Fourier Transform Infrared Transmission Spectral Study on the Thermal Stability of Multilayer Langmuir-Blodgett Film of Stearic Acid

Fourier transform infrared (FTIR) transmission spectra at elevated temperatures of the 21-layer LB film of stearic acid have been recorded to study the thermal-induced order-disorder transition of the film. The transition was found to take place around 65°C (T _c), which results in the introduction of gauche conformers into the ordered zigzag alkyl chains, and the transformation of the trans dimers of the carboxylic groups to the cis ones while both configurations coexist below T _c. It was found that the mobility and the orientational disorder of the alkyl chains begin to increase before their conformational disorder occurs., The temperature-dependent spectra of the bulk stearic acid were also measured for comparison. The cyclic thermal treatment of the LB film indicates that the structural changes caused by the order-disorder transition is largely irreversible, although the conformation of the alkyl chains and the dimeric configuration of the carboxylic groups return to the corresponding states at room temperature.