Effect of Volume Fraction on the Order−Disorder Transition in Low Molecular Weight Polystyrene-block-polyisoprene Copolymers. 2. Order−Disorder Transition Temperature Determined by Small-Angle X-ray Scattering

Small-angle X-ray scattering (SAXS) was employed to investigate the order−disorder transition of low molecular weight polystyrene- block -polyisoprene (SI diblock) copolymers with widely varying composition. The SI diblock copolymers investigated in the present study have molecular weight varying from 15 000 to 41 000 and the volume fraction of PS block varying from 0.186 to 0.811. Low-temperature resolution SAXS experiments, which involved a temperature increment of ≥10 °C, did not allow us to accurately determine the order−disorder transition temperatures ( T ODT ) of the SI diblock copolymers, but high-temperature resolution SAXS experiments, which involved measurements of the scattering profiles across T ODT with a small temperature increment of ≤1 °C, enabled us to determine the values of T ODT with accuracy to within ±1 °C. T ODT was determined to be the temperature at which the reciprocal of the maximum intensity and the square of the half-width at half-maximum of the first-order scattering maximum change discontinuously with the reciprocal of absolute temperature. We compared the values of T ODT determined from SAXS experiments with those determined from rheological measurements reported in part 1 of this series.