A small-angle X-ray scattering study of the phase behavior of diblock copolymer/homopolymer blends

The order-disorder transition (ODT), microdomain morphology, and phase beha vior in mixtures of polystyrene-block-polyisoprene (SI) diblock blended wit h homopolystyrene (HPS) were investigated. SI with a total molecular weight of 2.0 x 10(4) and volume fraction of polystyrene (PS) of 0.51 (designated SI-11/9) was blended with a homopolystyrene of molecular weight 6.1 x 10(3 ) (designated S-6). Binary mixtures of diblock copolymer and homopolymer we re prepared by solvent casting. The ODT was quantitatively identified using the discontinuity observed in a plot of the reciprocal of the peak small-a ngle X-ray scattering (SAXS) intensity, I-m(-1), as a function of the recip rocal of the absolute temperature, 1/T, except for the mixtures showing the disordered sphere morphology for which we determined the temperature of th e demicellization/micellization transition (DMT) instead of the ODT by the disappearance of the form factor peak with increasing temperature. We syste matically measured the ODT or DMT temperature as a function of the volume f raction of homopolymer. SAXS data were also used to investigate the microdo main structure of the blends. Furthermore, for two blends of SI-11/9 and S- 6 with volume fractions of SI of 0.77 and 0.71, we observed an order-order transition (OOT) from a cylindrical structure to a gyroid structure on heat ing above 110 degrees C for the 0.77 volume fraction blend and 100 degrees C for the 0.71 volume fraction blend. However, the reverse transition from gyroid to cylinder on cooling the 0.77 volume fraction blend to below 110 d egrees C was not observed even after annealing at temperatures below 110 de grees C for more than 10 h, possibly due to kinetic effects. Slow cooling ( 2-3 h) of the blend from the disordered state led to the gyroid structure e ven below 110 degrees C, while the low-temperature cylindrical phase could only be accessed by fast cooling (1.5 h) from the disordered state. Experim entally determined ODTs or DMTs are compared with predictions based on mean field theory. The predicted effect of homopolymer concentration on the ODT or DMT temperature was quantitatively consistent with that found experimen tally. The phase diagram of the diblock copolymer/homopolymer blend was fou nd to show the same complexity as and similar features to phase diagrams of pure diblock copolymers.