Reversible conformation-driven order–order transition of peptide-mimic poly(n-alkyl isocyanate) in thin films via selective solvent-annealing

Abstract

We report for the first time the conformational and structural details of peptide-mimic poly(n-hexyl isocyanate) (PHIC). PHIC is a representative poly(n-alkyl isocyanate)s, which have received significant attention because of their unique stiff chain characteristics and potential applications in various fields. A well-ordered hexagonal close packing structure of PHIC with 83 helical conformation was clearly observed in the nanoscale thin films that were selectively annealed with carbon disulfide (CS2). A well-ordered multi-bilayer structure of the polymer with β-sheet conformation was also clearly formed in the films that were selectively annealed with toluene. In addition, a fully reversible transformation between these two self-assembled structures was demonstrated by consecutive annealings with CS2 and toluene. A family of peptide-mimic polymers — polyisocyanates with linear alkyl side groups — are known to have a stiff helical backbone. These structural features have garnered attention for the development of chiral recognition, optical switches, liquid crystals or degradable materials. Yet the exact conformation the chains adopt, and how they pack in the solid state, remains unclear. Through detailed characterization, Moonhor Ree, Jae-Suk Lee and co-workers in Korea have now confirmed that, in a thin film, the polymer ‘poly(n-hexyl isocyanate)’ forms one of two different, well-ordered structures. The two morphologies were obtained, and reversibly converted into each other, by annealing under different solvent vapours. In the presence of carbon disulfide, helical chains self-assembled into a hexagonal close packing structure, whereas the use of toluene yielded a multi-bilayer lamellar structure consisting of polymer chains in β-sheet conformation. This intriguing transformation arises from the solvent molecules' different affinities with the backbone and side groups of the polymer chain. Peptide-mimic poly(n-hexyl isocyanate) (PHIC) with stiff chain characteristics demonstrated to selectively form a well-ordered hexagonal close packing structure with 83 helical conformation in the nansocale thin films annealed with carbon disulfide. Moreover, this polymer showed to selectively form a well-ordered multi-bilayer structure with β-sheet conformation in the thin films annealed with toluene. These two self-assembled structures were reversibly transformed by consecutive annealing with carbon disulfide and toluene. These chain conformations and self-assembled structures were confirmed by synchrotron grazing incidence X-ray scattering analysis.