Department of Engineering, University of Leicester, Leicester, LE1 7RH, UK.
J Mech Behav Biomed Mater. 2012 Jan;5(1):224-30. doi: 10.1016/j.jmbbm.2011.09.002. Epub 2011 Sep 13.
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.
本文通过分子动力学研究了半晶态聚合物在链断裂降解过程中杨氏模量的变化，这与生物降解聚合物力学性能的研究有关。我们使用一种简单的聚合物模型，其结构和力学性能与常用的生物降解聚合物聚（乙醇酸）相似，结合分子动力学和蒙特卡罗模拟了两个聚合物晶体之间存在非晶区的体系。在非晶区中的聚合物链随机被切断以模拟水解链断裂。在一系列虚拟拉伸测试中，对具有不同链断裂数的体系进行单向变形。我们发现，在低于模型聚合物玻璃化转变温度的温度下，体系的杨氏模量随链断裂数的增加迅速降低，而在玻璃化转变温度以上时，杨氏模量的降低滞后于聚合物链断裂。这一观察结果支持了 Wang 等人提出的无定形聚合物的熵-弹簧模型，该模型表明玻璃化转变温度以上的杨氏模量主要由体系的内能决定，而在玻璃化转变温度以下则主要由无定形相的熵决定。因此，这项数值研究为广泛观察到的半晶态生物降解聚合物的行为提供了分子水平的理解。
