Two-dimensional near-infrared correlation spectroscopy study of premelting behavior of nylon 12

The premelting behavior of Nylon 12 has been investigated by two-dimensional (2D) Fourier-transform (FT) near-infrared (NIR) correlation spectroscopy. FT-NIR spectra of Nylon 12 in a cast film have been measured over a temperature range of 30-150 degrees C. The 2D NIR correlation analysis reveals that there are at least eight bands in the 6800-6100 cm(-1) region where the first overtone of an NH stretching mode of Nylon 12 is expected to appear. They may be ascribed to free- and hydrogen-bonded NH groups in various environments. The asynchronous 2D NIR correlation spectrum in the above region indicates that the amide group with a free carbonyl oxygen (structure C) appears first and then unassociated free amide (structure A) and amide with free NH (structure B) follow as the temperature is increased. In the 6000-5500 cm(-1) region of the synchronous spectrum, four dominant autopeaks corresponding to the first overtones of the CH2 stretching modes are observed at 5840, 5770, 5680, and 5640 cm(-1), and negative cross peaks are found between the 5770 cm(-1) band and the rest. This observation, together with the careful inspection of the dynamic NIR spectra, suggests that the band at 5770 cm(-1) is due to the contribution of the CH stretching vibration of an ordered or highly associated form of Nylon 12, which decreases with temperature, while other NIR bands correspond to more disordered forms. The corresponding asynchronous spectrum indicates that the intensity variation of the bands at 5840, 5680, and 5640 cm(-1) actually occurs at a lower temperature compared to the onset of the intensity decrease of the 5770 cm(-1) band. A substantial amount of disordered or dissociated components corresponding to the above three bands start appearing before the disappearance of more ordered components represented by the 5700 cm(-1) band. Probably, they appear as the premelting precursors (or even possibly as the indirect cause) to the precipitous decrease of the ordered components associated with the melting of Nylon 12 occurring at a much higher temperature.