Single Crystals of the Frustrated β-Phase and Genesis of the Disordered α'-Phase of Poly(l-lactic acid).

Abstract

The metastable, frustrated β-phase of poly(l-lactic acid) (PLLA) had been obtained so far only by drawing fibers from amorphous PLLA or the stable α-phase. This phase has now been obtained by crystallization in thin films in the form of snow-flake-like crystals produced at high Tc (140 °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 °C takes place in a transient, frustrated βPLLA phase that converts rapidly to α'PLLA. The disorder in α'PLLA stems from the incompatible cell symmetries of the trigonal, three-chains βPLLA and orthorhombic, two-chains αPLLA unit-cells. The disorder in α'PLLA involves shifts of domains along the chain axis and three azimuthal orientations 120° apart that both reflect the initial trigonal/hexagonal symmetry of the parent βPLLA. Recognition of this solid-state transformation provides a logical framework that explains many unusual characteristics of the α'PLLA phase (structural disorder, lamellar thickness and possibly growth rate vs Tc).