Molecular Dynamic Simulations of Amorphous Systems under Stress

聚合物晶体中的非晶区：我们开发的用于研究橡胶弹性的分子动力学程序被修改为计算凝聚纠缠系统的化学势μ和刚性G。结果发现，当纠缠在无定形区域中凝聚时，它变得坚硬，其G值大约是普通橡胶的几倍。还发现自由链中元素的化学势μ_f高于固定在片层上的链的化学势μ_t。μ_f与μ_t之差随无定形区域内纠缠浓度x的增大而增大。这表明在晶化过程中，游离链和悬挂链被迅速地从无定形结构域中排出。非晶态二氧化硅体系：利用Feuston和Garofalini提出的三体相互作用势对硅酸盐草进行了分子动力学模拟。系统的温度从9000K逐步降低到室温，间隔200K。在每个温度下，系统都要进行0.1nsec的退火热处理。结果表明，随着温度的升高，体系的体积V增加到一定的温度，高于某一温度时，体积V开始减小，最后又增大。V的这种复杂行为与实验相符。模拟计算的等温压缩系数与Saito和Ikushima的实验结果定性地吻合。

Amorphous Domains in Polymer Crystals : The molecular dynamic program, which we have developed for the study of rubber elasticity, was modified to calculating chemical potentials μ and rigidity G of condensed entanglement systems. It is found that, as entanglements are condensed in the amorphous domain, it becomes harden and its G , ca.several times as large as ordinary rubbers. It is also found that chemical potential μ_f of elements in free chains is higher than μ_t of chains fixed to the lamella. The difference between μ_f and μ_t, increases with increase of concentration x of entanglements in the amorphous domain. This shows that free and dangling chains are expelled rapidly from the amorphous domain in the process of crystallization. It is suggested that this may make the period of stacking lamella structure unstable and lead formation of spherite.Amorphous Silica Systems : Molecular dynamic simulations are done for silicate grasses using the three-body interaction potential proposed by Feuston and Garofalini. The temperature of systems are lowered from 9000K to the room temperature, step-by-step, with an interval Of 200K.At each temperature, the system is annealed for 0.1 nsec. It is found that, as the temperate increases, volume V of the system increases up to a certain temperature, above it, V begins to decreases and finally it increases again. This complicated behavior of V agrees with experiments. It is also found that the isothermal compressibility computed in the present simulation agree qualitatively with that determined experimentally by Saito and Ikushima.