Organic/Inorganic Langmuir−Blodgett Films Based on Metal Phosphonates. 4. Thermal Stability1

Abstract

The thermal stability of metal phosphonate Langmuir−Blodgett (LB) films formed from the amphiphiles octadecylphosphonic acid (OPA) and an azobenzene-derivatized phosphonic acid, (4-(4‘-tetradecyloxyphenyldiazenyl)phenyl)butylphosphonic acid (A4), with divalent and trivalent metal ions has been examined. These films are characterized by the formation of a metal phosphonate continuous lattice in the polar region of the transferred bilayers. The behavior of the films upon heating and with temperature cycling was studied with attenuated total reflectance (ATR)-FTIR, transmission FTIR, absorbance spectroscopy, and X-ray diffraction. A reversible transition is observed between 40 and 50 °C in each film, and this process is assigned as a premelting transition in analogy to similar transitions observed in previously studied LB films. Irreversible disordering of the organic networks is observed for each film at higher temperatures, occurring near 150 °C in the lanthanum octadecylphosphonate film, near 140 °C in the manganese octadecylphosphonate film, and near 120 °C in lanthanide films of A4. Despite disordering of the organic network, FTIR and X-ray diffraction indicate that the inorganic network remains intact to above 180 °C. The inorganic continuous lattice network is shown to greatly enhance the thermal stability of the metal phosphonate LB films relative to that of the traditional carboxylic acid-based films.