Retardation of enzymatic degradation of microbial polyesters using surface chemistry: effect of addition of non-degradable polymers

The effects of addition of non-degradable polymers on the rate of enzymatic erosion for the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] have been studied at 37 °C and pH 7.4 in the aqueous solution of an extracellular PHB depolymerase from Alcaligenes facalis. Polystyrene (PS) or poly(methyl methacrylate) (PMMA) was selected as a minor component (5 wt%) in a blend because of their non-enzymatic activity. Enzymatic degradation behaviors of the “as-cast” and “annealed” blend films were investigated using atomic force microscopy and weight loss measurements. Although the spherulites of P(3HB-co-3HV) cover all blend film surfaces throughout, the retardation of biodegradation in the P(3HB-co-3HV)/PS blend films was detected from morphological observation and weight loss measurement for both as-cast and annealed blend films while there was little difference between the P(3HB-co-3HV)/PMMA blend and pure P(3HB-co-3HV). Since the enzymatic degradation of P(3HB-co-3HV) initially occurs by a surface erosion process, these degradation behaviors were explained by the surface structure of blend films measured by X-ray photoelectron spectroscopy. The surface of P(3HB-co-3HV)/PS blend films revealed an excess of PS, whereas the surface of P(3HB-co-3HV)/PMMA blend films was nearly covered by P(3HB-co-3HV). It was concluded, therefore, that the PS, which exists within P(3HB-co-3HV) spherulites at surface acts as a retardant of enzymatic attack to the surface of the blend film.