Multi-Block Copolymers in Selective Solvent: A Brownian Dynamics Simulation
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Multi-Block Copolymers in Selective Solvent: A Brownian Dynamics Simulation Yongsheng Liu, 1 Huifen Nie, 1 Rama Bansil1 Zhenli Zhang,2 Sharon Glotzer2 1
Center of Polymer Studies and Department of Physics, Boston University, Boston, Massachusetts 02215
2
Department of Chemical Engineering and Materials Science & Engineering, University of Michigan, Ann Arbor, Michigan 48109
ABSTRACT We performed Brownian Dynamics simulations of multiblock copolymers of A and B polymers in a solvent selective for the A block at a volume fraction of 20%. Tri-, penta- and heptabocks were simulated. Fourier transformation reveals micellar clusters arranged in a BCC lattice, in agreement with scattering experiments. The clusters were analyzed using a percolation approach and we observed larger clusters when the outermost block was in the poor solvent condition. The ratio of number of loops to bridges decreases as the number of blocks in the copolymer increases, as does the polydispersity. Increased penalty of looping as the number of blocks increases leads to a larger number of smaller clusters with more bridges. INTRODUCTION: The structure and dynamics of block copolymer micelles in selective solvents has been extensively studied experimentally and theoretically, but much less attention has been given to simulations of these systems [1]. Computer simulation enables us to examine details of structure including features such as loops and bridges, which can form in copolymers containing three or more blocks, that are difficult to observe directly in experiments. The association behavior changes qualitatively in going from a diblock in a selective solvent to a BAB triblock in a selective solvent for the A block, as the micelles containing B blocks in the cores can be connected by bridges of the soluble A chains. Competition between looping and bridging of the A chains becomes important in determining whether networks or isolated flower-like micelles will form. Previous Brownian Dynamics (BD) studies have examined diblock copolymers in a selective solvent in 2-dimensions [2]. We have modified this model to simulate multiblock copolymers in a selective solvent in 3-dimensions. Brownian Dynamics is a variant of Molecular Dynamics (MD) which is better suited for simulating the diffusive motion in solution environment [2]. Furthermore by using a simplified particle description of a monomer which neglects all interactions within the bead one can run the simulation to longer times [3]. In this paper we present results on the association of multi-block copolymers made up of A and B polymers in a solvent selective for the A block and examine the dependence of structural features such as loops and bridges on the number of blocks, and on the solubility of the outermost of the copolymer. MODEL: We simulated TriBlock (ABA and BAB), PentaBlock (ABABA and BABAB), and HeptaBlock (ABABABA and BABABAB) copolymers in a selective solvent good for the A block and poor for the B block. The simulation consisted of 200 bead-spring chains placed in a c
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