Morphology and biodegradability of a binary blend of poly((R)-3-hydroxybutyric acid) and poly((R,S)-lactic acid)

The miscibility, morphology, and biodegradability of a binary blend of bacterial poly((R)-3-hydroxybutyric acid) (P((R)-3HB); Mn = 300 000) with atactic poly((R,S)-lactic acid) (P((R,S)-LA); Mn = 9000) were studied by means of differential scanning calorimetry, optical microscopy, scanning electron microscopy, and hydrolysis with and without enzyme. Differential scanning calorimetry revealed that a P((R)-3HB)–P((R,S)-LA) blend had a single glass-transition temperature for all proportions of the components. The spherulites of P((R)-3HB) were volume filled in the blend films, indicating the inclusion of amorphous P((R,S)-LA) within the spherulites. The spherulitic growth rate decreased with an increase in the content of P((R,S)-LA). These results indicate that the P((R)-3HB)–P((R,S)-LA) blend is miscible in the melt and in the amorphous state. The enzymatic hydrolysis of P((R)-3HB)–P((R,S)-LA) blend films was carried out at 37 °C for 19 h in 0.1 M potassium phosphate buffer (pH 7.4) with an extracellular poly(hydroxybutyrate) depolymerase from Alcaligenes faecalis T1. The rate of enzymatic surface erosion decreased with increasing P((R,S)-LA) content in the blend films. The simple hydrolysis of P((R)-3HB)–P((R,S)-LA) blend films without enzyme was also conducted at 37 °C in a 0.01 M potassium phosphate buffer (pH 7.4) for 150 days. The hydrolytic scission of P((R)-3HB) polymer chains was accelerated by blending with P((R,S)-LA). However, the rate of enzymatic hydrolysis was much faster than the rate of nonenzymatic hydrolysis.Key words: poly((R)-3-hydroxybutyric acid), poly((R,S)-lactic acid), miscibility, morphology, biodegradability.