Chava Bhargav Sai, Thorn Eva K, Das Siddhartha
Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA.
Air Systems Group, NAWCAD, Air Vehicle Engineering Department, 48086 Shaw Road, Building 2188, Patuxent River, MD 20670, USA.
Phys Chem Chem Phys. 2021 Nov 10;23(43):24634-24645. doi: 10.1039/d1cp02321g.
Nanofiller-based epoxy inks have found extensive use in fabricating 3D printed nanocomposites for applications in aerospace, automobile, and marine systems. In this paper, we employ an all-atom molecular dynamic (MD) simulation to atomistically explore the mechanisms dictating the shear-thinning behavior of the graphene flake-infused (GFI) epoxy inks. We compare our findings with those for pure epoxy inks: non-equilibrium MD (NEMD) simulations reveal that both the GFI epoxy ink and pure epoxy ink demonstrate shear thinning behavior, , their viscosities decrease with an increase in the shear rate. However, interestingly, the viscosity of the GFI epoxy ink is larger than that of pure epoxy for smaller shear rates, while for higher shear rates, the viscosities of these two materials are similar. This indicates a much more favorable viscosity profile for the GFI epoxy inks in the context of 3D printing. From the context of exploring the nanoscale mechanism, we identify the tendency of the bisphenol F molecules (the key constituent of the epoxy inks) and the graphene flakes (for the case of GFI epoxy inks) to align along the shear planes (in the presence of a shear flow) allowing the dissipation of viscous force among them ensuring shear-thinning behavior for both pure epoxy and GFI epoxy inks. In this context, we also identify that the bisphenol F chains prefer to localize along a given shear plane to reduce the effect of tension forces: such an alignment ensures that the radius of gyration for the bisphenol F molecules (for both pure epoxy and GFI epoxy inks) is larger for the case of finite shear and has a non-monotonic variation with the shear rate. Finally, the equilibrium MD (EMD) simulations establish that the presence of the graphene flakes significantly slows down the rotational dynamics of the bisphenol F molecules that are adsorbed to these graphene flakes and, as a result, causes the zero-shear viscosity of the GFI epoxy to be three orders of magnitude larger than that of the pure epoxy. This difference provides a qualitative justification of the viscosity of the GFI epoxy being larger than that of pure epoxy at smaller shear rate values.
基于纳米填料的环氧油墨在制造用于航空航天、汽车和船舶系统的3D打印纳米复合材料方面有广泛应用。在本文中，我们采用全原子分子动力学（MD）模拟，从原子层面探索决定石墨烯薄片注入（GFI）环氧油墨剪切变稀行为的机制。我们将研究结果与纯环氧油墨的结果进行比较：非平衡MD（NEMD）模拟表明，GFI环氧油墨和纯环氧油墨都表现出剪切变稀行为，即它们的粘度随剪切速率的增加而降低。然而，有趣的是，在较小剪切速率下，GFI环氧油墨的粘度大于纯环氧油墨，而在较高剪切速率下，这两种材料的粘度相似。这表明在3D打印背景下，GFI环氧油墨的粘度分布更为有利。从探索纳米尺度机制的角度来看，我们确定了双酚F分子（环氧油墨的关键成分）和石墨烯薄片（对于GFI环氧油墨而言）在剪切流存在时沿剪切平面排列的趋势，这使得它们之间的粘性力得以消散，从而确保纯环氧和GFI环氧油墨都具有剪切变稀行为。在此背景下，我们还发现双酚F链倾向于沿特定剪切平面定位，以减少张力的影响：这种排列确保了在有限剪切情况下，双酚F分子（对于纯环氧和GFI环氧油墨）的回转半径更大，并且随剪切速率呈非单调变化。最后，平衡MD（EMD）模拟表明，石墨烯薄片的存在显著减缓了吸附在这些石墨烯薄片上的双酚F分子的旋转动力学，结果导致GFI环氧的零剪切粘度比纯环氧大三个数量级。这种差异从定性上解释了在较小剪切速率值下GFI环氧的粘度大于纯环氧的原因。
