Crystallinity Determination of Polylactide by FT-Raman Spectrometry

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Abstract
The complete Raman spectra of isotropically crystalline and amorphous polylactide (PLA) have been successfully obtained with the use of a Fourier transform instrument equipped with a near-IR laser source. The Raman spectra of PLA were recorded at a resolution of 4 cm−1 from a backscattering sampling geometry. The changes in band intensity and shape of Raman spectra in the crystalline PLA samples facilitated the quantification of the crystallinity, which was primarily determined by the change in enthalpy via differential scanning calorimetry (DSC). A comparison of quantitative analysis of crystallinity by a multivariate technique, partial least-squares (PLS), and a univariate method at the carbonyl stretching band is described. Quantitative analysis of PLA crystallinity was performed after the data preprocessing with the standard normal variate (SNV) method in the combined spectral regions 3100–2800 cm−1 and 2000–200 cm−1 that were determined by the analysis of the correlation spectrum. The use of the calibration model containing mostly 1% d-PLA samples to separately analyze the samples with 1% and 5% d-PLA is discussed. Superior results obtained from the multivariate over the univariate method make PLS the preferred method of choice in quantification of crystallinity. The standard error of prediction (SEP) for the analysis of 1% d-PLA samples with the use of the calibration model described is 0.85 (—J/g), and the SEP for 5% d-PLA samples is 1.35 (—J/g) with the outliers excluded. The discrepancy in analyzing these two types of PLA samples with the use of the same calibration model is also discussed.