Due to the widespread use of composite materials in aerospace, automobile, and other fields, their vibration and noise reduction performance is becoming more and more important. Damping is a key parameter in the control of structural vibration and noise [1
]. The damping properties of composites are largely derived from the matrix [2
]. Therefore, it is feasible to improve the damping properties of composites by modifying the damping of the matrix. For the damping integration of composite structure, the filler should be able to simultaneously improve the mechanical properties and damping properties of the matrix, without affecting the light weight of the composite material.
Carbon nanotubes (CNTs) are ideal damping fillers. CNTs were first discovered by Japanese scholar Iijima under a high-resolution transmission electron microscope in 1991 [3
]. Subsequently, it was found that the modulus of carbon nanotubes was about 1 TPa and the strength was about 14 GPa using simulation [4
], indirect measurement [6
], and direct measurement [7
]. Carbon nanotubes are characterized by a large aspect ratio, large specific surface area, pore structure, quasi-one-dimensional structure, and nanometer size. The mechanical properties of polymers can be significantly improved by the addition of a small amount of CNTs. Chang et al. [8
] prepared single-walled carbon-nanotube-reinforced thermoplastic fibers (polypropylene, PP) and found that the modulus of the fiber was significantly improved from 0.4 to 1.4 GPa. In the work of Xu et al. [9
], the mechanical properties of epoxy resin were reinforced by multi-walled carbon nanotubes (MWCNTs) and a significant increase in flexural modulus from 4.2 to 5 GPa was observed with the addition of only 0.1 wt. % of MWCNTs. In terms of damping properties, Xu et al. [10
] found that, compared with silicone rubber, carbon nanotubes had excellent viscoelasticity properties over a wide range of temperatures. Moreover, they produced more energy dissipation than silicone rubber under the conditions of periodic strain. In addition, Zhou et al. [11
] reported that the stick-slip frictional motion between the nanotubes and the resin could increase energy consumption. Buldum [12
] also pointed out that debonding slippage occurred between carbon nanotubes and the matrix under external force, resulting in interface friction, which in turn led to energy dissipation. Nanometer-sized carbon nanotubes (CNTs) could increase the interfacial area dramatically between fillers and the polymer matrix. The pull-out failure similar to that of scabbard was observed in the matrix reinforced by multi-walled carbon nanotubes, which indicated that there was also sliding between the inner and outer walls of multi-walled carbon nanotubes [13
]. Therefore, multi-walled carbon nanotubes (MWCNTs) show the potential to synchronously enhance the mechanical properties and damping properties of the polymer matrix.
However, there are three problems that still need to be solved to realize the integrated design of structural damping using MWCNTs.
Firstly, which dimensions of MWCNTs should we choose to achieve synchronous enhancement of mechanical properties and damping properties? Many researches were carried out to study the effect of MWCNT dimensions on the mechanical properties of nanocomposites. Dubnikova et al. [14
] studied the effect of MWCNT dimensions on the morphology, mechanical reinforcement, and electrical properties of PP-based composites. The influence of MWCNT dimensions on the mechanical properties and electrical conductivity was evaluated by Su et al. [15
]. Singh et al. [16
] investigated the effect of length on the mechanical, electrical, and electromagnetic interference shielding of MWCNT/epoxy nanocomposites with two different lengths of MWCNTs. Nevertheless, there are few studies on the effect of MWCNT dimensions for the synchronous enhancement of mechanical properties and damping properties of nanocomposites. Similarly, the influence of agglomeration on mechanical properties was frequently reported, but studies on the influence of agglomeration on damping properties are relatively scarce.
Secondly, the dispersion is always an urgent problem in the application of carbon nanotubes. A variety of dispersion methods, such as ultrasonic, ball milling, and mechanical agitation were previously applied. The solution processing of composites which mixed the polymer and nanotube in a suitable solvent and cooperated with the above dispersion methods was developed to improve dispersion [17
]. Arun et al. [20
] reported an approach by simultaneously applying ultrasonic waves and shear force generated by an axial flow impeller to obtain a superior level of dispersion of multi-walled carbon nanotubes in epoxy. Then, more approaches were developed to improve the dispersion of carbon nanotubes, among which the functionalization of carbon nanotubes was the most effective. Strong acids and other oxidizing agents were used to generate various functional groups such as carboxyl, ketone, etc., and amination, fluorination, etc. were introduced via further reactions. In addition, non-covalent functionalization approaches, such as special polymers [21
] and sodium dodecyl sulfate (SDS) [22
], were also used to improve the dispersion of MWCNTs. Studies focusing on the dispersion of CNTs using functionalization were frequently reported in recent publications. Gröschel et al. [23
] reported that the selective adsorption of Janus micelles (JMs) on MWCNTs changed the compatibility between CNTs and the matrix, which facilitated the dispersion of MWCNTs. Parveen et al. [24
] utilized Pluronic F-127 as a novel dispersing agent to achieve a short dispersion route in cementitious composites. In our experiment, a solution-based high-speed shear dispersion method was developed, and functionalized MWCNTs and a surfactant (BYK 9077) were also applied in the dispersion process.
Finally, the enhancement effect is greatly affected by the dispersion state of MWCNTs. Song et al. [17
] compared the properties of MWCNT-reinforced epoxy nanocomposites prepared via direct dispersion and solvent dispersion. The results showed that the properties of MWCNT/epoxy nanocomposites with good dispersion (solvent dispersion) were superior to the nanocomposites with poor dispersion (direct dispersion). In our experiment, the difficulty degree of dispersion of MWCNTs with different dimensions is different using certain dispersion processes, and non-uniform dispersion may occur. The influence of non-uniform dispersion must be considered when analyzing the enhancement effect of MWCNT dimensions. Microscopic observation is a common method of determining the uniformity of dispersion. Rahman et al. [25
] studied the dispersion state of amino-functionalized MWCNTs in epoxy resin using TEM. Zeiler et al. [26
] analyzed the length distribution of MWCNTs using TEM in the study of the withholding effect of fiber on CNTs. In addition, the micromorphology of fracture surfaces obtained with SEM was used to roughly assess the state of CNTs in a large number of studies [27
]. In this paper, a method combining theoretical analysis and experimental verification was applied to evaluate the dispersion state of carbon nanotubes with different dimensions. The cohesion and van der Waals forces were used to characterize the difficulty of dispersion of MWCNTs with different dimensions. Then, the prediction of the difficulty degree of dispersion was confirmed, and it was verified whether the uniform dispersion of different dimensions was achieved by comparing the experimental and predicted values of tensile modulus.
In summary, in order to synchronously enhance the mechanical properties and damping properties, five kinds of carboxylic MWCNTs (C-MWCNTs) with different dimensions were selected to prepare MWCNT-reinforced epoxy nanocomposites by mixing nanotubes with epoxy resin using high-speed shear dispersion. Considering the significant influence of dispersion on the properties, a method combining theoretical analysis and experimental verification was used to evaluate the dispersion state of carbon nanotubes with different dimensions. The mechanical properties, damping properties, and glass transition temperature of nanocomposites were studied experimentally. The effects of C-MWCNT loading, length, diameter, aspect ratio, and agglomeration on the mechanical properties, damping properties, and glass transition temperature of the nanocomposites were also investigated.
In this paper, the effects of dimensions and agglomerations of MWCNTs on the synchronous enhancement of mechanical properties and damping properties were studied. A reasonable dispersion process to ensure the uniformity and stability of dispersion was selected, and then the study on the dispersion mechanism was carried out using theoretical analysis. The difficulty of dispersion of MWCNTs with different dimensions was evaluated. The effect of synchronous enhancement was tested experimentally. Furthermore, the effects of carbon nanotube loading, aspect ratio, specific surface area, and functional groups on the properties of nanocomposites were analyzed.
The morphologies of agglomerates of MWCNTs prepared by CVD with different lengths were considerably different. The van der Waals force and cohesion strength were attributed to the degree of difficulty of dispersion via the mechanisms of erosion and rupture. On the whole, the dispersion of carbon nanotubes with a diameter of 8–15 nm and a length of 50 μm was the worst. The minimum size of agglomerates obtained through the mechanism of rupture was determined to be 1–10 μm by back-calculation. The ranking of dispersion difficulty was verified by the deviations between predicted and experimental values of tensile modulus. Among all the samples, No. 3 (10–30, 20–30, 0.5) and No. 5 (0.5–2, 8, 0.5) had the best effects of synchronous enhancement, but their Tg showed the maximum decrease. It was found that a large aspect ratio and high specific surface area improved the mechanical properties and damping properties synchronously, but decreased the Tg of nanocomposites. The agglomerations had a negative effect on the synchronous enhancement of the nanocomposites, but reduced the negative impact on Tg. Overall, C-MWCNTs with appropriate technical parameters can simultaneously improve the mechanical properties and damping properties of epoxy resin under appropriate dispersion conditions, but the Tg will decrease to a certain extent.