopy experiments shows that the PCL within a worm-like micelle is predominantly rubbery while within a vesicle it tends to be more rigid indicating possible crystallinity. The morphological and molecular weight dependence of PCL crystallization under nanoscale confinement are currently being investigated using differential scanning calorimetry. This fundamental characterization of PEO-b-PCL supramolecular structures will aid in the design of robust drug delivery systems.