Ring-shaped morphology in H-shaped block copolymer thin films

Abstract

The formation of ring-shaped structures in an H-shaped block copolymer [a poly(ethylene glycol) backbone with polystyrene branches, i.e., (PS)₂PEG(PS)₂] thin film was investigated when it was annealed in saturated PEG-selective acetonitrile vapor. Our results clearly indicate that ring formation is determined by the initial morphology of the spin-coated film, the solvent vapor selectivity and the environmental temperature of the solvent-annealing process. Only the films with the initial core–shell cylindrical structure in strongly PEG-selective acetonitrile vapor could form the ring-shaped structures. Environmental temperature has an influence on the formation mechanisms. It was proven that the rings arising from the core–shell cylindrical structure of the pristine spin-coated (PS)₂PEG(PS)₂ film were caused by the migration of the PEG shells to the top surface in the process of PEG-selective vapor annealing at the environmental temperature of 15 °C. While at a higher environmental temperature (25 °C), the degree of swelling of the film and the chains’ mobility increased and the formation mechanism of the rings in this situation was similar to that in dilute solution.