Near-surface structure formation in chemically imidized polyimide films

Free-standing polyimide films are manufactured by the chemical imidization of linear, soluble polymeric precursors. The reactive solution is coated onto a heated substrate, peeled off after partial imidization, and then dried and cured as a free-standing film. Adhesive bonds to the cast side of the final film more strongly than to the air side. Near-surface elastic moduli of film samples were measured with a nanoindentation setup. Samples were annealed at different final temperatures. The air side of the samples annealed at 400°C had a higher modulus of 1.4 GPa than the 0.8 GPa of the casting side. This difference diminished as the annealing temperature was raised to 460°C. Polyamic acid and polyimide exhibit phase transitions from disordered, isotropic solutions to ordered, liquid-crystalline states. A theoretical model of drying and curing demonstrates formation of a gradient in conversion and ordering: the air side vitrifies at a lower solvent content, lower conversion, and higher ordering; the casting side, at a greater solvent content, higher conversion, and less ordering. Subsequent high-temperature drying and curing of the free-standing films removes solvent, completes reaction, and nematically orders both sides. However, longer times and higher temperature annealing are needed to bring the two sides to their common equilibrium state of nematic order.