Chain Exchange Kinetics of Asymmetric B₁AB₂ Linear Triblock and AB₁B₂ Branched Triblock Copolymers

Abstract

Equilibrium chain exchange of asymmetric B₁AB₂ and AB₁B₂ branched triblock copolymers in a B selective solvent has been studied via dissipative particle dynamics simulations. Hybridization simulations are performed using B₁AB₂ and AB₁B₂ branched triblock copolymers at varying levels of asymmetry and compared with equivalent AB diblock copolymers. It is found that B₁AB₂ triblocks exchange ∼1 order of magnitude faster than diblocks (persisting over various values of χ<i><i>N</i></i>_core and total corona length), while AB₁B₂ triblocks exchange ∼4 times faster than diblocks. The dependence on asymmetry is weak, with very asymmetric triblocks (<i>N</i>_B₁ ≪ <i>N</i>_B₂) exchanging only 2 or 3 times faster than symmetric triblocks. Two causes are found for this: (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. Additionally, unlooped core blocks pull out bead-by-bead with multiple activations, whereas the looped core blocks tend to aggregate near the micelle surface and pull out as a single entity, potentially further reducing the energy penalty of pullout. Because of this difference in mechanism, the looped core triblocks pull out more rapidly than the unlooped core triblocks even at identical aggregation numbers