Effects of morphology, conformation and configuration on the IR and Raman spectra of various poly(lactic acid)s

Morphology, conformation and configuration of a series of aliphatic polyesters of the polylactide-type were studied by C-13 n.m.r. and vibrational spectroscopies. Investigations were performed on the stereocomplex (PLAcomplex) formed by cocrystallisation of poly(L-lactic acid) and poly(D-lactic acid), on poly(D,L-lactic acid) and on poly(meso-lactide) stereocopolymers. Comparison was made with poly(L-lactic acid) PLA100, taken as a reference. Fundamental vibrations were correlated to bands characterising morphology, conformation and configuration of these different polymers. Factor group analysis based on the 10(3) and 3(1) helical conformations of semi-crystalline PLA100 and PLAcomplex, respectively, led to good agreement with information collected from X-ray diffraction. It was further shown that difference in tacticity between amorphous 'preferentially isotactic' poly(D,1-lactic acid) and 'preferentially syndiotactic' poly(meso-lactide) can be detected from the vibrational position of delta CH and symmetric delta CH3 bending modes. Solid state C-13 n.m.r. confirmed the different conformations of semi-crystalline PLA100 and PLAcomplex, but did not allow us to discriminate the different tacticities of the amorphous stereocopolymers, contrary to Raman spectroscopy. (C) 1997 Elsevier Science Ltd.