Highly Enhanced Mesophase Formation in Glassy Poly(l‑lactide) at Low Temperatures by Low-Pressure CO₂ That Provides Moderately Increased Molecular Mobility

The mesophase structuring in melt-quenched poly­(l-lactide) (PLLA) treated in low-pressure CO₂ at 2 MPa and 0–35 °C was investigated by using infrared spectroscopy, differential scanning calorimetry (DSC), temperature-modulated DSC, and atomic force microscopy (AFM). It was found that the mesophase formation in glassy PLLA was significantly enhanced, in particular at lower temperature (0 °C), which promoted a distinctly faster formation rate. AFM results revealed that the CO₂-enhanced mesophase exhibited nodular morphology with dramatically increased nucleation density. A framework of multistage model in combination with the moderately improved molecular mobility exerted by CO₂ was proposed to explain the main findings. Because of the moderate molecular mobility, a tremendous number of metastable mesomorphic layers were formed and stabilized by the accompanying development of the rigid amorphous fraction (RAF), leading to the immobilization of the remaining mobile amorphous fraction (MAF). The mesomorphic phases were converted to more stable crystals via cooperative structural reorganization upon the devitrification of the RAF, requiring high chain mobility, showing time and temperature dependence. Consequently, the amorphous PLLA transiently transformed to the mesophase before transforming into the crystal during treating at a relatively high temperature (35 °C). Alternatively, upon heating, the mesophase underwent disordering–reorganization to form active crystallite, profoundly promoting the cold crystallization of the surrounding restored MAF, resulting in obviously depressed cold-crystallization temperatures. The present results have important implications in understanding and regulation of the crystallization of polymers