Studies on the enzymatic degradation of solution-grown lamellar crystals of poly[(R)-3-hydroxybutyrate]:: Defects in crystals

Morphological changes during the enzymatic hydrolysis of solution-grown lathlike lamellar crystals (SGCs) of poly[(R)-3-hydroxybutyrate] (P(3HB)) attached on a silicon wafer have been studied in order to elucidate the initial stage of enzymatic attack by an extracellular PHB depolymerase purified from Alcaligenes faecalis T1. These changes may be interpreted in terms of the preferential erosion of less-ordered regions within the lamellae, parallel to the b-axis, resulting in breakup of lamellar crystals. The erosion continued along the a-axis from the edges of newly created crystal fragments. This initial hydrolytic behavior was confirmed by quenching samples from a series of temperatures (T-Q < melting temperature). At lower To, ridges first appeared on the lamellar surface running parallel to the b-axis (width of crystal) and developed with increasing temperature. The ridge formation reflects the chain compression induced by the thermal molecular motion of less-ordered chains in lamellar crystals. The deformation of SGCs on poly(ethylene terephthalate) film by elongation has also been used to investigate the defect region in lamellar crystals. The multicracks of lamellae parallel to the b-axis were clearly observed after a a-axis elongation, while a a-axis compression accompanied by the elongation along the b-axis led to the ridge formation parallel to the b-axis on the lamellar surface, owing to pulling out of chains. All results suggest that the less-ordered chains within the lamellar crystals of P(3HB) exist along the b-axis mainly due to the lower regularity between chain-end groups.