Phase Behavior of Binary Blends of High Molecular Weight Diblock Copolymers with a Low Molecular Weight Triblock

Binary blends of four different high molecular weight poly(styrene-b-isoprene) (SI) diblock copolymers with a lower molecular weight poly(styrene-b-isoprene-b-styrene) (SIS) triblock copolymer were prepared, and their morphology was characterized by transmission electron microscopy and ultra-small-angle X-ray scattering. All the neat block copolymers have nearly symmetric composition and exhibit the lamellar morphology. The SI diblock copolymers had number-average molecular weights, M̅n, in the range 4.4 × 105−1.3 × 106 g/mol and volume fractions of poly(styrene), ΦPS, in the range 0.43−0.49, and the SIS triblock had a molecular weight of M̅n ∼ 6.2 × 104 g/mol with ΦPS = 0.41. The high molecular weight diblock copolymers are very strongly segregating, with interaction parameter values, χN, in the range 470−1410. A morphological phase diagram in the parameter space of molecular weight ratio (R = Mndiblock/1/2Mntriblock) and blend composition was constructed, with R values in the range between 14 and 43, which are higher than previously reported. The phase diagram revealed a large miscibility gap for the blends, with macrophase separation into two distinct types of microphase-separated domains for weight fractions of SI, wSI R ∼ 30, morphological transitions from the lamellar to cylindrical and bicontinuous structures were also observed