Diblock Copolymer Membranes: What Can We Learn from Computer Simulation?

Block copolymer membranes offer a bottom-up approach to form isoporous membranes that are useful for ultrafiltration of functional macromolecules, colloids, and water purifcation. Experimentally, the fabrication of isoporous block copolymer membranes from a mixed film of an asymmetric block copolymer and two solvents involves two stages: First, the volatile solvent evaporates, creating a dense polymer skin, in which the block copolymer self-assembles into an isoporous top layer via evaporation-induced self-assembly (EISA). This top layer imparts selectivity onto the membrane. Subsequently, the film is brought in contact with a nonsolvent (coagulation bath), and the exchange between the remaining nonvolatile solvent and the nonsolvent through the porous top layer results into macrophase separation between copolymer and nonsolvent (nonsolvent-induced phase separation, NIPS). Thereby a macroporous support for the functional top layer is fabricated that imparts mechanical stability onto the system.

To optimize permeability, selectivity, longevity, and cost, and to rationally design fabrication processes, direct insights into the spatiotemporal structure evolution are necessary. Because of a multitude of nonequilibrium processes in polymer membrane formation, theoretical predictions remain a challenge. Using a single highly coarse-grained, particle-based simulation technique we study EISA and NIPS, focussing on the complex interplay between evaporation and solvent− nonsolvent exchange, micro- and macrophase separation, and dynamic arrest. The simulations identify a narrow process window, which allows for the successful fabrication of isoporous diblock copolymer membranes, and provide insights into the role of the different thermodynamic (e.g., solvent selectivity for the block copolymer components), structural (e.g., copolymer architecture) and kinetic (e.g., plasticizing effect of the solvent) characteristics.

References

Simulation of solvent evaporation from a diblock copolymer film: Orientation of the cylindrical mesophase, O. Dreyer, G. Ibbeken, L. Schneider, N. Blagojevic, M. Radjabian, V. Abetz, and M. Müller, Macromolecules 55, 7564 (2022)

Nonequilibrium processes in polymer membrane formation: theory and experiment, M. Müller and V. Abetz, Chem. Rev. 121, 14189 (2021)