Extended Data Fig. 3: Spectroscopic analysis of the development of films.

a, Reflectance spectrum of the developed polystyrene (28 kDa) films and a model photonic multilayer film spectrum produced using the transfer matrix method. b–d, Real-time spectroscopy of the immersion process in acetic acid at various conditions. b, Polystyrene film crosslinked and developed in an acetic acid bath (20 °C). The film initially undergoes a series of sharp changes in spectra, indicating changes in film thickness, and then after formation of a Bragg peak (green arrows), the spectrum persists. c, Polystyrene film crosslinked and developed in an acetic acid bath (30 °C). Unlike in b, the final Bragg peak decays. d, Polystyrene film crosslinked, and then annealed at 160 °C for 2 days and developed in an acetic acid bath (30 °C). The heat map shows a smooth change in spectra, indicating a gradual increase towards a final swollen film without microfibril formation. e, After crosslinking, films were post-annealed at a range of temperatures (60–160 °C) to remove stresses in the films (thermal de-stressing). Clear stepwise changes in the spectrum are seen in films that were thermally de-stressed below the glass transition temperature (approximately 110 °C). After thermal de-stressing at higher temperatures the stepwise expansion effect is reduced and the films simply expand smoothly. f, The film thickness expansion over time is derived from real-time spectra where 1.0 corresponds to the initial film thickness. After microstructure expansion finishes the films relax to equilibrium thickness. Without any stress or external force, crazes in glassy polymers undergo collapse under certain conditions⁴⁰, as seen by the decay of the expansion ratio.