Residual changes and thickness effects in glass-forming polymer thin films after solvent vapor annealing

Residual solvent in polymer coatings caused by solvent vapor annealing (SVA) may be used as an indicator of the SVA effectivity for block-copolymer self-assembly and other applications. To analyze the residual changes, temperature-programmed desorption (TPD) technique has been applied to 6-200 nm thick glassy polymer coatings processed by SVA. This method is based on the film thickness contraction and expansion measurements by ellipsometry in vacuum as the temperature of the sample ramps up. The measurements are corrected for the polymer thermal expansion. The polymers used are polystyrene, poly (methyl methacrylate), and poly (2-vinyl pyridine). The TPD curves exhibit up to 2 distinct desorption peaks. One peak appears in the vicinity of the film's glass transition temperature Tg. This feature shows up only if the coating has reached a solvent-induced glass transition during the preceding vapor annealing. The other desorption event is a broad feature appearing at the temperatures below Tg. This peak is observed even for the coatings without previous solvent vapor exposure, although the SVA enhances the effect. The magnitude of the desorption effect corresponds to 1% of the residual solvent content or less for about 30 nm thick films. The desorption demonstrates a clear thickness effect and becomes more pronounced for thinner films, but not exceeding a few % of the residual solvent in 5-8 nm thick coatings. It is supposed that the desorption event in the Tg vicinity is mainly caused by the loss of the solvent or voids from the polymer due to the increased molecular mobility, while the low temperature feature is mostly a desorption of residual gases from the sample surface. Application of the TPD data and sorption-desorption isotherms to the optimization of the SVA procedure and creating a process window plot is demonstrated.

Automated summary

Residual solvent in polymer coatings caused by solvent vapor annealing (SVA) can be analyzed using temperature-programmed desorption (TPD) technique. The TPD curves show distinct desorption peaks, with one peak related to the coating's glass transition temperature and another peak appearing at lower temperatures. The desorption effect is more pronounced in thinner films.