Effects of chain microstructures on mechanical behavior and aging of a poly(L-lactide-co-ε-caprolactone) biomedical thermoplastic-elastomer

Abstract

Three statistical poly(L-lactide-co-ε-caprolactone) (PLCL) copolymers of 70% L-lactide content having different chain microstructures ranging from moderate blocky to random (R=0.47,0.69 and 0.92, respectively) were characterized by DSC, GPC and (1)H and (13)C NMR. The results demonstrate that higher randomness character (R→1) limits the capability of crystallization of LA-unit sequences shifting the melting temperature of the copolymers to lower values and reducing the crystallinity fraction substantially. The effect of different distributions of sequences of PLCL on crystallization and phase behavior was also studied for different storage times at room temperature (21±2°C) by DSC. The mechanical properties were evaluated by tensile tests during aging. The PLCL showing a random character closest to the Bernoullian distribution of sequences (l(LA)=1/CL) was found to exhibit higher strain capability and strain recovery values and is less prone to supramolecular arrangements. However, as a result of aging, L-lactide sequence blocks in the other PLCLs of smaller randomness character tend to crystallize prompting to a double T(g) behavior indicative of the existence of phase separation into two compositionally different amorphous phases. Physical aging leads also to dramatic changes in tensile behavior of the moderate blocky PLCLs that evolved from being an elastomeric to be partly a glassy semicrystalline thermoplastic, and, thus, can eventually condition its potential uses for medical devices.