Mechanical detection of liquid evaporation in thin polymer films using a quartz crystal microbalance

The physics of thin, deposited polymer films are more complex than their bulk or macroscopic counterparts. Often these films are deposited by spin-coating from solutions, where the solvent evaporates and leaves behind the polymer. However, evaporation can be inhibited by the solvent mobility within the drying film, and solvent molecules can be trapped indefinitely. The presence of these residual solvent molecules can affect material properties. We used a quartz crystal microbalance (QCM) to explore the timescales and mass loss during evaporation in thin polystyrene films deposited by spin-coating from toluene solutions. Monitoring the mechanical resonance frequency of the QCM reveals sub-monolayer changes in mass. We spin-coated polystyrene films of thickness < 1 micron on the QCMs and found that 10-25% of the residual toluene solvent remained in the film. Part of the solvent left the samples through evaporation, and the rest through heating the sample above the glass transition temperature and annealing in vacuum. The change in mass vs. time can be fit well with two exponential timescales. For a 1 micron thick film, these timescales are 0.7 hours and 4.8 hours. If we interpret the data with a 1D diffusion model with a homogenous diffusion coefficient, we estimate that D ~ 10^-16 - 10^-17 m²/s