a block copolymer
Most polymers are made of one kind of repeat unit. A copolymer mixes two, and a block copolymer joins them in a very particular way: a long run of one type of monomer bonded end-to-end to a long run of another type, all in one molecule. Picture a train made of fifty red carriages coupled directly to fifty blue carriages — one continuous train, but red at one end and blue at the other. The simplest case, one A-block joined to one B-block, is a diblock, written A-b-B; there are also triblocks (A-b-B-b-A) and stars.
The interesting physics comes from a tension built into the molecule. Usually the two blocks dislike one another chemically — like oil and water, they would rather not mix. If they were separate molecules they would simply demix into two bulk layers (macrophase separation). But here they are tied together by a covalent bond at the join, so they cannot get away from each other; the A ends can only huddle with other A ends, and the B ends with other B ends, out to a distance no larger than the chains are long. The result is that the material splits into a regular pattern of tiny A-rich and B-rich domains, each only 10-50 nanometres across — microphase separation. Which pattern forms is controlled mostly by the relative sizes of the two blocks (the volume fraction f): roughly equal blocks stack into alternating flat lamellae, a minority block of about a third rolls into cylinders packed in the majority, and a small minority block gathers into spheres, with a labyrinthine gyroid network in between.
Block copolymers matter because they build ordered nanostructures by themselves, with no mask or template — a gift for nanotechnology. Their self-assembled patterns are used to make ultrafine templates for computer-chip lithography, nanoporous filtration membranes, and photonic materials, all at length scales below what light can pattern. They are also everyday materials: thermoplastic elastomers such as SBS rubber (a polystyrene-polybutadiene-polystyrene triblock) get their springiness from hard glassy polystyrene spheres that pin the ends of soft rubbery chains, giving a rubber that melts and can be re-moulded — the basis of shoe soles and countless flexible parts.
A polystyrene-b-polyisoprene diblock with roughly equal blocks self-organizes, on annealing, into a stack of alternating polystyrene and polyisoprene lamellae about 30 nm apart — a striped nanostructure you can photograph in the electron microscope. Shift the balance so polystyrene is only a quarter of the chain and the same chemistry instead makes polystyrene cylinders packed in a hexagonal array. Composition alone dials the shape.
A block copolymer joins unlike blocks in one chain; unable to fully separate, they form 10-50 nm ordered domains.
A block copolymer is not a random copolymer. In a random copolymer the two monomers are scrambled along the chain; only contiguous blocks can microphase-separate into ordered nanodomains, because the join tethers the incompatible parts together.