Dynamics and phase separation during spin casting of polymer films

Spin coating is process often used to make thin films. A substrate covered with a polymer solution is rotated rapidly. The solution spreads due to the centrifugal force and a film is obtained once the solvent has evaporated. In organic devices, films are often deposited via spin coating. The efficiency of such devices depends on the morphology of the film, which can be controlled by changing the concentration of the solution, the polymer ratio, the solvent, the vapour pressure and the temperature. A full understanding of the dynamics of spin coated films is necessary in order to control the structure of the film. The thinning rate of spin cast films of polysterene (PS) and poly(methyl methacrylate) (PMMA) was investigated using time-resolved optical reflectivity. Spin coating is usually modelled by accounting for the centrifugal forces and a constant evaporation rate. We show that an accurate modelling of the early stage of the process requires a consideration of the speed difference between the fluid and the substrate (inertial forces). We propose a model for spin coating of polymer solutions which accounts for these inertial forces, the centrifugal forces, a constant evaporation rate and a concentration depend viscosity (Huggins viscosity). This model is in good agreement with the experimental data and enables a modelling of the polymer concentration during the coating. In-situ light scattering was used to monitor development of a structure during the coating of solution of PS and PMMA in toluene solution with solvent volume fraction of 90, 86, and 88% at 21◦C. The phase separation was less pronounced as the toluene volume fraction increases. A mean field theory (Flory-Huggins) was applied, and it revealed that despite the different structure the thermodynamics of the process is unchanged and the drying rate increases with the solvent concentration. We studied how the interactions between the PS and PMMA chains affect the structure of the film by controlling the temperature prior and during coating. The experiment was performed at four temperatures: 21, 15, 7 and 0◦C. The polymer solution studied had an equal amount of PS and PMMA with toluene volume fraction of 90, 86, and 88%. UV-visible spectroscopy showed that at 0◦C these solutions enter the two phase region. There seem to be little correlation between the morphology of the film and the miscibility of the solution. The results are discussed in term of the evaporation rate, the thickness of the film and instabilities in the film due to different surface gradient (the Marangoni effect)