A Phase Diagram for the Binary Blends of Nearly Symmetric Diblock Copolymers. 2. Parameter Space of Temperature and Blend Composition

Abstract

A phase diagram in the parameter space of temperature and blend composition that contains order−disorder transition (ODT) and thermoreversible macrophase separation between constituent copolymers was constructed for binary blends of polystyrene-block-polyisoprene (PS−PI) copolymers. The constituent copolymers, designated as H102 and FS-1, have nearly symmetric copolymer compositions and rather different molecular weights; i.e., number-average molecular weight (Mn) and volume fraction of PS-block (fPS) are 1.0 × 105 and 0.47 for H102 while 2.1 × 104 and 0.40 for FS-1. The ODT was observed on the neat FS-1 and two blend specimens whose weight fractions of H102, ΦH102, are 0.1 and 0.2. Counterintuitively, the ODT temperature (TODT) decreased with increasing the average molecular weight of the specimen, namely, TODT for FS-1 neat > TODT for the blend of ΦH102 = 0.1 > TODT for the blend of ΦH102 = 0.2. Then TODT abruptly increased between the blends of ΦH102 = 0.2 and 0.3 and became inaccessibly high temperature (i.e., > 200 °C). The macrophase separation between H102 and FS-1 was observed on the blend of ΦH102 = 0.2. This macrophase separation was thermoreversible but accompanied by a hysteresis between cooling and heating processes. Although both H102 and FS-1 neat copolymers showed lamellar morphology, PS cylindrical morphology was observed on certain blend specimens. The origin of PS cylindrical morphology may be attributed to the slight asymmetry in the fPS of FS-1.