等规聚丙烯注塑成型冷却固化分子机制研究

Macromolecular Solidification Mechanism of the Isotropic Polypropylene(iPP) Material During the Injection Cooling Stage

针对等规聚丙烯(iPP)在注塑成型冷却过程中的液-固相变与大分子形构问题,采用分子模拟实验的方法,研究了iPP材料在注塑冷却过程中的分子固化与形态演化的分子机制。建立了聚合度依次为24、36、51、72、120,链数为36、24和16的iPP胞元体系,基于Smart Minimizer与SA算法实现了能量初始化;基于周期性边界并引入COMPASS力场及Velocity-Verlet算法,实现了体系冷却固化过程的分子模拟实验。结果表明:不同模拟体系的T_g结果符合Flory自由体积理论;固化前后iPP大分子链主链键长、键角等结构参数均呈高斯分布,温度越低键长与键角分布越集中,符合无外力加载条件下的缠结大分子链的冷却松弛过程;〈h〉和〈R_g〉均随聚合度的增大而增大,降温过程中〈h〉呈高斯分布状态,而〈R_g〉分布趋于集中,一方面表明冷却固化过程中大分子链由初始取向排布逐步恢复至缠结状态,另一方面体现了固化结晶过程;材料聚合度越大,降温过程中分子迁移或固化速率越小,且当温度高于T_g时,聚合度越小,降温过程体系扩散系数下降梯度越大,而温度下降至T_g以下则无明显规律。

In response to the micro-mechanism of the liquid-solid phase transition and macromolecular structure evolution of the isotropic polypropylene(iPP)material during the injection molding cooling process,the macromolecular solidification and configuration evolution mechanism was studied based on the molecular dynamic simulation method.The different iPP cell systems were built,of which the polymerization degree were 24,36,51,72,120 and chain number were 36,24 and 16,successively.The energy initialization was performed using the smart minimization method followed by the simulated annealling(SA)method.Coupling with the periodic boundary conditions,COMPASS force field and the velocity-verlet algorithm,the cooling simulation was launched.The results indicate that the glass transition temperature(T_g)of each simulated cell coincides well with the Flory's free volume theory.The bond length as well as bond angle presents a Gaussian distribution,the lower the temperature,the more concentrated distribution of them,agreeing with the cooling free relax process of the macromolecular.The〈h〉and〈R_g〉of the polymer chains are all increasing with the polymerization degree.〈h〉shows a Gaussian distribution during the declining of the temperature,while〈R_g〉exhibits a shrinkage trend,which indicates the macromolecular chains configuration tangle again during this stage from the orientated phase,and another crystallize phase is taken place.The migration and solidification rate give a negative correction with the increasing polymerization degree.And the diffusion coefficient exhibits steeper declining trend with higher polymerization degree when the temperature is above the glass transition temperature.