(576e) Molecular Simulation of Micellar Chain Exchange Kinetics of Asymmetric B1AB2 Linear Triblock and AB1B2 branched Triblock Copolymers

Authors: 
Peters, A., Louisiana Tech University
Lodge, T. P., University of Minnesota
Block copolymer micelles have found application in a wide variety of areas including drug delivery, viscosity modification, and polymer blend stabilization. Critical to controlling polymer self-assembly and behavior is a thorough understanding of the chain exchange processes that are essential to achieving equilibrium. While diblock chain exchange has been extensively studied, exchange of triblocks or more complicated polymer architectures has received significantly less attention. We have studied the exchange of asymmetric B1AB2 and AB1B2 branched triblock copolymers in a B selective solvent using dissipative particle dynamics simulations. These two architectures differ only in which end of the A block the two B blocks are attached to: AB1B2 polymers have both B blocks attached to the same end of the A block, while B1AB2polymers have each B block attached to opposite ends of the A block. This allows a direct comparison of the effect of looped vs unlooped core blocks on micelle size and chain exchange rate. It is found that B1AB2 triblocks exchange ∼10 times faster than diblocks while AB1B2 triblocks exchange ∼4 times faster than diblocks. The dependence on asymmetry is weak, with very asymmetric triblocks (NB1 ≪ NB2 ) exchanging only 2 or 3 times faster than symmetric triblocks. Two main mechanisms are responsible for this behavior: : (1) increases in the density of corona beads near the micelle core for triblocks, resulting in greater stretching penalties and lower aggregation numbers, and (2) looped core blocks (B1AB2) spending more time near the surface of a micelle core than unlooped core blocks (AB1B2 and AB), resulting in a lower energy benefit of insertion. In addition, the unlooped core blocks require multiple activations for removal, while the looped core blocks as a single entity.
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