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

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journal contribution
posted on 19.05.2015, 00:00 by Bernard A. Lotz
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).