Solvent vapor annealing of diblock copolymer thin films with a solvent mixture*

Solvent vapor annealing (SVA) is an attractive method to control the morphology of block copolymer thin films. Using mixtures of vapors from solvents having different selectivities for the blocks enhances the possibilities greatly. We used a setup with two bubblers to investigate thin films from a cylinder-forming polystyrene-block-poly(dimethylsiloxane) (PS-b-PDMS) diblock copolymer in mixtures of the vapors of toluene, which is nearly non-selective for the PS matrix and the PDMS cylinders, and n-heptane, which is selective for PDMS [1]. We varied the vapor composition in a stepwise manner and monitored the overall film thickness in-situ using spectral reflectance. The film morphologies were determined using in-situ real-time grazing-incidence small-angle X-ray scattering (GISAXS). In dependence on the mixing ratio of the two solvent vapors, randomly or hexagonally ordered cylinders and lamellae were observed. This novel type of experiments allows probing trajectories in the state diagram of the PS-b-PDMS/n-heptane/toluene mixture, which are drawn in dependence on the polymer volume fraction Φ_P and the effective volume fraction of the swollen PDMS nanodomains, f_eff. These are determined from the overall degree of swelling and from the intensity of a Bragg reflection in the 2D GISAXS maps. Comparing the trajectories from several protocols of swelling and vapor exchange [2], we find that, apart from the solvent vapor composition, the mobility of the PS blocks forming the matrix is a key factor for the time scales of re-structuring and for the sequence of morphologies.

[1] A. V. Berezkin, F. Jung, D. Posselt, D.-M. Smilgies, C. M. Papadakis, Adv. Funct. Mater. 28, 1706226 (2018)
[2] F. A. Jung, A. V. Berezkin, T. B. Tejsner, D. Posselt, D.-M. Smilgies, C. M. Papadakis, Macromol. Rapid Commun. 41, 2000150 (2020)