拉伸分子动力学方法研究聚乙烯单链的力学行为

采用拉伸分子动力学方法(steered molecular dynamics,SMD)研究一端固定的聚乙烯单链(singlepolyethylene chain)在被拉伸过程中的力学性质.在拉伸过程中发现平均拉力〈f〉受拉伸速度v的影响,当v<0.05 nm/ps时,〈f〉在250 pN附近会出现一个拉力平台.聚乙烯单链各部分的形状因子在拉伸过程中表现出一定的规律性,总是头部和尾部的形状因子〈δh〉、〈tδ〉先增加然后才是中间部分〈δm〉增加.如果按顺序再释放被拉开的聚乙烯单链,就会出现力学回滞现象,这与Kellermayer等的力学回滞曲线实验是一致的.力学回滞曲线面积表示耗散能〈Ed〉,与速度v满足方程〈Ed〉=a+b×e-cv,而且在v<0.005 nm/ps和v>0.005 nm/ps两个速度区域有不同的特性.〈Ed〉在不同的分子热运动温度区域,也表现出不同的规律性,当温度T>220 K时,〈Ed〉随着温度T的升高而减小,这与Pegoretti等的实验一致,当T<220 K时〈Ed〉随着温度T的升高而增加.

A Steered Molecular Dynamics Study on Elastic Behavior of Polyethylene Chains

This study presents the elasticity of single polyethylene(PE) chains with one end fixed based on the steered molecular dynamics(SMD) method.During the tensile process,we find that the average pulling force 〈f〉 displays a dependence of pulling velocity v.When the pulling velocity v<0.05 nm/ps,the average pulling force 〈f〉 increases first and then keeps a plateau of a constant force around 250 pN.The shape factors of the polyethylene(PE) chain are investigated during the pulling process.Here,〈δ_h〉、〈δ_m〉 and 〈δ_t〉 are the shape factors of the head segment,middle segment and the tail segment of the single polyethylene(PE) chain.We find that 〈δ_h〉 and 〈δ_t〉 increase obviously in the low extension area,but 〈δ_m〉 keeps unchanged.However,〈δ_m〉 increases rapidly in the high extension area.We conclude that the shape of the head segment and tail segment of the polyethylene(PE) chain change first,and then the shapes of the middle segment changes during the pulling process.If we sequentially release the stretching chain,the polyethylene(PE) chain exhibits force hysteresis.This means that the stretching and releasing follow different paths.The results agree well with the experimental results of the force hysteresis by Kellermayer et al.The force extension curves display hysteresis because the pulling velocity at which the single polyethylene(PE) chain is stretching and the release exceedal the velocity of molecular thermal fluctuation.The area enclosed by the force hysteresis represents the dissipated energy〈E_d〉,which has a relationship with pulling velocity v.Behavior of 〈E_d〉-v can be fitted with a curvilinear equation of 〈E_d〉=a+b×e~(-cv),and shows two distinct cases:a weak dependence as v<0.005nm/ps,〈E_d〉 almost keeps unchanged with the increase of the pulling velocity v,and a strong dependence as v>0.005 nm/ps,〈E_d〉 increases obviously with the pulling velocity v.Furthermore,at different thermodynamics temperature areas of single polyethylene(PE) chain,〈E_d〉 shows different behaviors.At high temperature areas T>220 K,〈E_d〉 decreases with the increase of the temperature,which agrees well with the experimental results of energy dissipated in a loading-unloading cycle of poly(butylene terephthalate) by Pegoretti et al.However,the experiment can't be realized at low temperature.Our computer simulations can overcome the unaccessible condition in real experiments.We find that 〈E_d〉 decreases with the decrease of the temperature at low temperature area T<220 K.All these investigations may provide some insights into the elastic behavior of polymer chains.