Blends of aliphatic polyesters. II. Hydrolysis of solution-cast blends from poly(L-lactide) and poly(E-caprolactone) in phosphate-buffered solution

Blend films were prepared from poly(L -lactide) (PLLA) and poly(E-caprolactone) (PCL) with different PLLA contents [X_PLLA (w/w) = PLLA/(PCL + PLLA)] by the solution-casting method and their hydrolysis behaviors were investigated up to 20 months in a phosphate-buffered solution of pH 7.4 at 37°C by gel permeation chromatography, tensile testing, differential scanning calorimetry, and gravimetry. Polarizing microscopic observation and dynamic mechanical analysis revealed that PCL and PLLA were phase-separated in blend films before hydrolysis. The mass remaining, molecular weight, and tensile strength of the blend films with X_PLLA of 0.5 and 0.75 decreased more rapidly by hydrolysis than those of the nonblended PLLA, while the elongation at break of the blend film of X_PLLA = 0.25 decreased the slowest. The rate constant for hydrolysis (k) calculated from the M_n change during hydrolysis was higher for blend films of X_PLLA = 0.5 and 0.75 than those expected from k of nonblended PLLA and PCL. The melting temperature (T_m) of PLLA in the blend and nonblended films of X_PLLA = 0.5, 0.75, and 1 decreased from 179 to 161, 160, and 175°C upon hydrolysis for 20 months, respectively, while that for X_PLLA = 0.25 slightly increased from 176 to 177°C. On the other hand, T_m and the crystallinity of PCL was significantly increased by hydrolysis for 20 months, irrespective of X_PLLA. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 405–415, 1998