【摘要】 以聚甲基丙烯酸甲酯(Polymeric methyl methacrylate,PMMA)为实验材料,基于分子动力学模拟实验研究了注塑成型聚合物充模流动与力学行为的分子机制。构建包含10条聚合度为20的无规PMMA分子链所构成的链团模型,基于能量最小化与SA算法实现了体系能量初始化;基于周期性边界,引入COMPASS从头算分子力场及Velocity-Verlet算法,实现了PMMA胞元在恒温平面流场中的流态与力学行为模拟实验。结果表明,PMMA充模与形变过程首先需要克服包含体系内能、分子链松弛与解缠在内的"活化能",且存在时间和应力阈值,前者体现了瞬时加载内能协调效应,后者对应于高剪切力作用下分子松弛与解缠现象。体系C原子回转半径分布表明剪切力的作用使得高分子沿流场方向取向排布,剪切力越大则取向越明显,剪切力过大则分子链将断裂而弹性恢复。MSD结果揭示了熔态聚合物充模流动的实质是大分子链定向迁移和取向排布协调运动的结果,且进一步验证了"活化能"的存在,克服这一制约之后大分子链的迁移效应才变得明显,且迁移速率随剪切应力的增大呈非线性增大变化。更多还原

【Abstract】 With the example of the amorphous polymethyl methacrylate(PMMA)polymer material,the molecular dynamics simulation experiments were performed to study the flowing morphology and mechanical behavior of the macromolecular during the injection molding produce.A cubic PMMA cell consisting of 10 molecular chains of 20 units was constructed.The energy initialization was performed using the energy minimization method followed by the Simulated Anneal method.By coupled with the periodic boundary conditions,COMPASS(condensed-phase optimized molecular potentials for atomistic simulation studies)force field and the Velocity-Verlet algorithm,the simulation was launched with isothermal conditions.The results indicate the deformation of the chains cell need first to overcome the"activation energy",involving the internal energy,relation and unwrapping energy.There was a time and a stress threshold during the filling stage.The former presents the coordination course of the internal energy with a transitional loaded condition,and the later reveals the relaxation and disentanglement of the flexible chains with a higher shear stress loaded.The radius of gyration distributions of the C atom in main chains show the macromolecular gives an orientation direction with the flowing direction under the loaded conditions.The bigger of the shear stress,the more sensible of the orientation.While the excessive shear stress leads to the macromolecular break then elastic recovery.Furthermore,the mean square displacement(MSD)data indicate that the flowing essence of the molten polymer is the directional migration and orientation arrangement of the macromolecule chains.The results also confirm that the"activation energy"is the principal obstacle for chains motion.After overcoming this obstacle,the migration rate gives a nonlinear increase trend with the rise of the shear stress.更多还原