Structural evolution of the amorphous phase during crystallization of poly(l-lactic acid): A synchrotron wide-angle X-ray scattering study

Initially amorphous poly(l-lactic acid), PLLA, was crystallized at 80 °C, during which time-resolved wide-angle X-ray scattering measurements (WAXS) were performed using synchrotron radiation. The amorphous halo of the initial material (AM) and fully transformed semi-crystalline PLLA (SC) could be approximated by single Gaussian functions, and exhibited different features: for SC-PLLA the amorphous halo is found to be broad and shifted to higher 2θ values, i.e. featuring a more compact/oriented arrangement than in AM-PLLA. Such results are consistent with previous dielectric and DSC results on PLLA that showed that the glass transition dynamic of SC-PLLA is characterized by a higher Tg and by a broader distribution of characteristic times. For intermediate values of crystallization times, it was found that the amorphous halos of the WAXS profiles could be described by simple linear combinations of contributions of the AM and SC amorphous halos, in which the first one progressively vanishes, while a concomitant increase of the later is detected. Such results are in agreement with the description of the dielectric behavior found in PLLA during crystallization and provide an example where structural information could be linked with the segmental dynamic characteristics in semi-crystalline systems.