Collapse and Phase Separation of Miscible Diblock Copolymers in Single Good Solvents

We study the phase behavior of a miscible AB diblock copolymer with chain length N in a single good solvent. When the solvent is neutral and good for both blocks, and the AB copolymer itself is miscible, the single chain can collapse in the dilute solution with the transition χeff – 1/2 ∼ N–1/2b3/2/v01/2, where χeff = χASfA + χBSfB – χABfAfB is the effective χ parameter between monomer and solvent. This collapse can subsequently drive the macrophase separation of the miscible diblock copolymer in the good solvent. Moreover, when the solvent exhibits strong selectivity toward the B block, we show that the diblock copolymer can undergo microphase separation even though the AB blocks are miscible. Our results can provide a fundamental understanding of the phase behavior of a miscible diblock copolymer in the single good solvent, showing great potential guidance for studying the DNA or protein folding and the corresponding phase separation.