Thermal restraint on PEG-EG mixtures by FTIR investigations and wavelet cross-correlation analysis

The aim of the present work is to investigate the thermal response of PolyEthylene Glycol 1000 (PEG1000) and of its mixtures with the monomer Ethylene Glycol (EG). On purpose Attenuated Total Reflectance Infra-Red (ATR-IR) spectra were collected, in the spectral range spanning from 400 cm−1 to 4000 cm−1, on PEG1000 and on its mixtures with EG, as a function of concentration and temperature, through positive thermal scans, i.e. by increasing temperature. It will be shown that ATR-IR technique reveals a powerful tool for the characterization of the thermal response in polymeric systems. The registered spectra have been analyzed both on the whole investigated spectral range, as well as, separately, on the restricted intramolecular OH stretching vibrational contribution region. In the first case the displacement of the spectral features from the spectrum at the lowest temperature, taken as reference spectrum, shows a lower dependence for the mixture. As far as the intramolecular OH vibrational contribution is concerned, besides a conventional analysis in terms of band components, three different data analysis procedures have been applied, i.e. the characterization of the temperature dependence of the intramolecular OH stretching center frequency, of the spectral distance and of the wavelet cross correlation coefficient. The three applied data analysis approaches indicates that the addition of a small amount of pure EG to PEG1000 significantly influences the OH vibrational properties of the PEG1000 polymeric matrix. The three different methods furnish a unique coherent interpretative picture which supports the validity of the applied approaches. Furthermore, the analyses show the presence of a higher thermal restraint for the PEG + EG mixture which confirms that, within the three-dimensional networks of hydrogen bonded EG-PEG1000 mixtures, a key role is played by EG in determining an increase of the hydrogen bond network density.