A molecular dynamics study of Young's modulus change of semi-crystalline polymers during degradation by chain scissions

This paper presents a molecular dynamics study on the change in Young's modulus of semi-crystalline polymers during degradation by chain scissions, which is relevant to the study of mechanical properties of biodegrading polymers. Using a simple polymer model whose structural and mechanical properties are similar to that of a commonly used biodegrading polymer poly(glycolic acid), we combine molecular dynamics and Monte Carlo to model a system of two polymer crystals separated by an amorphous region between them. The polymer chains in the amorphous region are cut randomly to mimic hydrolysis chain scissions. In a series of virtual tensile tests, the systems with various numbers of chain scissions are subjected to a unidirectional deformation. We find that at temperatures below the glass transition temperature of the model polymer, the Young's modulus of the system reduces quickly with the number of chain scissions, while at temperatures above the glass transition temperature, the Young's modulus reduction lags behind the polymer chain scissions. This observation supports the entropy-spring model of amorphous polymers proposed by Wang et al., which suggests that Young's modulus above the glass transition temperature is dominated by the internal energy of the system, while below the glass transition temperature it is dominated by the entropy of the amorphous phase. The numerical study therefore provides a molecular understanding of the widely observed behaviours of semi-crystalline biodegradable polymers.