Tagging the oligomer-to-polymer crossover on EG and PEGs by infrared and Raman spectroscopies and by wavelet cross-correlation spectral analysis

Infrared and Raman spectra on pure ethylene glycol (EG) and on low molecular weight polyethylene glycols (PEGs) with increasing chain length have been collected and analysed in a wide spectral range in order to determine the oligomer-to-polymer regime crossover. Previous spectroscopic crossover determinations reported in literature dealt only with Raman investigation of the Disordered Longitudinal Acoustic Mode (D-LAM). The IR and Raman contribution features, when analysed in different spectral ranges, show a dramatic change as a function of the polymer chain length, signaling an oligomer-polymer crossover for a polymerization degree of m = 13. An innovative Wavelet Transform (WT) approach allows to confirm the above reported physical picture. In particular WT, differently from standard Principal Component Analysis (PCA) allows a multiscale analysis which permits to quantify the spectral shape modifications of different spectral regions of the collected IR and Raman spectra. It is shown that the wavelet cross-correlation parameters evaluated between each couple of the registered data, from EG to PEG 1000, change as a function of the polymer chain length, confirming that a plateau value is reached for m = 13.