Surface Segregation and Self-Assembly of Block-Copolymer Separation Layers on Top of Homopolymer Substructures in Asymmetric Ultrafiltration Membranes from a Single Casting Step

Surface segregation in blended polymer films has attracted much interest in fundamental research as well as for practical applications. A variety of methodologies have been proposed for controlling surface segregation. They often require long annealing times, however, to achieve thermodynamic equilibrium. Here, a strategy and proof-of-principle experiments are described to control surface segregation of thin block-copolymer (BCP) layers on top of a homopolymer in a single casting step from blended BCP/homopolymer solutions. The surface coverage by the minor constituent BCP (2-10 wt%) is accomplished despite almost identical surface energies of BCP and homopolymer constituents. Immersing this casted solution into water for nonsolvent induced phase separation (NIPS), a nonequilibrium process, affords solidified bilayer ultrafiltration membranes composed of a thin porous surface layer of self-assembled BCP atop an asymmetric porous homopolymer substructure. Key to successful BCP surface segregation is the choice of a binary solvent system based on careful considerations of solvent surface energies and polymer-solvent interaction parameters. Furthermore, stabilizing the BCP micellar structure by a divalent metal additive is also essential. The approach provides a cost-effective method for fabricating bilayer-type asymmetric ultrafiltration membranes with uniform BCP self-assembly based selective top surface pore layers in a single casting step.

Automated summary

Surface segregation in blended polymer films has attracted significant interest, and a strategy to control this phenomenon in thin block-copolymer layers has been described in this study. By utilizing nonsolvent induced phase separation, solidified bilayer ultrafiltration membranes with self-assembled BCP top layers atop homopolymer substructures can be fabricated in a single casting step. Key factors for successful BCP surface segregation include careful selection of solvent systems and stabilization of micellar structures.