Influence of crystal polymorphism on crystallinity calculation of poly(l-lactic acid) by infrared spectroscopy

Vibrational spectroscopic methods have been used to determine the crystallinity of poly(l-lactic acid) (PLLA), which is the most popular bio-plastic today. However, it is found that influence of crystal polymorphism of PLLA on the quantitative method proposed for crystallinity calculation based on IR technique has seldom been considered. Herein, by preparing a set of PLLA specimens with different crystallinity and crystal forms, the absorption coefficient ratios between amorphous and crystal bands for evaluating the absolute crystallinity of PLLA α and α′ crystal form have been derived, respectively. In addition, a comparison of the proposed IR method with other techniques used to analyze crystallinity of PLLA such as X-ray diffraction (XRD) and differential scanning calorimetry (DSC) is presented. The origin of the disagreement on the results of quantitative measurements of crystallinity by different methods is also discussed. This study provides a simple spectral method to determine the crystallinity of PLLA with various crystal modifications.