microphase separation
Pour oil into water and the two separate completely, into a bulk layer of each — that is ordinary (macro)phase separation, and the domains grow as large as the container allows. Now imagine every oil molecule were chemically chained to a water-loving molecule so they could never fully part company. They would still try to separate, but they could only get as far apart as the chain is long. The compromise they reach is a regular pattern of tiny oil-rich and water-rich regions, each only nanometres across. That frustrated, nanoscale demixing is microphase separation, and it is what a block copolymer does.
The physics is a tug-of-war between two energies. Contact between the unlike A and B segments is unfavourable, measured by the Flory interaction parameter chi (bigger chi means the blocks dislike each other more), and this drives them apart. Opposing that, pulling the blocks into separate domains stretches and confines the chains, which costs entropy. Which side wins is governed by the single combination chi times N, where N is the number of segments in the chain: for a symmetric diblock the domains order when chi times N exceeds about 10.5 (the order-disorder transition), and below that the melt is a disordered blob. Once ordered, the domain spacing grows with chain length roughly as N to the two-thirds power, landing in the 10-50 nm range, and the shape — spheres, cylinders, gyroid, or lamellae — is set by the volume fraction of the minority block. The pattern has genuine long-range order, like a crystal, but with a repeat unit thousands of times larger than an atomic lattice.
Microphase separation is prized because it lays down a periodic nanoscale pattern with no external template, purely by letting the molecules find their own equilibrium. This is the engine behind directed self-assembly for semiconductor lithography, where block-copolymer domains define features finer than optics can draw; behind nanoporous membranes made by etching away one domain; and behind photonic and optical films whose colour comes from the regular domain spacing. It is the polymer world's answer to how you get order at ten nanometres without building it atom by atom.
Coat a wafer with a symmetric block copolymer whose blocks separate at about 25 nm, guide it with a coarse surface pattern, and anneal: the copolymer self-organizes into perfectly regular 25 nm stripes or dots, subdividing the guide lines into features finer than the lithography could draw. This directed self-assembly, driven entirely by microphase separation, is used to push chip feature sizes downward.
Microphase separation is nanoscale demixing of tethered blocks; order sets in when chi times N exceeds about 10.5.
The micro is historical: the domains are actually nanoscale, about 10-50 nm. And they never coarsen into macroscopic phases the way oil and water do, because the covalent join between the blocks forbids full separation.