A spectroscopic analysis of conformational distortion in the α′ phase of poly(lactic acid)

Simulations in conjunction with Raman scattering were performed to determine the conformational distortion of the α′ form of poly(lactic acid) (PLA). Based on packing constraints of an overall helical structure, the chain conformation can only be approximated by slight deviations from a tg't chain conformation. Models were generated with different types of random departures from the expected dihedral angles. Raman active vibrations in the 700 cm−1 region that are sensitive to changes in chain conformation were analyzed. The model that showed the best fit to the experimental data contains a majority (80%) of tg't-103 sequences with randomly distributed tg't-31 units. Departure of the O-Cα dihedral angle was also proved. The mechanism of order formation was established. As PLA chains crystallize at low temperatures, only the α′ phase with a disordered helix can be formed which at elevated temperatures can transform to the more ordered α phase. The conformation distortion is then due to the inability for some chains to complete the transition to 103 helicity, thus resulting in the formation of the distorted α′ structure.