Dependence of Solvent Diffusion on Hydrophobic Block Length within Amphiphilic-Hydrophobic Block Copolymer Membranes

Pore networks and water diffusion within model (amphiphilic-hydrophobic) diblock copolymer membranes in the presence of 16 vol % water is studied by dissipative particle dynamics in combination with Monte Carlo tracer diffusion calculations. The amphiphilic block (parent architecture (A[A₃C])₁₀) is composed of a backbone that contains 10 consecutively connected hydrophobic A beads; to each A bead, a side chain is grafted composed of three connected A beads and a pendant hydrophilic C bead. Hydrophobic blocks are constructed from x covalently bonded A beads, with x = 20, 30, or 50. Water diffusion through the pores is modeled by Monte Carlo tracer diffusion within more than 500 mapped morphologies. Long range water diffusion within the amphiphilic-hydrophobic ((A[A₃C])₁₀-A_x) diblock architectures increases with hydrophobic block length. Diffusion increases with Q = ⟨N_bond⟩|C||1 - C|^-1, where C is the hydrophilic C bead fraction and ⟨N_bond⟩ the average number of bonds that A beads are separated from the nearest C bead. These trends are also anticipated for amphiphilic parent architectures (ACA₃)₁₀, (A₂[C]A₂)₁₀, and (A₂[AC]A)₁₀. This is explained by the squeezing of water from the hydrophobic phase into the amphiphilic phase. Two characteristic distances are observed: The shorter distance corresponds to the interpore (or intercluster) separation within the "parent architecture-water" phase and obeys the earlier obtained linear relation between intercluster distance and ⟨N_bond⟩_amphi of the amphiphilic parent architecture. The longer distance is governed by the phase separation between the amphiphilic-water phase and hydrophobic blocks.