Raman spectra of high-density, low-density, and linear low-density polyethylene pellets and prediction of their physical properties by multivariate data analysis.
Raman spectra have been measured for pellets of five samples of high-d. polyethylene (HDPE), seven samples of low-d. polyethylene (LDPE), and six samples of linear low-d. polyethylene (LLDPE). The obtained Raman spectra have been compared to find out characteristic Raman bands of HDPE, LDPE, and LLDPE. Principal component anal. (PCA) was applied to the Raman spectra in the 1600-650 cm-1 region after multiplicative scatter correction (MSC) to discriminate the Raman spectra of the three different PE species. They are classified into three groups by a score plot of PCA factor 1 vs. 2. HDPE with high d. and high crystallinity gives high scores on the factor 1 axis, while LDPE with low d. and low crystallinity yields neg. scores on the same axis. It seems that factor 1 reflects the d. or crystallinity. A PC wt. loadings plot for factor 1 shows six upward peaks corresponding to the bands arising from the cryst. parts or all-trans -(CH2)n- groups and seven downward peaks ascribed to the bands of the amorphous or anisotropic regions and those arising from the short branches. Partial least-squares (PLS-1) regression was applied to the Raman spectra after MSC to propose calibration models that predict the d., crystallinity, and m.ps. of the polyethylenes. The correlation coeff. was calcd. to be 0.9941, 0.9800, and 0.9709 for the d., crystallinity, and m.p., resp., and their root-mean-square error of cross validation (RMSECV) was found to be 0.0015, 3.3707, and 2.3745, resp. The loadings plot of factor 2 for the prediction of m.p. is largely different from those for the prediction of d. and crystallinity.
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