PHYSICAL-PROPERTIES AND ENZYMATIC DEGRADABILITY OF POLYMER BLENDS OF BACTERIAL POLY[(R)-3-HYDROXYBUTYRATE] AND POLY[(R,S)-3-HYDROXYBUTYRATE] STEREOISOMERS

Three stereocopolymers of poly[(R,S)-3-hydroxybutyrate] (P[(R,S)-3HB]) containing both R and S units; atactic P[(R,S)-3HB] with 70% (R)-3-hydroxybutyrate ((R)-3HB) (ata-P[70%(R)-3HB]), syndiotactic P[(R,S)-3HB] with 50% (R)-3HB (syn-P[50%(R)-3HB]), and atactic P[(R,S)-3HB] with 50% (R)-3HB (ata-P[50%(R)-3HB]), were prepared by ring-opening polymerization of (R)- and (S)-beta-butyrolactone in the presence of 1-ethoxy-3-chlorotetrabutyldistannoxane catalyst or diethylzinc/water (1.0/0.6) catalyst. The physical properties of binary blends of bacterial P[(R)-3HB] with chemosynthetic P[(R,S)-3HB] were investigated by means of DSC, optical microscopy, X-ray diffraction, and tensile test. The glass-transition temperatures of each polymer blend were 5 +/- 2 degrees C, independently of P[(R,S)-3HB] content. The equilibrium melting temperature of binary blends decreased from 191 to 174 degrees C as P[(R,S)-3HB] content was increased from 0 to 75 wt %, suggesting that the P[(R)-3HB] and P[(R,S)-3HB] polymers are miscible in the melt and in the amorphous state. The degrees of crystallinity of P[(R)-3HB]/P[(R,S)3HB] blend films decreased with an increase in the P[(R,S)-3HB] content. The enzymatic degradation of blend films were carried out in a 0.1 M potassium phosphate buffer (pH 7.4) at 37 degrees C in the presence of PHB depolymerase from Pseudomonas pickettii. The enzymatic hydrolysis of polymer chains took place on the surface of P[(R)-3HB] film, while little hydrolysis occurred on the surface of atactic and syndiotactic P[(R,S)-3HB] stereoisomers. However, when P[(R,S)-3HB] stereoisomers were blended with P[(R)-3HB], the enzymatic erosion of films were accelerated, and the highest rates of enzymatic hydrolysis were observed at around 50 wt % of P[(R,S)-3HB] content. Water-soluble products liberated during the enzymatic degradation of P[(R)-3HB]/P[(R,S)-3HB] blend films were characterized by HPLC analysis. Bacterial P[(R)-3HB] film produced a mixture of monomer and dimer of S-hydroxybutyric acid, while P[(R)-3HB]/[(R,S)-3HB] blend films gave a mixture of monomer, dimer, trimer, and tetramer, which suggests that atactic and syndiotactic P[(R,S)-3HB] components are hydrolyzed by PHB depolymerase in the presence of P[(R)-3HB] component. A model of enzymatic hydrolysis of P(3HB) chains by PHB depolymerase is proposed on the basis of above results.