Understanding the Impact of Chain Mobility on Conformational Evolution and Kinetics of Mesophase Formation in...

Although the mesomorphic phase as an intermediate state has been introduced to understand polymer crystallization, the understanding of the mesomorphic phase is far from complete. Here, the effect of chain mobility on the mesophase structuring in melt-quenched poly(ʟ-lactide) (PLLA) treated in low-pressure CO₂ at 1.6-2.0 MPa and 0 °C was investigated using infrared (IR) spectroscopy, differential scanning calorimetry (DSC), and atomic force microscopy (AFM). The IR and AFM results demonstrated that the final degree of order and the kinetics of structural evolution during the CO₂-induced mesophase formation were critically dependent on the CO₂ pressure. This was attributed to the distinct dynamics of conformational evolution (gg to gt conformer transition) due to the different CO₂ pressures. The thermal behavior from the DSC results showed that CO₂ pressure dominated both the scale and dynamics of the chain motion of PLLA. At a lower CO₂ pressure of 1.6 MPa, smaller-scale segmental motion was not replaced by the larger-scale cooperative motion that occurred at a relatively higher CO₂ pressure of 2 MPa, which was favorable for faster mesophase formation. Consequently, by inhibiting direct crystallization under limited mobility conditions, it was demonstrated that different chain mobility controlled by CO₂ pressure and thus CO₂ solubility impacted the dynamics of the mesophase formation of PLLA. The present results have implications for understanding the role of chain mobility in determining the intermediate structural phases in semicrystalline polymers.