Cryogenic-temperature transmission electron microscopy (cryo-TEM) is now accepted as an almost standard tool in the study of complex liquids, i.e., liquid systems with aggregates or building blocks on the nanometric scale. Recent developments in high-resolution scanning electron microscopy (HR-SEM) have made it an ideal tool for the study of nanoparticles and colloids in viscous systems or in systems containing large objects, hundreds of nanometers and larger, in which small (nanometric) features are to be imaged, e.g., hydrogels or biological cells. I will describe the application of electron microscopy and related methodologies to elucidate the mechanism of self-aggregation of block copolymers from solution, through casting, all the way to solid nanoporous membranes. Following a brief description of the methodologies, I will describe a study of the self-assembly of isoporous membranes of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP), which involves many parameters: the block copolymer composition, the solvent, and its interactions with the respective blocks, composition of the casting solution, solvent evaporation, and immersion into a water bath. We characterized the self-assembly of PS-b-P4VP in solution in nearly neutral or P4VP-selective solvent, and on the surface of a cast film. We used a combination of room temperature and cryogenic high-resolution scanning electron microscopy (cryo-HR-SEM) to study copolymer micellization in dried films, in solution, and on the membrane surface during evaporation. We investigated the solutions, with and without addition of small water amounts, by cryogenic transmission electron microscopy (cryo-TEM) and small-angle neutron scattering (SANS). To enhance contrast between the polymer blocks we used selective electron-beam etching. We have shown that the starting point for membrane formation is a microphase-separated copolymer solution with the P4VP within the micelle core. As water is introduced, this structure is preserved as long as the system is far from equilibrium. Closer to equilibrium the PS blocks form the micelle cores.
