Vibrational spectroscopy as a tool for the investigation of polymer bases in motion picture films: A comparison between mid-infrared, near-infrared and Raman techniques

In the present work, numerous samples of motion picture films from different brands and spanning a wide chronological range were examined with the aim of studying their polymeric support materials using various vibrational spectroscopic techniques. The bases of the films investigated included cellulose nitrate, cellulose acetate, polyethylene terephthalate (PET), and cellophane, the support material of the unique Ozaphan films. Regarding Fourier-transform infrared (FTIR) spectroscopy, the external reflection (ER) technique was employed, both in the mid-infrared (MIR) range and in the longer-wavelength portion of the near-infrared (NIR) region. For Raman spectroscopy, the sequentially shifted excitation (SSE™) technique was used to minimize issues related to potential fluorescence emission. The information provided by each technique was carefully considered, particularly in terms of penetration depth and specificity towards certain molecular structures. Furthermore, diffuse reflectance spectroscopy in the entire NIR range was combined with partial least squares (PLS) regression of the spectral data to estimate the degree of substitution (DS) of the polymer in cellulose acetate bases. This parameter is influenced both by the historical period in which the films were produced and possibly by degradation phenomena.