Stereocomplex formation through reorganization of poly(L-lactic acid) and poly(D-lactic acid) crystals

The annealing behavior of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) single crystals was investigated in order to reveal the mechanism of chain diffusion in their solid states, below their melting points. The structural changes of the sedimented single-crystal mats on continuous heating were followed in real time by synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAX-D) measurements. The single crystal of homopolymer showed a discontinuous thickening at an annealing temperature below each melting point. The experimental evidence indicated that the lamellar thickening occurs by partial melting and recrystallization. On the other hand, it was discovered that the binary mixture of PLLA and PDLA single crystals exhibits a different reorganization at the same annealing temperature. During their lamellar thickening, stereocomplex of PLLA and PDLA appeared. This provides that the molecular chains, which are registered in each homocrystal, mutually diffuse and rearrange. The dominant orientation of the reorganized stereocomplex crystals was the same as that of the original crystals. However, the stereocomplex crystals with the orientation perpendicular to the original ones were also found to form. This peculiar phenomenon supports that the lamellar thickening is due to not chain sliding through the crystal lattice but partial melting and recrystallization.