Imidisation of Langmuir–Blodgett films using a supercritical medium

Chemical imidisation of Langmuir–Blodgett (LB) films of polyamic acid-n-octadecyl amine salt was performed in the presence of carbon dioxide at supercritical conditions. The supercritical medium constituted a replacement for benzene as the solvent vehicle for transporting the dehydrating agent (acetic anhydride) and catalyst (pyridine) to the polyimide precursor. FTIR spectroscopy showed that imidisation was complete and that the spectral characteristics were similar to that of films imidised by conventional thermal and chemical methods. However, the reaction is completed in a much shorter time than that required for imidisation in benzene. Use of the supercritical medium facilitated flushing of the reaction vessel and ensured complete removal of the unused reagents and reaction products, thereby obviating the need for post-imidisation rinse and drying. The application of this method in preparing composite LB films containing organic guest species is demonstrated by the fabrication of functional polyimide–copper butoxy phthalocyanine and composite LB films. Thermal imidisation of the composites at temperatures above 250 °C and chemical imidisation in benzene resulted in loss of the guest species, possibly due to evaporation and dissolution, respectively. On the other hand, the guest molecules were retained in the LB films that were chemically imidised in supercritical CO2. Atomic force microscopy showed that phthalocyanine tended to form crystalline aggregates in the thermally imidised LB film and not in the CO2-imidised one. XPS investigations did not show the presence of copper in the CO2-imidised film, suggesting that the chromophore may be present in a diluted/de-aggregated form on the surface.