Measurement of polyethylene pellets near the glass transition temperature to enhance Raman spectral selectivity among samples and improve accuracy for density determination
A simple and effective strategy for improving the accuracy of the multivariate determination of polyethylene (PE) density using Raman spectroscopy has been demonstrated. This strategy is based on the possibility that varied polymeric structures of the PE samples, especially at a sub-zero temperature range, would enhance their spectral selectivity, thereby potentially improving the multivariate correlation with their pre-determined physical properties such as density. For the evaluation, Raman spectra were collected at regular intervals during continuous increase of the PE temperature from cryogenic to near room temperature. Then, using partial least squares (PLS) regression, calibration models were developed to correlate the Raman spectral features collected at each time period with the reference PE density values. Interestingly, the accuracy was improved when the temperature of the PE pellets was −35 °C, near the glass transition temperature (Tg). To explain the improved accuracy, a two-dimensional (2D) correlation analysis was employed to detail the spectral variation induced by temperature change. Diverse segmental chain motions (so called micro-Brownian motion) predominantly occurring in the amorphous section of the PEs around Tg greatly enhanced the spectral selectivity among PE samples. In addition, minor β-relaxation occurring around this temperature was an additional source of the enhanced spectral selectivity. In parallel, differential scanning calorimetry (DSC) curves of the samples were also examined to check the existence of the phase transitions.