The effect of heat treatment on the optical properties of thin films of polyazomethine with flexible-side chains

Thermal stability of a soluble derivative of a basic aromatic polyazomethine: poly(1,4-(2,5-bisoctyloxy-phenylene-methylenenitrilo)-1,4-phenylene-nitrilomethylene) BOO-PPI, in thin films, has been studied by “in situ” UV-Vis-NIR transmission measurements during the heat treatment. Optical investigations of the BOO-PPI films have been completed by the wide angle X-ray diffraction (WAXRD) and atomic force microscopy (AFM) studies. Absorption spectra of the BOO-PPI films did not change during the first annealing, like as for unsubstituted PPI films. Study of temperature dependences of the absorption edge parameters (the Urbach energy EU and energy gap EG) allowed us assessment of changes in structural order, conformation and conjugation of the BOO-PPI polymer chain during the successive heating and cooling runs. The value of EG (2.19 eV) was constant, while EU changed linearly in the range 71–140 meV, within the temperature range 25–250 °C. This reversible and recurrent dependence of EU is connected with the structural changes of flexible bis-octyloxy side chains, while the conjugation in the main chain is preserved (constant EG). Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy studies confirmed invariable conjugation: C═N stretching vibration band was unchanged during the heat treatment.