Ordered Structure in Blends of Block Copolymers. 5. Blends of Lamella-Forming Block Copolymers Showing both Microphase Separation Involving Unique Morphological Transitions and Macrophase Separation

It is pointed out theoretically that the ordered morphology in the binary mixtures of A−B type diblock copolymers, ((A−B) 1 /(A−B) 2 ), depends not only on the compositions of block copolymers (i.e., the volume fraction of the A block, f A1, f A2 ) but also on the relative chain-length differences of the constituent copolymers ( α j for jth ( j = A or B) block where α j is defined by α j = ( − )/ and ( i = 1 or 2) denotes the polymerization index of the jth block chain in the i th block copolymer (A−B) i ) and blend composition (i.e., the mole fraction of (A−B) 2 having a larger molecular weight, q 2 ), whereas the morphology in the single component A−B diblock copolymer system in a strong segregation limit solely depends on the composition ( f A ). We explored ordered morphology of binary mixtures of lamella-forming polystyrene (PS)- block -polyisoprene (PI), (PS−PI), with given block compositions ( f PS1 = 0.44, f PS2 = 0.46) and given relative chain-length differences ( α PS = 6.18, α PI = 5.68) as a function of q 2 . Our results revealed both the regime where only microphase separation occurred ( q 2 = 0 and 0.179 ≤ q 2 ≤ 1) and the regime where macrophase separation occurred as well (0.003 ≤ q 2 ≤ 0.127). In the microphase separation regime, we found a unique morphological transition between lamellar and cylindrical morphology with q 2 whose trend is consistent with the theoretical prediction by Lyatskaya et al. ( Polymer 1992, 33, 343). In the macrophase separation regime we found the two coexisting macrophases with cylindrical and lamellar morphology. The miscibility gap of the two PS−PIs, with respect to q 2 for the macrophase separation, was found to be consistent with a theoretical prediction by Matsen ( J. Chem. Phys. 1995, 103, 3268).