拉伸分子动力学方法研究高分子单链的吸附现象

A STEERED MOLECULAR DYNAMCS STUDY OF ADSORBED POLYMER CHAINS

采用拉伸分子动力学方法研究聚乙烯单链从一个强吸附表面被拉伸的动力学过程.当聚乙烯单链被拉伸时,从理想弹簧测量到的平均力<f>与形变的曲线上出现了一个力的平台,这与实验结果非常一致.在研究这个平台力<f>与拉伸速度的关系时发现,当速度v<0.001 nm/ps时,<f>不随速度的变化而变化;当速度v>0.001 nm/ps时,<f>随着拉伸速度的增加而迅速上升,这表明拉伸速度0.001 nm/ps是平衡态拉伸和非平衡态拉伸的临界点,并与链长无关.按顺序拉伸和释放这条链,观察到有趣的力回滞现象,并与实验结果进行了比较.

The dynamics of the detachment of single polyethylene （PE） chains from a strong adsorption surface isinvestigated by the steered molecular dynamics method （SMD）. Our simulations use the united-atom （UA） model of PE chains,which regards each CH2 and CH3 group as a monomer. Due to the significant reduction of the number of the degrees of freedom, the UA model allows one to explore longer chains or a larger number of systems within the limited computational time. In the previous investigations with SMD, the simulation is performed only once, and thestatistical properties can not be discussed. In this work, the statistical behavior of single PE chains pulled from the strong adsorption surface is investigated with the SMD and MC method. There exists a long force plateau for the curve of average force probed by spring 〈 f 〉 , which agrees well with the experimental results. The relationship between the average force 〈 f 〉 and the pulling velocity v shows two distinctive regions：a weak dependence region as v 〈0.001 nm/ps, and a strong dependence region as v 〉 0. 001 nm/ps. Remarkably, the PE chain manifests force hysteresis under sequential stretching and releasing. The area of the hysteresis also increases with the velocity v. Force-extension curves display hysteresis because the velocity at which the polymer chain is stretched and released exceeds the velocity of unfolding and folding of the polymer at equilibrium at that extension. If the stretching-releasing had been done at a very low velocity used in the experiment, the stretching or releasing curves will convergetoward a single intermediate curve representing the equilibrium process. Some results in our simulation agree well with the available experimental results, while the others may provide some insights into the related processes and further experimental investigations.