Fourier Transform Raman Spectra of Linear Low-Density Polyethylene and Prediction of Their Density by Multivariate Data Analysis

Fourier transform Raman spectra have been measured for pellets of sixteen kinds of linear low-density polyethylene (LLDPE) with short branches and one kind of PE without any branch. Before we tried chemometrics analysis, the Raman spectra of LLDPE were investigated by comparing them with the spectrum of PE in order to explore the effects of the branches on the Raman spectra. Partial least-squares (PLS) regression was applied to the Raman spectra in the 1600–600 cm −1 region after multiplicative scatter correction (MSC) to propose a calibration model that predicts the density of LLDPE. The correlation coefficient was calculated to be 0.968, and the root mean square error of cross validation (RMSECV) was found to be 0.0018 g/cm 3 . The loadings plot of regression coefficients for the calibration model shows that, not only a sharp upward peak at 1417 cm −1 corresponding to the CH 2 bending mode reflecting the crystallinity, but also a broad downward peak near 1308 cm −1 corresponding to the amorphous board band of LLDPE plays a key role in the prediction of their density. The chemometrics study has deepened the analysis of the Raman spectra of LLDPE. For example, the detailed analysis of the principal component weight loadings plots has elucidated the existence of bands due to the CH 3 groups of branches and those arising from amorphous parts of LLDPE that are almost missing or hidden by other intense bands. In other words, the chemometrics analysis has enhanced spectral resolution.