Surface-Grafted Polymeric Ionic Liquids with Tunable Morphology via In/Ex Situ Cross-linking Methods

Surface-grafted poly(ionic liquid) (PIL) films were prepared by both in and ex situ cross-linking methods with reversible addition-fragmentation chain transfer (RAFT) polymerization. Cross-linked brushes are more stable than linear brushes without sacrificing the surface functionality and, therefore, have increased potential for applications in biomedicine and materials chemistry. The two methods, in situ via a bifunctional cross-linker and ex situ via thermal cross-linking, were systematically compared on silicon-wafer substrates. Films obtained through in situ cross-linking were superior to films derived from our ex situ cross-linking technique with respect to responsive behavior and controlling the formation of polymer brushes on the surface. Alternatively, more stable layers were obtained by the ex situ cross-linking method using a cross-linker based on Meldrum's acid, where the film structure could be changed from a brush to collapsed film morphologies with an increasing cross-linker ratio.