Jen Yi-Ming, Chang Hao-Huai, Lu Chien-Min, Liang Shin-Yu
Department of Mechanical and Mechatronic Engineering, National Taiwan Ocean University No. 2, Pei-Ning Rd., Keelung 20224, Taiwan.
Polymers (Basel). 2020 Dec 28;13(1):84. doi: 10.3390/polym13010084.
Even though the characteristics of polymer materials are sensitive to temperature, the mechanical properties of polymer nanocomposites have rarely been studied before, especially for the fatigue behavior of hybrid polymer nanocomposites. Hence, the tensile quasi-static and fatigue tests for the epoxy nanocomposites reinforced with multi-walled carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) were performed at different temperatures in the study to investigate the temperature-dependent synergistic effect of hybrid nano-fillers on the studied properties. The temperature and the filler ratio were the main variables considered in the experimental program. A synergistic index was employed to quantify and evaluate the synergistic effect of hybrid fillers on the studied properties. Experimental results show that both the monotonic and fatigue strength decrease with increasing temperature significantly. The nanocomposites with a MWCNT (multi-walled CNT): GNP ratio of 9:1 display higher monotonic modulus/strength and fatigue strength than those with other filler ratios. The tensile strengths of the nanocomposite specimens with a MWCNT:GNP ratio of 9:1 are 10.0, 5.5, 12.9, 23.4, and 58.9% higher than those of neat epoxy at -28, 2, 22, 52, and 82 °C, respectively. The endurance limits of the nanocomposites with this specific filler ratio are increased by 7.7, 26.7, 5.6, 30.6, and 42.4% from those of pristine epoxy under the identical temperature conditions, respectively. Furthermore, the synergistic effect for this optimal nanocomposite increases with temperature. The CNTs bridge the adjacent GNPs to constitute the 3-D network of nano-filler and prevent the agglomeration of GNPs, further improve the studied strength. Observing the fracture surfaces reveals that crack deflect effect and the bridging effect of nano-fillers are the main reinforcement mechanisms to improve the studied properties. The pullout of nano-fillers from polymer matrix at high temperatures reduces the monotonic and fatigue strengths. However, high temperature is beneficial to the synergistic effect of hybrid fillers because the nano-fillers dispersed in the softened matrix are easy to align toward the directions favorable to load transfer.
尽管聚合物材料的特性对温度敏感，但此前很少有人研究聚合物纳米复合材料的力学性能，尤其是混合聚合物纳米复合材料的疲劳行为。因此，本研究对多壁碳纳米管（CNT）和石墨烯纳米片（GNP）增强的环氧纳米复合材料在不同温度下进行了拉伸准静态和疲劳试验，以研究混合纳米填料对所研究性能的温度依赖性协同效应。温度和填料比例是实验方案中考虑的主要变量。采用协同指数来量化和评估混合填料对所研究性能的协同效应。实验结果表明，单调强度和疲劳强度均随温度升高而显著降低。MWCNT（多壁CNT）与GNP比例为9:1的纳米复合材料比其他填料比例的纳米复合材料具有更高的单调模量/强度和疲劳强度。MWCNT与GNP比例为9:1的纳米复合材料试样在-28、2、22、52和82℃时的拉伸强度分别比纯环氧树脂高10.0%、5.5%、12.9%、23.4%和58.9%。在相同温度条件下，该特定填料比例的纳米复合材料的 endurance limits 分别比原始环氧树脂提高了7.7%、26.7%、5.6%、30.6%和42.4%。此外，这种最佳纳米复合材料的协同效应随温度升高而增强。CNT 桥接相邻的 GNP，构成纳米填料的三维网络，防止 GNP 团聚，进一步提高所研究的强度。观察断口发现，纳米填料的裂纹偏转效应和桥接效应是改善所研究性能的主要增强机制。高温下纳米填料从聚合物基体中的拔出降低了单调强度和疲劳强度。然而，高温有利于混合填料的协同效应，因为分散在软化基体中的纳米填料易于沿有利于载荷传递的方向排列。
