Infrared spectroscopic determination of lactide concentration in polylactide: An improved methodology

In this study, a new Fourier transform infrared (FTIR) spectroscopy method for measuring lactide concentration in a polylactide (PLA) matrix is presented. The lactide ring breathing mode peak at 935 cm(-1) is rationed against the asymmetric bending mode of methyl groups at 1454 cm(-1); because the methyl group is present both in lactide monomer and the polymer, this ratio provides a direct measure of lactide concentration. Using well-defined mixtures of lactide and PLA, a calibration curve is established. For lactide concentrations corresponding to conversions above 75%, a linear relationship between conversion and the peak area ratio exists. Below 75% conversion, the development of a double peak at 1454 cm(-1) leads to deviations from linearity; data can then be accurately represented by a second-order polynomial relating conversion to the peak area ratio. Kinetic data for the bulk polymerization of lactide at different temperatures are also obtained and independently verified using rigorous NMR techniques. Finally, conversion as a function of time calculated using the FTIR method is found to be in good agreement with the polymerization kinetics model of Witzke, Narayan, and Kolstad.