NMR analysis and tacticity determination of poly(lactic acid) in C5D5N

Poly(lactic acid), (PLA), is a well known bioplastic derived from agri-based renewable materials. Because PLA's material properties depend largely on its tacticity, an improved test method for PLA tacticity would be useful. NMR analysis of PLA tacticity is revisited in this work, especially the assignments of NMR peaks from CH carbon and CH3 proton at the tetrad level in deuterated pyridine. The methyl protons are better resolved in pyridine due to solvent effects such as ring current shielding of the aromatic ring and electric field effect from the nitrogen lone pair. As an aid for the tacticity assignments, two-dimensional NMR spectra of poly(DL-lactic acid) (L/D = 50/50) and relative intensities of PLA samples have been used. The new assignments provide more detailed understanding of the 1H and 13C NMR spectra of PLA.