First-Order Disordered-to-Lamellar Phase Transition in Lithium Salt-Doped Block Copolymers

We study the microphase separation of block copolymer electrolytes containing lithium salts. Taking poly(ethylene oxide)-b-polystyrene (PEO-b-PS) as an example, we show that in the presence of lithium salts the disordered-to-lamellar phase transition becomes first-order even at the level of mean-field theory, with a moderate range of temperature in which both the disordered and lamellar phases coexist, and different salt concentration in the coexisting phases. The coexistence arises from the different partitioning of the salt ions between the disordered phase and the lamellar phase, driven primarily by the solvation energy of anions. A striking consequence of the coexistence is that heating a lamellar phase into the coexistence region leads to increased order in the remaining lamellar phase.