Comparison of thermal characteristics and degradation properties of epsilon-caprolactone copolymers

The objective of this study was to enhance the degradation of the homopolymer of epsilon-caprolactone (epsilon-CL) through its copolymerization with a second lactone monomer: delta-valerolactone (delta-VL), gamma-butyrolactone (gamma-BL), gamma-valerolactone (gamma-VL), and gamma-caprolactone (gamma-CL). The influences of the molar ratio and the structure of the second lactone monomers on the molecular weight, melting temperature, crystallinity, and degradation rate of the copolymers were compared and elucidated. The enhancement of degradation of the epsilon-CL polymers was achieved by the addition of a second lactone monomer during the copolymerization process, which prominently depended on the initial molar ratio and the chemical structure of the monomers. The changes in the molecular weight and the melting temperature and the enthalpy of fusion of the copolymers were directly related to the initial molar ratio of two monomers, which further affected the magnitude of the enhancement of the polymer degradation. The degradation rates of copolymers of epsilon-CL/delta-VL with initial molar ratios of 75/25, 80/20, and 85/15% were 7.72, 3.60, and 2.05%/day, respectively. However, the degradation rate of the homopolymer of epsilon-CL was 1. 85%/day. The degradation rates of copolymers of epsilon-CL and three five-member ring lactone monomers were in the order of gamma-BL > gamma-VL > gamma-CL. The number of carbon atoms in the side chain of the lactone monomers determined the hydrophobicity of the polymers, which was not favorable for water penetration causing degradation.