Relevance of lamellar to lamellar-catenoid to double gyroid phase transitions to biomembranes

Microspheres containing a polystyrene-polydimethylsiloxane (PS-PDMS) diblock copolymer were made by a membrane emulsion strategy and suspended in water. The as-formed structure is a set of nested, concentric bi-layers (lamellar). A small amount of a PS preferential solvent was very slowly indirectly infiltrated into the sphere via transport across the surrounding water phase to induce a series of order-order phase transitions depending on the infiltration time. The structure evolves from the initial lamellar to lamellar-catenoid to double gyroid due to the lowering of the effective Chi parameter and shift in the relative component volume fractions due to the preferential solvent. Rapid drying of the microsphere vitrifies the structures. Slice and view scanning electron microscopy enables high resolution 3D tomograms of the boundary region between the transforming phases. Detailed information on the geometry, topology and crystallographic relationships of the microdomains, their component mesoatoms and the shapes and curvatures of the inter-material diving surface during the transformation are analyzed. The nature of the transformations are compared to biomembrane fusion and transport pathways in drug release.