PVDF单轴拉伸的第一性原理和分子动力学模拟

Ab initio and molecular dynamics study of uniaxial tensile behaviors of PVDF

为得到聚偏二氟乙烯(PVDF)的大变形拉伸特性,应用第一性原理方法模拟了β相PVDF分子链的在单轴拉伸下的能量及构型变化,得到了β相分子链拉伸过程中发生断裂时的临界键长、临界断裂能量、临界拉力以及分子链弹性系数的变化规律.接着用一种新颖的方法构造了无定形PVDF的立方元胞模型并用于单轴拉伸的分子动力学模拟.模拟中分别考虑链不被拉断和链允许被拉断两种情况.第1种情况下,拉伸应变率分别为1010,109和108s-1,得到了应力应变关系和构型变化;第2种情况下,拉伸应变率分别为109s-1,5×108s-1和准静态,得到了应力应变关系和断裂前后的构型.同时分析了一些关键的形变信息和影响模拟结果的一些关键因素.模拟结果表明,不仅对于理解PVDF的拉伸变形机制具有较重要意义,而且对于其他无定形材料的拉伸模拟具有参考价值.

To obtain the behaviors of Poly （ vinylidene difluoride） （PVDF） under large tensile deformations, this paper investigates the tensile behavior of a single molecular chain of rod-like PVDF with β phase structure at zero temperature using a Density Functional Theory （DFF） calculation. We obtain the critical bond length, critical fracture energy, critical tensile force and the elastic constant when the]3 phase PVDF chain is stretched to break. Then a novel method is proposed to build the cell model of the amorphous polymer, which is used for the molecular dynamics （MD） simulations of uniaxial tensile of PVDF. During the simulations, two cases that the chains will never break and are permitted to break are considered. In the first case, the simulations with three different strain rates 10~~ s-~ , 109 S-1 , and l0s s-~ were carried out and the strain rate dependence of the stress-strain relationship, the structure deformation were analyzed as well. In the second case, the simula- tions with a strain rate of 109 s-I , 5 ~ 108 s-1 and a quasistatic strain loading were performed and the stress- strain relationship, the cell structures before and after the breaking were researched. Some key factors to influ- ence the simulation results and some important deformation information were analyzed. This is helpful to eluci- date the inherent deformation mechanisms during the tensile procedure.