Aramoon Amin, Breitzman Timothy D, Woodward Christopher, El-Awady Jaafar A
Department of Mechanical Engineering, Whiting School of Engineering, The Johns Hopkins University , Baltimore, Maryland 21218, United States.
Air Force Research Laboratory, Wright-Patterson Air Force Base , Fairborn, Ohio 45433, United States.
J Phys Chem B. 2017 Sep 7;121(35):8399-8407. doi: 10.1021/acs.jpcb.7b04147. Epub 2017 Aug 23.
A new algorithm is developed to quantify the free-volume hole distribution and its evolution in coarse-grained molecular dynamics simulations of polymeric networks. This is achieved by analyzing the geometry of the network rather than a voxelized image of the structure to accurately and efficiently find and quantify free-volume hole distributions within large scale simulations of polymer networks. The free-volume holes are quantified by fitting the largest ellipsoids and spheres in the free-volumes between polymer chains. The free-volume hole distributions calculated from this algorithm are shown to be in excellent agreement with those measured from positron annihilation lifetime spectroscopy (PALS) experiments at different temperature and pressures. Based on the results predicted using this algorithm, an evolution model is proposed for the thermal behavior of an individual free-volume hole. This model is calibrated such that the average radius of free-volumes holes mimics the one predicted from the simulations. The model is then employed to predict the glass-transition temperature of epoxy polymers with different degrees of cross-linking and lengths of prepolymers. Comparison between the predicted glass-transition temperatures and those measured from simulations or experiments implies that this model is capable of successfully predicting the glass-transition temperature of the material using only a PDF of the initial free-volume holes radii of each microstructure. This provides an effective approach for the optimized design of polymeric systems on the basis of the glass-transition temperature, degree of cross-linking, and average length of prepolymers.
开发了一种新算法，用于在聚合物网络的粗粒度分子动力学模拟中量化自由体积孔分布及其演变。这是通过分析网络的几何结构而非结构的体素化图像来实现的，以便在聚合物网络的大规模模拟中准确有效地找到并量化自由体积孔分布。通过拟合聚合物链之间自由体积内的最大椭球体和球体来量化自由体积孔。结果表明，由该算法计算得到的自由体积孔分布与在不同温度和压力下通过正电子湮没寿命谱（PALS）实验测得的结果高度吻合。基于使用该算法预测的结果，提出了单个自由体积孔热行为的演变模型。对该模型进行校准，使得自由体积孔的平均半径与模拟预测值相符。然后使用该模型预测具有不同交联度和预聚物长度的环氧聚合物的玻璃化转变温度。预测的玻璃化转变温度与模拟或实验测得的温度之间的比较表明，该模型仅使用每个微观结构初始自由体积孔半径的概率密度函数就能成功预测材料的玻璃化转变温度。这为基于玻璃化转变温度、交联度和预聚物平均长度对聚合物体系进行优化设计提供了一种有效方法。
