Crystal polymorphism of polylactides and poly(Pro-alt-CO): The metastable beta and gamma phases. Formation of homochiral PLLA phases in the PLLA/PDLA blends

Abstract

Among the various crystalline phases of polylactides, an elusive orthorhombic gamma phase (¿PLLA) had been obtained so far only by epitaxial crystallization on hexamethylbenzene. A recent work by J. Shao et al. (Macromolecules, 2013, 46, 6933) reports on two “novel modified crystallites” of PLLA obtained “under the confinement of stereocomplex PLLA/PDLA crystallites” produced on cooling blends of PLLA and PDLA from the melt at specific rates. These novel modified crystallites are metastable and transform to the stable aPLLA on heating. Reinterpretation of Shao et al.'s DSC and X-ray data suggests that one novel modified crystallite is the elusive ¿PLLA. The second form could be a disordered hexagonal phase that blends PDLA and PLLA three-fold helices, although the structural evidence is less compelling. In a parallel line of analysis, a crystal modification of the related alternating copolymer of propene and carbon monoxide (poly(Pro-alt-CO)) described as an orthorhombic structure very similar to ¿PLLA by Anokhin et al. (Polymer Science, Ser. A, Vol. 46, No. 1, 2004, pp. 52–60) must be reinterpreted as a trigonal frustrated phase, a phase that has already been observed in single crystals of this polymer.