In vitro hydrolysis of poly( -lactide) crystalline residues as extended-chain crystallites. Part I: long-term hydrolysis in phosphate-buffered solution at 37°C

Poly( -lactide) (PLLA) crystalline residues or extended-chain crystallites containing a trace amount of chains in an amorphous state were prepared by hydrolysis of crystallized PLLA films in a phosphate-buffered solution (PBS) at 97°C for 40 h. Their in vitro hydrolysis behavior and mechanism were investigated in PBS at 37°C for the periods of time up to 512 days using gel permeation chromatography (GPC), gravimetry, differential scanning calorimetry, and wide angle X-ray diffractometry. The GPC results showed that the hydrolysis of the crystalline residues proceeded from their surface composed of very short chains with a free end along the chain direction but the hydrolysis from their lateral surface could not be traced. The average hydrolysis rates of PLLA crystalline residues estimated from the changes of number-average and peak top molecular weights (Mn and Mt, respectively) were 5.31 and 5.01 g mol−1 day−1, respectively. Such low hydrolysis rates mean that the PLLA crystalline residues can remain for a long period such as ca. 2×103 days (ca. 5.5 years) in the human body even after PLLA loses its functions as biomaterials. The increased melting temperature (Tm) and crystallinity (Xc) (in 320 days), and the decreased half-widths of X-ray diffraction peaks (in 320 days) during hydrolysis were attributed to the decreased lattice disorder of the PLLA crystalline residues. On the other hand, the decreased Xc for the hydrolysis period exceeding 320 days was ascribed to the decreased thickness or Mn of the crystalline residues, which increased the effect of the surface having a large surface energy.