Morphology and Enzymatic Degradation of Poly(l-lactic acid) Single Crystals

Abstract

Lamellar single crystals of poly(l-lactic acid) (PLLA) were grown from a 0.05% solution of p-xylene at 90 °C. The hexagonal, truncated-lozenge and lozenge-shaped single crystals with spiral growth were prepared simultaneously, and these crystals were relatively stable under conditions of electron bombardment. The hexagonal and lozenge-shaped crystals gave well-resolved electron diffractograms from which the reciprocal lattice parameters a* = 0.935 nm-1, b* = 1.626 nm-1, and γ* = 90° could be determined, and these lattice parameters corresponded with those of the α structure of PLLA fibers. Accordingly, the hexagonal crystal was considered as pseudo-hexagonal symmetry, but actually with orthorhombic packing of the PLLA chains; i.e., the ratio of the a and b cell parameters is quite equal to √3. The enzymatic degradation of PLLA single crystals with a proteinase-K from the mold Tritirachium album in 50 mM Tris−HCl buffer at pH 8.5 and 37 °C was investigated by means of transmission electron microscopy, atomic force microscopy, high-performance liquid chromatography, and gel permeation chromatography. Enzymatic degradation progressed from the edges of lamellar crystals to yield a rounded shape without decreasing the molecular weights and lamellar thicknesses. It has been concluded that the single crystals were enzymatically hydrolyzed at the disordered chain-packing region of crystal edges rather than chain-folding surfaces of single crystals. As the degradation time increased, in a rare case the diamond holes were observed on the crystal surfaces of lozenge-shaped single crystals.