注射成型发泡过程中温度和剪切速率对CO2扩散行为影响的分子动力学研究

为研究临界CO₂辅助聚乳酸（PLA）注射成型发泡过程中，温度和剪切速率对CO₂扩散行为影响。应用分子动力学模拟方法，以PLA和CO₂为研究对象，基于周期性边界条件和SA算法，引入CAMPASS力场，构建PLA/CO₂扩散模型，对模型进行能量最小化处理。从温度和剪切速率两个方面研究在注射成型发泡过程中PLA主链活跃性、体系能量响应、均方回转半径对CO₂分子在PLA中扩散行为的影响。结果表明，CO₂扩散系数在不受剪切力作用时，随着温度升高而增大，温度达到388 K时，CO₂扩散系数达到0.219 8×10^-5 cm²/s；受剪切力作用时，随着剪切速率增大而增大；温度为378 K，剪切速率为1 ps^-1时，扩散系数达到17.743 6×10^-5 cm²/s；PLA分子链所处环境温度不断提高，分子链能量升高，活跃性提高，CO₂分子扩散路径增多，从而提高CO₂扩散速率；分子链所受剪切力不断增大，分子链能量升高，活跃性提高，单个分子链从缠结网络中剥离出来，沿剪切力方向形成流动取向，从而提高CO₂扩散速率。

Molecular Dynamics Study on Effects of Temperature and Shear Rate on CO2 Diffusion Behavior in Foaming Process of Injection Molding

This paper focused on the effects of temperature and shear rate on CO₂ diffusion behavior in the foaming process of critical CO_2-assisted injection molding. Taking poly（lactic acid） （PLA） and CO₂ as research objects， a PLA/CO₂ diffusion model was constructed by molecular dynamics simulation along with the CAMPASS force field on the basis of periodic boundary conditions and SA algorithm， minimizing the energy of the model. From two aspects of temperature and shear rate， the effects of PLA main chain activity， system energy response and radius of gyration on the diffusion behavior of CO₂ molecules in PLA during the foaming process of injection molding were investigated. Mean square displacement （MDS） analysis indicated that the CO₂ diffusion coefficient increased with an increase in temperature， but there was no influence from shear force. The CO₂ diffusion coefficient reached 0.219 8×10^-5 cm²/s a temperature of 388 K， and it also increased with an increase in shear rate. Moreover， a diffusion coefficient 17.743 6×10^-5 cm²/s was achieved at a temperature of 378 K and a shear rate of 1 ps^-1. According to the calculated results， the activity and system energy of the PLA molecular chain increased due to a rise in environmental temperature， resulting in a increase in the diffusion paths of CO₂ molecules together with an increase in the CO₂ diffusion rate. The energy and activity of the molecular chain increased with a continuous increase in the shearing force on the molecular chain. A single molecular chain was separated from the entangled network to form a flow orientation along the direction of the shear force， increasing the CO₂ diffusion rate accordingly.