Molecular dynamics simulation of thermodynamic properties of polypropylene modified by nano-ZnO
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摘要: 聚丙烯(PP)作为环境友好型热塑性材料，在超特高压电缆线路的应用中具有巨大潜力。但其在实际应用中，热、力学性能难以协同提升，所处的潮湿环境加速了水树枝的引发，严重掣肘了PP的快速发展。为此，基于分子动力学模拟技术，搭建纯PP、半径分别为0.4 nm和0.6 nm的球型纳米粒子ZnO/PP复合模型及含水分子的模型，利用热、力学性能等参数指标的变化情况，从分子层面研究球型纳米ZnO对PP性能的影响。结果表明：纳米ZnO能够在提高PP热稳定性的同时，改善其力学性能，且不同半径的ZnO颗粒提升效果有所不同。其中，半径为0.6 nm的纳米ZnO使PP的玻璃化转变温度提高56 K，能够明显降低其杨氏模量和剪切模量，对PP分子链运动和水分子运动具有较好的抑制作用。水分子使均方位移有明显的提高，玻璃化转变温度和力学参数均下降，扩散能力随着温度的升高不断增强；由于纳米ZnO与水分子之间的氢键作用，该复合体系中水分子运动受限，从而保持良好的热稳定性。研究结果对抑制PP水树枝生长、老化及开发实用型和环境友好型电缆料提供了微观层面的参考。Abstract: Polypropylene (PP), as an environment-friendly thermoplastic material, has great potential in the application of ultra high voltage cable lines. However, in practical applications, the thermal and mechanical properties are difficult to improve in coordination, and the humid environment accelerates the initiation of water branches, which seriously hampers the rapid development of PP. To this end, based on molecular dynamics simulation technology, pure PP, composite model of ZnO/PP with sphericalnanoparticles with radius of 0.4 nm and 0.6 nm and water-containing molecular models were built, and the effects of spherical nano-ZnO on the properties of PP were studied from the molecular level by using the changes of thermal and mechanical properties and other parameters. The results show that nano-ZnO can improve the thermal stability of PP and its mechanical properties, and the lifting effect of ZnO particles with different radii is different. Among them, the glass transition temperature of PP increases by 56 K with nano-ZnO with a radius of 0.6 nm, which can significantly reduce its Young's modulus and shear modulus, and has a good inhibition effect on the movement of PP molecular chain and water molecules. Water molecules significantly increase the average azimuthal shift, reduce the glass transition temperature and mechanical parameters, and enhance the diffusion ability with the increase of temperature. Due to the hydrogen bonding between nano ZnO and water molecules, the movement of water molecules in the composite system is limited, thus maintaining good thermal stability. The research results provide micro-level reference for inhibiting the growth and aging of PP water tree and developing practical and environment-friendly cable materials.
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图 1 (a) 聚丙烯(PP)模型分子构象；(b) PP/10H2O模型分子构象；(c) ZnO(0.4 nm)/PP模型分子构象；(d) ZnO(0.4 nm)/PP/10H2O模型分子构象；(e) ZnO(0.6 nm)/PP模型分子构象；(f) ZnO(0.6 nm)/PP/10H2O模型分子构象
Figure 1. (a) Polypropylene (PP) model molecular conformation; (b) PP/10H2O model molecular conformation; (c) ZnO(0.4 nm)/PP model molecular conformation; (d) ZnO(0.4 nm)/PP/10H2O model molecular conformation; (e) ZnO(0.6 nm)/PP model molecular conformation; (f) ZnO(0.6 nm)/PP/10H2O model molecular conformation
图 2 PP/10H2O模型(a)、ZnO(0.4 nm)/PP/10H2O模型(b)及ZnO(0.6 nm)/PP/10H2O模型(c)的玻璃化转变温度(Tg)拟合图
Figure 2. Fitting diagram of glass transition temperature (Tg) of PP/10H2O model (a), ZnO(0.4 nm)/PP/10H2O model (b) and ZnO(0.6 nm)/PP/10H2O model (c)
图 3 纯PP模型自由体积分数示意图
Figure 3. Schematic diagram of free volume fraction of pure PP model
图 4 不同温度下纯PP (a)和ZnO(0.4 nm)/PP体系(b)的均方位移(MSD)曲线及250 K温度下PP及其复合材料的MSD曲线(c)
Figure 4. Mean square displacement (MSD) curves of pure PP (a) and ZnO(0.4 nm)/PP models (b) at different temperatures, and MSD curves of PP and its composites at 250 K (c)
图 5 PP及其复合材料的杨氏模量(a)和剪切模量(b)
Figure 5. Young's modulus (a) and shear modulus (b) of PP and its composite materials
图 6 PP/10H2O体系(a)和ZnO(0.4 nm)/PP/10H2O体系(b)的 MSD曲线
Figure 6. MSD curves of PP/10H2O model (a) and ZnO(0.4 nm)/PP/10H2O model (b)
表 1 250 K温度下PP及其复合材料的自由体积VFree、占有体积VOcucupied和自由体积分数f
Table 1. Free volume VFree, occupied volume VOccupied and free volume fraction f of PP and its composite materials at 250 K
System VFree/
(10−3 nm3)VOccupied/
(10−3 nm3)f/% PP 5463.79 20708.08 20.88 PP/10H2O 5627.56 20821.13 21.28 ZnO(0.4 nm)/PP 4458.44 21770.09 17.00 ZnO(0.4 nm)/PP/10H2O 4843.20 21428.15 18.44 ZnO(0.6 nm)/PP 3957.96 22485.71 14.97 ZnO(0.6 nm)/PP/10H2O 4605.22 22645.34 16.90 
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表 2 PP及其复合材料的扩散系数
Table 2. Diffusion coefficient of PP and its composite materials
Model Diffusion coefficient /(10−11 m2/s) 100 K 150 K 200 K 250 K 300 K 350 K 400 K 450 K 500 K 550 K PP/10H2O 16.44 21.12 52.00 57.43 148.20 372.77 703.50 454.08 3434.14 3070.43 ZnO(0.4 nm)/PP/10H2O 0.76 1.28 2.65 36.51 66.43 216.91 364.04 400.47 581.43 2466.70 ZnO(0.6 nm)/PP/10H2O 0.22 1.14 1.88 21.39 18.41 75.34 85.99 98.19 740.30 865.94
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