In the context of block copolymers, which of the following best describes the driving force behind their self-assembly into distinct nanoscale structures?

The main driving force is the energetic penalty that arises when two chemically different blocks are forced to mix; each block prefers to be surrounded by its own kind, so the polymer tends to separate on the nanoscale. This segregation is opposed by the covalent bond that keeps the blocks linked, creating a balance that fixes the size of the domains. Because the blocks are tethered, the system settles into a regular, periodic structure rather than macroscopic phase separation. For example, a diblock of polystyrene and polybutadiene will form alternating lamellae of a few nanometers thick, each layer containing only one polymer type. Thus, the competition between interfacial energy and chain connectivity drives the self‑assembly.