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 k inds 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 spectru m of PE in order to explore the effects of the branches on the Raman spectr a. 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 cor relation coefficient was calculated to be 0.968, and the root mean square e rror of cross validation (RMSECV) was found to be 0.0018 g/cm(3) The loadin gs plot of regression coefficients for the calibration model shows that, no t only a sharp upward peak at 1417 cm(-1) corresponding to the CH2 bending mode reflecting the crystallinity, but also a broad downward peak near 1308 cm(-1) corresponding to the amorphous hoard band of LLDPE plays a keg role in the prediction of their density. The chemometrics study has deepened th e analysis of the Raman spectra of LLDPE. For example, the detailed analysi s of the principal component weight loadings plots has elucidated the exist ence of bands due to the CH3 groups of branches and those arising from amor phous parts of LLDPE that are almost missing or hidden by other intense ban ds. In other words, the chemometrics analysis has enhanced spectral resolut ion.