Degradation of composite materials composed of tricalcium phosphate and a new type of block polyester containing a poly(L-lactic acid) segment

Degradation of a new type of poly(L-lactic acid)/poly(ethylene; hexamethylene/ sebacate) block polyester and its composite containing 10 and 30 wt% tricalcium phosphate (TCP) were studied in vitro. Film specimens of thickness 100 and 250 microm for each of the three materials were immersed in phosphate buffered saline (pH 7.4) at 37 degrees C for up to 24 weeks. At appropriate intervals, water absorption, dry and wet tensile strength, molecular weight, and thermal properties of the specimens were measured by weighing, tensile strength testing, size exclusion chromatography, and differential scanning calorimetry, respectively. The decrease in tensile strength was greater in the unblended and thicker polymer film than in the other five films. The retention of tensile strength after 24 weeks increased with increasing TCP content. This trend was also noticed in the retention of molecular weight. The tensile strength of the materials having molecular weights below 5 x 10(4)-6 x 10(4) Mw or 2 x 10(4)-3 x 10(4) Mn dropped substantially and the materials became fragile. Blending of TCP to the PLLA block polyester retarded degradation, suggesting that TCP neutralized the carboxyl end groups formed by hydrolysis of ester bonds.