Single Crystals of the Frustrated β-Phase and Genesis of the Disordered α′-Phase of Poly(L-lactic acid)

The metastable, frustrated beta-phase of poly(L-lactic acid) (PLLA) had been obtained so far only by drawing fibers from amorphous PLLA or the stable alpha-phase. This phase has now been obtained by crystallization in thin films in the form of snow-flake-like crystals produced at high T-c (140 degrees C). They display the characteristic single crystal diffraction pattern of frustrated polymers with a three-chains, trigonal unit-cell. Also, hko electron diffraction patterns indicate that crystallization of PLLA at 90 degrees C takes place in a transient, frustrated beta PLLA phase that converts rapidly to alpha'PLIA. The disorder in alpha'PLLA stems from the incompatible cell symmetries of the trigonal, three-chains beta PLLA and orthorhombic, two-chains alpha PLLA unit-cells. The disorder in alpha'PLLA involves shifts of domains along the chain axis and three azimuthal orientations 120 degrees apart that both reflect the initial trigonal/hexagonal symmetry of the parent beta PLLA. Recognition of this solid-state transformation provides a logical framework that explains many unusual characteristics of the alpha'PLLA phase (structural disorder, lamellar thickness and possibly growth rate vs T-c).