Disorder-Order-Crystalline State Transitions of PEO-b-PPO-b-PEO Copolymers and their Blends: SAXS/WAXS/DSC Study

The structure in the symmetrical triblock copolymers PEO-b-PPO-b-PEO and their blend with PEO, studied by small angle x-ray scattering (SAXS), wide angle x-ray scattering (WAXS), and differential scanning calorimetry (DSC), pass from melt to the solid state on cooling. On subsequent heating back to the melt, they pass through disordered and ordered molten states, crystalline structure, and finally back to a disordered melt state. At high temperatures these systems are in the melt in the disordered state approximately described by the mean-field theory. The characteristic lengths of these systems, obtained from SAXS, are proportional to R _g . At lower temperatures, their structure changes to a disordered state which can be described by the concentration fluctuation theory. During cooling, the disordered melt structure changes abruptly into the ordered melt structure. The crystallization destroys this melt structure, forming a new lamellar structure with different periodicity. During heating near the melting point, lamellar periodicity increases very steeply. After melting, the crystalline structure transforms directly to the disordered state.