Crystal structure and biodegradation of aliphatic polyester crystals

Abstract

The biodegradability of aliphatic polyesters, which are produced by biosynthesis and chemo-synthesis, strongly correlates to the polymer morphology such as crystallinity, molecular orientation, chain packing and crystal surface, in addition to the chemical structure. For elucidation of biodegradation mechanism of crystal regions on an atomic level, lamellar single crystals of poly([R]-3-hydroxybutyrate) (P(3HB)) and its copolymers, poly([R]-3-hydroxyvalerate) (P(3HV)), poly(4-hydroxybutyrate) (P(4HB)), poly(L-lactic acid) (PLLA) and poly(ethylene succinate) (PES) were prepared from dilute solution by isothermal crystallization, and the crystal structures and morphologies were investigated by means of mainly transmission electron microscopy and atomic force microscopy. All single crystals were intramolecular single crystals, with spiral growth in some cases, and these crystals gave well-resolved electron diffractograms. The enzymatic degradation of lamellar crystals of P(3HB) and its copolymers was carried out with extracellular PHB depolymerases, and it was revealed that enzymatic degradation of lamellar crystals progressed from crystal edges and ends rather than the chain-folding surfaces of crystals, in spite of the homogeneous adsorption of enzyme molecules on the crystal surfaces by substrate-binding domain. Furthermore, in the case of PLLA single crystals, single crystals were also degraded from the edges to yield a rounded shape without decreasing the molecular weights and lamellar thickness. These results suggest that the attack by the active site of enzyme takes place at disordered regions of the crystals, that is, disordered lateral sites with high mobility of molecular chains are preferentially degraded.