Order-order phase transitions of block copolymers within microspheres

Various geometrical and topological transformational pathways have been proposed for order-order phase transitions (OOTs) between different block copolymer phases (e.g. lamellae ↔ double gyroid ↔ cylinder). Often, OOTs are studied as a function of temperature and/or solvent content using small angle x-ray scattering (SAXS). However, the specifics of the morphological evolution at the interface between the co-existing phases are challenging to determine from reciprocal space data. Tomographic electron microscopy imaging in real space can reveal the evolution of the intermaterial dividing surface (IMDS) shape across the boundary between the phases. However, tomograms from microtomed thin sections suffer from including only a relatively small number of unit cells from each phase as well as distortions arising from the microtome slicing. We employ microspheres of polystyrene-polydimethylsiloxane (PS-PDMS) diblock copolymers made by a membrane emulsion process to initially create a concentric layer lamellar structure. By very slowly infiltrating a preferential solvent into the microparticles, phase transitions are induced from lamellae to catenoid-lamellae to double gyroid. 3D tomograms of the interface between the transforming phases provide detailed information on the variation of the IMDS during the OOT.