Biodegradable lactone copolymers. III. Mechanical properties of ε-caprolactone and lactide copolymers after hydrolysis in vitro

Copolymers of ε-CL/L-LA and ε-CL/DL-LA and for comparison homopolymers PLLA, PDLLA, and PCL were allowed to age in a buffer solution of pH 7 at 23 and 37°C and studied for the changes in the mechanical properties taking place as a function of hydrolysis time. Tensile modulus measurements showed the copolymers to retain their modulus much longer than did the PLA homopolymers. The copolymers became stiffer with hydrolysis, while the elongation at break decreased gradually. For the amorphous P(CL60/L-LA40) copolymer, the tensile modulus and yield stress values increased dramatically in hydrolysis. The initial copolymer was soft and tough but became more brittle during hydrolysis, and it exhibited a plasticlike rather than a rubberlike deformation, though the stress values were still very low. After a short period of decrease at the beginning of hydrolysis, the tensile modulus of P(CL80/L-LA20) and P(CL40/L-LA60) copolymers to some extent increased. Yield stress values for these copolymers decreased during hydrolysis. The tensile modulus of PLLA and PDLLA began to decrease during the first days, i.e., the material became weaker. In the case of PCL, the tensile modulus remained almost the same during the 70 days of the test. The degradation was also studied by ¹³C-NMR. Caproyl homopolymeric sequences did not degrade significantly during hydrolysis.