Microphase Separation of Statistical Multiblock Copolymers

Random multiblock copolymers are characterized by a number of parameters, and their self-assembly is controlled, in particular, by correlations in monomer unit sequences. We investigate the microphase separation of random multiblock copolymers composed of A and B homo-oligomers (macromonomers) in melts. The copolymer synthesis and self-assembly are studied by computer simulations using the dissipative particle dynamics technique. The copolymers are formed by irreversible step-growth polymerization in an equimolar melt of end-functionalized macromonomers of length M up to a high conversion degree. Correlations between macromonomers of different types along the chains are controlled by the reaction probabilities. The molar mass and block mass distributions are described for linear macromolecules and for cycles. Macromonomer sequences in linear chains are well described by the first-order Markov statistics. With an increase in the Flory-Huggins parameter chi, microphase separation into a lamellar structure, for close-to-alternating block copolymers, and into irregularly arranged domains, for Bernoullian sequences of macromonomers, occurs. The transition chi M values are approximately twice those obtained with weak segregation theory [Fredrickson, G. H. Macromolecules1992, 25, 6341-6354] and approximately half the values for which microphases were previously detected in the simulations of random copolymer melts [Gavrilov, A. A. J. Chem. Phys.2013, 139, 224901]. Microphase separation is described by the static structure factor and local composition, besides the block positions are monitored visually. The lamellar thickness H is always between the unperturbed sizes of one- and two-macromonomer blocks, and we characterize the separation as a moderate segregation regime. For the close-to-alternating block copolymers at a fixed chi, the thickness scales with the average block length approximately as H similar to (N) over bar (2/3)(bl) that is in agreement with the theoretical description (Supporting Information) considering the elastic response of the shortest blocks only.