Determination of d-lactate content in poly(lactic acid) using polarimetry

A method has been developed to quantitatively determine the d-lactate content in poly (lactic acid) (PLA) using polarimetry. Various factors influencing test results, including monomer and other additives in samples, and temperature, have been considered in more detail. When the monomer content is 1.0% (w/w), the negative error is not more than 0.41%. For some additives without optical activity that are soluble in the solvent for PLA, when their content is 1.0%, the positive error produced is less than 0.50%. However, other additives that cannot be dissolved in the PLA solvent, such as inorganic components, can cause the indicator of the polarimeter to fluctuate considerably, leading to variation in test results. The absolute error caused by variation in temperature is not more than 0.1% and can be ignored at room temperature of from 15 to 35 °C. A precipitation method can be used to separate and remove interfering components from PLA. When all of the interferents have no optical activity, are soluble in a solvent for PLA and their content is known, the d-lactate content can be directly measured by polarimetry. This method is applicable to the determination of d-lactate content in PLA, including copolymers of l- and d-lactic acid, and blends of poly (l-lactic acid) and poly (d-lactic acid). The test results are consistent with those obtained by HPLC, as indicated by the residual standard deviation of the two methods:δˆ=0.56%. The relative standard deviation (RSD) of the measurements is 3.7%, indicating that this method is suitably precise.