Glassy structure affected cold-crystallization behavior and structure of poly(lactic acid)

Poly(lactic acid) (PLA) is a semicrystalline polymer with the lowest reported critical cooling rate to form glass, and therefore it is possible to track mesophase in its glassy state. The PLA glass was prepared by quenching the melt to various temperatures (5–60 °C) below its glass transition temperature (T_g), and the prepared samples were investigated by wide-/small-angle X-ray scattering (WAXS/SAXS) and differential scanning calorimetry (DSC) measurements before and after cold-crystallization. It was found that a mesophase is formed in glassy PLA, and the degree of order in the mesophase increases as quenching temperature (T_Q) rises up. In the subsequent heating, the non-isothermal cold-crystallization of glassy PLA is improved with the increase of T_Q, but the melting temperature (T_m) of the formed crystal decreases. For isothermal cold-crystallization at different temperatures (T_cc), the structural ordering degree of δ crystal increases firstly and then decreases with T_Q due to its metastability and low ordering degree, while that of the α crystal almost keeps unchanged with T_Q because of its high stability and ordering degree. Correspondingly, the variation with T_Q of the thickness of PLA lamellae composed of δ and/or α crystal exhibits similar behavior to that of microcrystal orderliness at the same T_cc ranges. A possible crystallization route is proposed to better understand the effect of glassy structure on PLA crystalline structures of cold-crystallization.