Self-organization of quasi-random copolymers

Copolymer macromolecules composed of chemi-cally different units are capable of forming partiallyordered spatial structures with diverse morphology [1,2]. The ability to self-assemble is dictated by a differentcharacter of interactions of copolymer units, on the onehand, and by covalent bonding of units within the samemacromolecule, on the other. The latter factor preventsthe separation of the system into homogeneous macro-scopic phases, which can, under definite conditions,stabilize some types of microdomain structures. As arule, a similar phenomenon is treated as microphaseseparation or order–disorder transition. The character-istics of this transition and morphological features ofnascent superstructures are determined by both theexternal parameters and the structure of the copolymeritself. The microphase separation theory mostly dealswith diblock or regular multiblock AB copolymerscomposed of two types of units (A and B) [1, 2]. Thephase behavior of AB copolymers with a random(“quasi-random”) distribution of units along the chainis of particular interest.In this paper, we report on the behavior of a specifictype of quasi-random copolymer in which the distribu-tion of A and B units is similar in a certain sense to theprimary sequence of amino acids in globular proteins.The sequence design scheme for generating such a pro-teinlike copolymer was suggested in [3, 4]. The essenceof this scheme is in chemical modification of the unitslying at the surface of a primary homopolymer globule.Through interaction with a dissolved reagent, some ofthe A units convert into B units. By analogy with globu-lar proteins, these units can be considered to be, respec-tively, hydrophobic and hydrophilic ones. The statisticalproperties of nascent AB sequences are characterized byspecific long-range correlations [5], which, as might beexpected, will influence the phase behavior of similarcopolymers. Such a macromolecule is characterized bythe following parameters: the length N ; the composi-tion, i.e., the fractions of A and B units ϕ