A phase diagram for the binary blends of nearly symmetric diblock copolymers. 2. Parameter space of temperature and blend composition

A phase diagram in the parameter space of temperature and blend composition that contains order-disorder transition (ODT) and thermoreversible macroph ase separation between constituent copolymers was constructed for binary bl ends of polystyrene-block-polyisoprene (PS-PI) copolymers. The constituent copolymers, designated as H102 and FS-1, have nearly symmetric copolymer co mpositions and rather different molecular weights; i.e., number-average mol ecular weight (M-n) and volume fraction of PS-block (f(PS)) are 1.0 x 10(5) and 0.47 for H102 while 2.1 x 10(4) and 0.40 for FS-1. The ODT was observe d on the neat FS-1 and two blend specimens whose weight fractions of H102, Phi (H102), are 0.1 and 0.2. Counterintuitively, the ODT temperature (T-ODT ) decreased with increasing the average molecular weight of the specimen, n amely, T-ODT for FS-1 neat > T-ODT for the blend Of Phi (H102) = 0.1 > T-OD T for the blend of Phi (H102) = 0.2. Then T-ODT abruptly increased between the blends Of Phi (H102) = 0.2 and 0.3 and became inaccessibly high tempera ture (i.e., > 200 degreesC). The macrophase separation between H102 and FS- 1 was observed on the blend Of Phi (H102) = 0.2. This macrophase separation was thermoreversible but accompanied by a hysteresis between cooling and h eating processes. Although both H102 and FS-1 neat copolymers showed lamell ar morphology, PS cylindrical morphology was observed on certain blend spec imens. The origin of PS cylindrical morphology may be attributed to the sli ght asymmetry in the f(PS) of FS-1.