Transition behavior of asymmetric polystyrene-b-poly(2-vinylpyridine) films: A stable hexagonally modulated layer structure

- Phase transitions in the bulk and films of asymmetric polystyrene-b-poly(2-vinylpyridine).
- Significant change in transitions of the films with decreasing film thickness.
- New observation of hexagonally modulated layer (HML) morphology in film geometry.
- A stable HML structure extended over the entire temperature range in the films ≤10Lo.

The phase transitions in the films of an asymmetric polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) were investigated by grazing incidence small-angle X-ray scattering (GISAXS) and transmission electron microscopy (TEM). Compared with the sequential transitions in the bulk, hexagonally perforated layer (HPL) - gyroid (GYR) - disorder (DIS) upon heating, the transitions in film geometry were dramatically changed with decreasing thickness due to the growing preferential interactions from substrate, resulting in a thickness-dependent transition diagram including four different morphologies of hexagonally modulated layer (HML), coexisting (HML and GYR), GYR, and DIS. Particularly in the films ≤10Lo, where Lo is d-spacing at 150 °C, a stable HML structure was identified even above the order-to-disorder transition (ODT) temperature of the bulk, which was attributed to the suppressed compositional fluctuations by the enhanced substrate interactions.