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Block copolymers are composed of long sequences ("blocks") of the same monomer unit, covalently bound to sequences of unlike type. The blocks can be connected in a variety of ways; schematics of AB diblock and ABA triblock structures are shown below.
The blocks can sometimes intermix freely at sufficiently high temperature, or when sufficiently diluted with solvent, generating the "disordered" structure shown above at right. However, it is common for the blocks to spontaneously self-assemble ("order", as shown above at left) into a diversity of mesophases, with the size scale governed by the chain dimensions (order tens of nanometers). In the mesophases, dissimilar blocks exist in distinct "microdomains" which are highly enriched in blocks of the same type, sometimes to the point of being essentially pure. The covalent bonds linking the dissimilar blocks are thus localized to the vicinity of the microdomain interfaces.