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 alpha' 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-10(3) sequences with randomly distributed tg't-3(1) units. Departure of the O-C(alpha) dihedral angle was also proved. The mechanism of order formation was established. As PLA chains crystallize at low temperatures, only the alpha' phase with a disordered helix can be formed which at elevated temperatures can transform to the more ordered alpha phase. The conformation distortion is then due to the inability for some chains to complete the transition to 10(3) helicity, thus resulting in the formation of the distorted alpha' structure. (C) 2011 Elsevier Ltd. All rights reserved.