Observation of Inverted Phases in Poly(styrene-b-butadiene-b-styrene) Triblock Copolymer by Solvent-Induced Order−Disorder Phase Transition

We report observation of inverted phases consisting of spheres and/or cylinders of the majority fraction block in a poly(styrene- b -butadiene- b -styrene) (SBS) triblock copolymer by solvent-induced order−disorder phase transition (ODT). The SBS sample has a molecular weight of 140K Da and a polystyrene (PS) weight fraction of 30%. Tapping mode atomic force microscopy (AFM) and transmission electron microscopy (TEM) were utilized to study the copolymer microstructure of a set of solution-cast SBS films dried with different solvent evaporation rates, R . The control with different R leads to kinetic frozen-in of microstructures corresponding to a different combination parameter χ eff Z of the drying films (where χ eff is the effective interaction parameter of the polymer solution in the cast film and Z the number of “blobs” of size equal to the correlation length one block copolymer chain contains), for which faster evaporation rates result in microstructures of smaller χ eff Z . As R was decreased from rapid evaporations (∼0.1 mL/h), the microstructure evolved from a totally disordered one sequentially to inverted phases consisting of spheres and then cylinders of polybutadiene (PB) in a PS matrix and finally reached the equilibrium phase, namely cylinders of PS in a PB matrix. We interpret the formation of inverted phases as due to the increased relative importance of entropy as χ eff Z is decreased, which may dominate the energy penalty for having a bigger interfacial area between the immiscible blocks in the inverted phases.