Molecular dynamics simulation of thermodynamic properties of polypropylene modified by nano-ZnO
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摘要:
聚丙烯材料以其绝缘性能优异、熔点高、耐腐蚀性强、易回收等优点，在高压直流输电电缆领域备受关注。但在应用实际当中，聚丙烯刚性大、韧性差的问题限制了其发展。因此，通过改性技术提高聚丙烯韧性的同时，使其绝缘性能和熔点仍然保持一个较高的状态是本文研究的重点。本文通过分子动力学模拟技术，将纳米ZnO颗粒掺杂于基体材料聚丙烯(PP)中，搭建PP、ZnO(0.4nm)-PP、ZnO(0.6nm)-PP、PP/10H2O、ZnO(0.4nm)-PP/10H2O以及ZnO(0.6nm)-PP/10H2O六个模型，计算上述模型的玻璃化转变温度、自由体积、均方位移、水分子运动以及力学参数，分析纳米ZnO和水分子对模型热稳定性和力学性能的影响。与纯PP模型相比，掺杂有纳米ZnO颗粒的模型在提高PP的玻璃化转变温度的同时，能够在无定形区与水分子之间建立氢键作用，抑制水分子对PP热稳定性的破坏。此外，复合体系的力学性能计算结果表明，纳米ZnO颗粒能够明显降低PP的力学模量，有效降低PP材料的刚性，而随着温度升高，分子链运动逐渐活跃，ZnO纳米团簇的固定阻碍能力凸显出来，使复合体系维持了较好的韧性。分子动力学模拟结果表明，ZnO/PP复合材料能够保持良好的热稳定性，提高纯PP材料的韧性，同时表现出一定的抗水树枝性能，对于开发基于聚丙烯的环保型电缆材料，实现其在电力电缆的大规模应用具有重要意义。 PP及其复合材料250K温度下运动状态(a)和杨氏模量(b) Abstract: Polypropylene, as an environment-friendly thermoplastic material, has great potential in the application of UHV 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 polypropylene. To this end, based on molecular dynamics simulation technology, pure polypropylene, composite model of ZnO/polypropylene 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 polypropylene 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 polypropylene and its mechanical properties, and the lifting effect of ZnO particles with different radii is different. Among them, the glass transition temperature of polypropylene increases by 56 K with nano-ZnO with a radius of 0.6nm, which can significantly reduce its Young’s modulus and shear modulus, and has a good inhibition effect on the movement of polypropylene 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 polypropylene water tree and developing practical and environment-friendly cable materials. -
图 1 PP模型分子构象模型分子构象(a)、PP/10 H2O模型分子构象(b)、ZnO(0.4 nm)/PP模型分子构象(c)、ZnO(0.4 nm)/PP/10 H2O模型分子构象(d)、ZnO(0.6 nm)/PP模型分子构象(e)以及ZnO(0.6 nm)/PP/10 H2O模型分子构象(f)
Figure 1. PP model molecular conformation (a), PP/10 H2O model molecular conformation (b), ZnO(0.4 nm)/PP model molecular conformation (c), ZnO(0.4 nm)/PP/10 H2O model molecular conformation (d), ZnO(0.6 nm)/PP model molecular conformation (e) and ZnO(0.6 nm)/PP/10 H2O model molecular conformation (f)
图 2 PP/10 H2O模型(a)、ZnO(0.4 nm)/PP/10 H2O模型(b)以及ZnO(0.6 nm)/PP/10 H2O模型(c)玻璃化转变温度(Tg)拟合图
Figure 2. PP/10 H2O model (a)、ZnO(0.4 nm)/PP/10 H2O model (b) and ZnO(0.6 nm)/PP/10 H2O model (c) fitting diagram of glass transition temperature(Tg)
图 3 纯PP模型自由体积分数示意图
Figure 3. Schematic Diagram of Free Volume Coefficient of Pure PP model
图 4 不同温度下纯PP和ZnO(0.4 nm)/PP体系的MSD曲线(a)、(b)以及250 K温度下PP及其复合材料的MSD曲线(c)
Figure 4. MSD curve of pure PP and ZnO(0.4 nm)/PP models at different temperatures ((a), (b)), 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)和体积模量(b)
Figure 5. Young's modulus (a) and bulk modulus (b) of PP and its composite materials
图 6 PP/10 H2O体系的MSD曲线(a)和ZnO(0.4 nm)/PP/10 H2O体系MSD曲线(b)
Figure 6. MSD curve of PP/10 H2O model (a) and ZnO(0.4 nm)/PP/10 H2O model (b)
表 1 250 K温度下PP及其复合材料的VFree、VOcucupied和f
Table 1. VFree, VOccupied and f of PP and its composite materials at 250 K
System
(At 250 K)VFree/
10−3nm3VOccupied/
10−3nm3f/% PP 5463.79 20708.08 20.88 PP/10 H2O 5627.56 20821.13 21.28 ZnO(0.4 nm)/PP 4458.44 21770.09 17.00 ZnO(0.4 nm)/PP/10 H2O 4843.2 21428.15 18.44 ZnO(0.6 nm)/PP 3957.96 22485.71 14.97 ZnO(0.6 nm)/PP/10 H2O 4605.22 22645.34 16.90 Notes: VFree is free volume；VOccupied is occupied volume；f is free volume fraction. 
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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 /(10-11m2/s) 100 K 150 K 200 K 250 K 300 K 350 K 400 K 450 K 500 K 550 K PP/10 H2O 16.44 21.12 52.0 57.43 148.20 372.77 703.50 454.08 3434.14 3070.43 ZnO(0.4 nm)/PP/10 H2O 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/10 H2O 0.22 1.14 1.88 21.39 18.41 75.34 85.99 98.19 740.30 865.94
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