Evidence of sequential ordering during cold crystallization of poly (l-lactide)

The structural development during cold crystallization of poly (l-lactide) has been explored by time-dependent Fourier transform infrared spectroscopy and depolarized light scattering, respectively. It is indicated that the conformation-sensitive 956 cm−1 band changes first during induction period, followed by formation of 103 helix sequence (921 cm−1 band) in the disordered crystals; after that, the inner structure of new-formed disordered crystals is further perfected, giving rise to frequency shift of 871 cm−1 band to higher wavenumber. Moreover, the formation and subsequent perfection of disordered crystals are also evidenced by the sharp transition of integrated scattering intensity revealed by depolarized light scattering measurements. It is strongly suggested that the cold crystallization of poly (l-lactide) follows a sequential ordering or multi-step process at atomic scale. Furthermore, such a sequential ordering is independent of crystallization temperature and the thermal history (melt cooling rate) of samples prior to cold crystallization. Increasing crystallization temperature or decreasing melt cooling rate just shortens the onset time related to above-referred each step.