COMPOSITIONALLY SYMMETRICAL DIBLOCK COPOLYMER BLENDS OF MODERATE POLYDISPERSITY

Recent experimental evidence and theoretical predictions indicate that binary blends of relatively monodisperse diblock copolymers remain mi scible if the molecular weight disparity of the constituent copolymers is not too great. In this work, we examine the effect of moderate cop olymer polydispersity on both the microstructural characteristics and phase behavior of blends prepared from four compositionally symmetric poly( styrene-b-isoprene) (SI) diblock copolymers ranging in polydispe rsity ((M) over bar(w)/(M) over bar(n)) from 1.02 to 1.30. Blend perio dicities, measured by small-angle X-ray scattering, compare favorably with predictions from a strong segregation theory proposed for lamella r diblock copolymer blends composed of monomolecular copolymers. Trans mission electron microscopy, employed to ascertain the real-space morp hological characteristics of these blends, reveals that a lamellar-->c ylindrical transition occurs in macrophase-separated blends. (C) 1997 John Wiley & Sons, Inc.