Phase behavior of symmetric linear multiblock copolymers

Molecular-dynamics simulations are used to study the phase behavior of a single linear multiblock copolymer with blocks of Aand B-type monomers under poor solvent conditions, varying the block length N , number of blocks n, and the solvent quality (by variation of the temperature T ). The fraction f of A-type monomers is kept constant and equal to 0.5, and always the lengths of A and B blocks are equal (NA =NB =N), as well as the number of blocks (nA = nB). We identify the three following regimes where: i) full microphase separation between blocks of different type occurs (all blocks of A-type monomers form a single cluster, while all blocks of B-type monomers form another); ii) full microphase separation is observed with a certain probability; and iii) full microphase separation cannot take place. For a very high number of blocks n and very high N (not accessible to our simulations) further investigation is needed. Copyright c © EPLA, 2011 The phase behavior of block copolymers has recently attracted much interest in experimental and theoretical studies [1–46]. For melts of multiblock copolymer chains the phase diagram almost resembles the phase diagram of that with diblock copolymer chains; the lamellar structure is expected for the most symmetrical case, an approach which is particularly valid in the strong segregation limit [33]. In the case of infinitely dilute solutions, it is sufficient to consider the behavior of isolated chains, where interactions (often of short range) within the chain are relevant. The case of a single linear multiblock copolymer chain [47–51] is very interesting even when only two different types (A, B) of blocks of the same length are considered (see ref. [51] for discussion). Interestingly, multiblock copolymer chains have also a close relation to the various toy-models (i.e., the HP model [52]) which try to mimic the behavior of various biomacromolecules, which are formed by periodically repeated structural units (“monomers”) along their chain, in order to understand complicated biological phenomena, i.e., protein folding [53], helical structures [54], etc. (a)E-mail: [email protected] Multiblock copolymers composed of two different types of blocks (A and B) under good solvent conditions form coil structures and the chain conformations are essentially dictated by the repulsive interactions between the different monomers [47,48]. In fact, the chemical difference of monomers should be kept responsible for an expansion in the chain dimensions with respect to the equivalent homopolymer chains (same total number of monomers) under the same thermodynamic conditions [48–50]. Also, it is expected that the spherical symmetry of the macromolecule should be distorted adopting a slightly ellipsoidal overall formation [48–51]. Nevertheless, the most interesting behavior is expected when the solvent quality is varied. Under poor solvent conditions the chain collapses, and different scenarios of microphase separation between the monomers A and B belonging to different blocks are conceivable depending on the block length and the number of blocks of the chain at a given temperature [47]. Identification of these different states could be based on the analysis of the formed clusters as monomers of the same, or different, blocks join together. For rather short chains (accessible to simulations), simulation techniques provide the best way to the understanding of the phase