Temporal Evolution of Optical Gradients during Drying in Cast Polymer Solutions

The development of optical anisotropy gradient as a result of solvent evaporation for poly(amide–imide) (PAI) solution in dimethylacetamide (DMAc) was investigated for the first time. Experiments were carried out using real-time optical measurement with spectral birefringence technique coupled with off-line optical techniques such as the Abbe refractometer and optical compensator method. We have shown that drying process induces temporal evolution of nonuniform out-of-plane birefringence profile through the thickness direction while in-plane birefringence remained zero. The highest birefringence was observed at the substrate–solution interface at early stages of drying. Beyond a critical time, the formation of highly oriented layer was observed at the air–solution interface. This oriented layer progresses through the thickness direction as the solvent concentration is disproportionately reduced in these regions. Abbe refractometer results confirmed the anisotropy is preserved at longer drying times, the air–solution interface birefringence becoming higher compared to the substrate–solution interface. Overall, observations obtained by real-time measurement system agreed with off-line measurements.