PLLA Mesophase and Its Phase Transition Behavior in the PLLA−PEG−PLLA Copolymer As Revealed by Infrared Spectroscopy

A detailed analysis of the IR spectrum of melt-quenched PLLA−PEG−PLLA sample and its change with temperature is presented. It was found that, in the initial state of the melt-quenched PLLA−PEG−PLLA sample, the PLLA constituent does not exist as a normal amorphous or crystalline phase but as a kind of mesophase, which is confirmed by the WAXD data. The IR data suggest that both chain conformation and chain packing are disordered to some extent in such a mesophase. Therefore, this mesophase is less thermally stable than the ordered α phase and even the disordered α phase (α′) of PLLA. In the heating process, the mesophase of PLLA in the PLLA−PEG−PLLA copolymer starts the partial melt first upon heating in a stepwise way, and subsequently it is reorganized directly into the stable α phase but not through the α′ phase of PLLA. These phase transition behaviors are examined and discussed at the molecular functional group level.