Two-dimensional correlation analysis of variable-temperature Fourier-transform near-infrared spectra of an amorphous polyamide

Near-infrared (NIR) spectra of a totally amorphous polyamide measured over the temperature range from 25 °C-200 °C were analyzed using generalized two-dimensional (2D) correlation spectroscopy. At least five distinct bands at 5690, 5810, 5900, 5980 and 6010 cm⁻¹ were identified in the region of the $v(CH)$ overtones (5400-6100 cm⁻¹). Among them, two bands at 5810 cm⁻¹ (aliphatic CH) and 6010 cm⁻¹ (aromatic CH) were found to be very sensitive to the temperature-induced structural changes of the polyamide under examination. In the $v(NH)$ overtone region (6300–6800 cm⁻¹), the asynchronicity of the bands assigned to the vibrations of the free and hydrogen-bonded NH groups indicates a complicated dissociation mechanism and the existence of different hydrogen-bonded species in the investigated totally amorphous polyamide sample. Owing to the spectral resolution enhancement in the 2D correlation spectra, a splitting of the first $v(NH)$ overtone of free NH groups into two components at 6780 cm⁻¹ (totally free) and 6740 cm⁻¹ (free-end) can be observed.