Porous thin films with hierarchical structures formed by self-assembly of zwitterionic comb copolymers

Abstract

Zwitterions exhibit high dipole moments and strong intra- and inter-molecular interactions. As a result, amphiphilic copolymers with zwitterionic repeat units easily self-assemble. Here, we explore the wide range of self-assembled morphologies formed by zwitterionic comb-shaped copolymers (ZCCs) comprising a hydrophobic backbone and zwitterionic side-chains. We study the effect of polymer structure, architecture, and film preparation method on self-assembled morphology. We synthesized ZCCs with varying zwitterionic side chain density and length, and applied each ZCC as a thin film on glass substrates using different methods. We compared the morphology of these films with ZCCs films coated on a porous substrate by non-solvent induced phase separation, creating mechanically supported thin films (MSTFs) and comparing them with films on non-porous substrates. The porous support led to distinct changes in film morphology, creating hierarchical features including ∼17 nm spherical micelles along with larger nanopores (∼85 nm). This feature size could be tuned using a zwitterionic homopolymer additive. MSTFs exhibited significant changes in water permeance and morphology upon exposure to saline solutions, known to stimulate expansion of the zwitterionic side-chains. These results demonstrate the wide range of morphologies accessible by this comb copolymer family and document the significant effect of a porous support on the film formation process.

Keywords

• Hierarchical
• Zwitterionic comb copolymer
• Self-assembly
• Porous thin film
• Ionic strength responsive