Alkaline Hydrolysis of Solution-Grown Poly[(R)-3-hydroxybutyrate] Single Crystals

Abstract

The changes in morphology and crystalline state of solution-grown single crystals of bacterial poly[(R)-3-hydroxybutyrate] (P(3HB)) in NaOH solution of various strengths were investigated by means of transmission electron microscopy, atomic force microscopy, and gel permeation chromatography. Lath-shaped P(3HB) single crystals, which had widths of 2−3 μm, were split to narrow ribbonlike crystals of around 0.1 μm at the initial stage of hydrolysis without a decrease in lamellar thickness. As hydrolysis time increased, ribbonlike crystals were degraded from the lateral side of the crystal to generate the notched morphology. Furthermore, the molecular weight of degraded P(3HB) crystals corresponded to equal the value calculated from lamellar thickness measured by atomic force microscopy, suggesting that the tight chain-folding region of P(3HB) molecules on the crystal surface was degraded by alkaline hydrolysis. This surface degradation was further supported by the disappearance in the electron micrograph of straight lines on the surface of the degraded crystal decorated with polyethylene. Despite the unselective degradation of alkaline hydrolysis from both crystal edges and crystal surfaces, narrow ribbonlike crystals were generated at the initial stage of hydrolysis. This observation suggests that single crystals with chain-folding have two regions of tight chain-packing region with adjacent reentry folds and a loosely chain-packing region with random reentry folds with interval of 100 nm or less. The model developed in this paper suggests that alkaline hydrolysis with NaOH solution first occurs at the loosely chain-packing region on crystal surface and then gradually progresses from both crystal edges and tight chain-packing region with chain-folding on the crystal surface.