In situ AFM study of low-temperature polymerization and network formation of thin film polyurea in ionic liquid

When a monomer/oligomer film composed of 4,4′-methylenebis(2-chlorophenyl isocyanate) (MBCI) and 2,7-diaminofluorene (DAF) stoichiometrically equivalent to each other is simply annealed in ionic liquid (IL), its polymerization and network formation take place at below 100 °C. It is in contrast to the case by annealing without IL in the air, in which much higher temperature is needed for the polymerization, but never happens such network formation. In situ topographic and nanomechanical atomic force microscopy (AFM), combined with separate ex situ experiments of Fourier transform infrared spectroscopy (FT-IR), is powerful to visualize this IL-assisted annealing process. The surface nanomechanical hardening behavior was first observed as a sign of the surface polymerization at a temperature as low as 45 °C prior to the formation of porous structures; many pits then appeared at around 50–55 °C, followed by a rapid emergence of porous, network structures above 60 °C.