Macro- and microphase transitions in binary blends of block copolymers with complementarily asymmetric compositions

A unique phase transition between the state of "two macrophase-separated di sordered phases" at higher temperatures and that of "a single phase with on ly a local order" at lower temperatures was observed in a binary mixture of polystyrene (PS)-block-polyisoprene (PI) copolymers by using small-angle X -ray scattering, light scattering, transmission electron microscopy, and op tical microscopy. The constituent block copolymers of the mixture have almo st same molecular weights but complementary compositions: one component (de signated as i-2K) has a number-average molecular weight (M-n) of 1.2 x 10(4 ) and its volume fraction of PS (f(PS)) is 0.81, and the other component (d esignated as s-3K) is M-n = 1.3 x 10(4) and f(PS) 0.21; both neat i-2K and neat s-3K are in disordered state at all temperatures covered in this exper iment. The macrophase transition temperature for the i-2K/s-3K = 50/50(wt % /wt %) mixture was determined to be 130 degreesC, above which the mixture s howed the two macrophase-separated disordered phases composed of i-2K-rich domains with micrometer size in disordered state and the s-3K-rich domains also in the disordered state. However, when the temperature was sufficientl y lower than 130 degreesC, i-2K and s-3K mixed on the molecular level witho ut involving the macrophase separation to form the single phase composed of locally ordered PS- and PI-rich domains with nanometer size. This phenomen on reveals a "pseudo-LCST type phase diagram" based on the polymer componen ts with a UCST type interaction parameter vs temperature relationship. This phenomenon is intriguing also from the viewpoint that the local segregatio n of the PS and PI segments induced by increasing the repulsive segmental i nteractions suppress the macroscopic phase separation between the two compo sitionally complementary block copolymers in disordered state.