Microphase and macrophase transitions in binary blends of diblock copolymers

The macro- and microphase transitions in binary blends of poly(styrene-b-is oprene) (SI) and poly(styrene-b-ethylenepropylene) (SEP) lamellar diblock c opolymers were investigated. by optical and electron microscopy and by rheo logical analysis. A variety of morphologies were produced as a function of blend composition and the rate of solvent evaporation. Numerous morphologie s are possible because of the interaction of macro- and microphase separati on. As one example, spherical droplets of SEP whose lamellae comprise conce ntric spherical shells were produced. At the phase boundaries of this and o ther morphologies, the polystyrene (S) microdomains associated with each ph ase were in contact. Additional phase separation occurred within the ordere d media. When the molecular weight of SI was small and its mobility high, a n SI domain was produced at the center of each spherical SEP droplet, there by reducing the curvature of the SEP lamellae. When the molecular weight of SI was larger and its mobility lower, individual SI lamellae formed within the ordered SEP-rich phase. The solubility of SI in SEP exceeded that of S EP in SI, because of the relative interaction parameters and molecular weig hts of the copolymers. SI that was dissolved within SEP localized at the mi crodomain interface and acted to shield the S and EP segments from one anot her. The lamellar ordering transition of the lower molecular weight SI dibl ock was determined theologically. In blends with SEP, the ODT of the SI was increased. This effect was shown to arise from interfacially induced order ing.