The drying behaviour of conjugated polymer solutions

The drying process of poly(octyl-thiophene) from chloroform and THF solutions has been studied using grazing incidence diffraction. Concentrated solutions combined with high ventilation rates in the sample cell promote the formation of a solid skin at the solution-air interface. This skin has a swollen unit cell and restricted correlation length compared with the dry films. The early skin formation could be reversed by switching off the ventilation. Conversely, dilute concentrations and low ventilation rates tend to give drying from the edges, i.e. solidification directly onto the substrate. It appears that the anisotropy of the films resulting from 'edge drying' is slightly higher than for the other films. A three-stage model of drying is appropriate for most of the cases studied. After an initial period of constant rate volume loss, the thickness reduction obeys exponential decay in a falling rate period. In the final third stage, the minute residual amounts of solvent leave the sample accompanied with an increasing internal order in the sample. However, both the 'edge drying' for dilute solutions and the skin formation for concentrated solutions modify this three-stage description.