Autocatalytic hydrolysis of amorphous-made polylactides: effects of l-lactide content, tacticity, and enantiomeric polymer blending

Poly(dl-lactide), i.e., poly(dl-lactic acid) (PDLLA), poly(l-lactide), i.e. poly(l-lactic acid) (PLLA), and poly(d-lactide), i.e., poly(d-lactic acid) (PDLA) were synthesized to have similar molecular weights. The non-blended PDLLA, PLLA, and PDLA films and PLLA/PDLA(1/1) blend film were prepared to be amorphous state, and the effects of l-lactide unit content, tacticity, and enantiomeric polymer blending on their autocatalytic hydrolysis were investigated in phosphate-buffered solution (pH7.4) at 37 °C for up to 24 months. The results of gravimetry, gel permeation chromatography (GPC), and tensile testing showed that the autocatalytic hydrolyzabilities of polylactides, i.e. poly(lactic acid)s (PLAs) in the amorphous state decreased in the following order: nonblended PDLLA>nonblended PLLA, nonblended PDLA>PLLA/PDLA(1/1) blend. The high hydrolyzability of the nonblended PDLLA film compared with those of the nonblended PLLA and PDLA films was ascribed to the lower tacticity of PDLLA chains, which decreases their intramolecular interaction and therefore the PDLLA chains are susceptible to the attack from water molecules. In contrast, the retarded hydrolysis of PLLA/PDLA(1/1) blend film compared with those of the nonblended PLLA and PDLA films was attributable to the peculiar strong interaction between PLLA and PDLA chains in the blend film, resulting in the disturbed interaction of PLLA or PDLA chains and water molecules. The X-ray diffractometry and differential scanning calorimetry (DSC) elucidated that all the initially amorphous PLA films remained amorphous even after the autocatalytic hydrolysis for 16 (PDLLA film) and 24 [nonblended PLLA and PDLA films, PLLA/PDLA(1/1) blend film] months and that the melting peaks observed at around 170 and 220 °C for the PLLA/PDLA(1/1) blend film after the hydrolysis for 24 months were ascribed to those of homo- and stereocomplex crystallites, respectively, formed during heating at around 100 and 200 °C but not during the autocatalytic hydrolysis.