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

Abstract

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 ...