Alkaline Hydrolysis of Solution-Grown Poly(L-lactic acid) Single Crystals.

The changes in morphology and crystalline state of solution-grown single crystals of poly(L-lactic acid)(PLLA) in aqueous NaOH solution of various strengths were investigated by means of transmission electron microscopy, atomic force microscopy and gel permeation chromatography. The degradations of hexagonal and lozenge shaped PLLA single crystals started from the lateral side of the crystal to generate the notched morphology at initial stage of hydrolysis. As hydrolysis proceeded, the molecular weight of degraded PLLA crystals corresponded to equal the value calculated from lamellar thickness measured by atomic force microscopy, suggesting that the tight chain-folding region of PLLA molecules on crystal surface was degraded by alkaline hydrolysis. Based on the results, alkaline hydrolysis with NaOH solution firstly occurs at the loosely chain-packing region on crystal edges, and then gradually progresses from both crystal edges and tight chain-packing region with chain-folding on the crystal surface.