Effect of MWCNTs Dispersion and Loading on the Rheological and Electrical Properties of MWCNTs / Silicone Composite

The dispersion of multiwalled carbon nanotubes (MWCNTs) in silicone was significantly affected by the method of mixing. In this study, rheology showed that there was an optimal mixing time to get the MWCNTs disperse well and high loading of MWCNTs made the suspension flow difficult. The stability of MWCNTs/silicone suspension was analyzed to estimate the degree of re-aggregation during the curing process. The dispersion and loading of MWCNTs affected significantly electrical properties of MWCNTs/silicone elastomer. With the extension of time stirred, the dielectric constant of 1 wt% MWCNTs/silicone elastomer increased from 17 to 216 while most high dielectric constant composites were only decades in number. The dielectric loss increased exponentially with the loading of MWCNTs, which made the high dielectric constant meaningless.


Introduction
In 1991, Iijima discovered carbon nanotubes (CNTs) [1] .The CNTs possess low mass density, high aspect ratio and degree of graphitization, which make CNTs endowed with excellent mechanical and electrical properties [2,3] .These excellent properties make CNTs promising candidates as filler material in the design of new composite systems.However, the aggregation of CNTs hamper significantly the mechanical and electrical properties of nanocomposites [4][5][6] .Therefore, researching effect of MWCNTs dispersion and loading on properties of composites made by MWCNTs has recently aroused more and more interests of scientists [7,8] .In this publication, we report that dispersion of multi-wall carbon nanotubes (MWCNTs) in silicone, which was significantly affected by the way of mixing through rheology.An optimized process was selected by comparing the samples' modulus and viscosity in an acceptable mixing time.MWCNTs silicone elastomer with different mixing time and different MWCNTs content were prepared to carry out the effect of MWCNTs on electrical properties.The side effect of high MWNCTs loading was estimated to ensure the composite we made were of practical significance.
Since the ultrasonicaton can cause unwanted heat and damage to MWCNTs [8,9] , mechanical mixing seems to be the appropriate way.The rotate speed is determined by the geometric dimensioning of container and agitator blade.Mixing time varied in order to study the effect of MWCNTs' dispersion on rheological and electrical properties of these composites. 1 wt% MWCNTs suspensions prepared by mechanical stirring for 10 minutes, 2 hours, 4 hours, 8 hours and 24 hours were labeled as suspension M10M, M2H, M4H, M8H, M24H respectively.1~6 wt% MWCNTs and 30 g silicone were mixed at the same criteria except that the stirring time were fixed at 8 hours.Changing MWCNTs content in order to research the effect of MWCNTs loading on rheological and electrical properties of these composites.Suspension M1wt, M2wt, M3wt, M4wt, M5wt and M6wt were shorted for 1~6wt% MWCNTs suspensions prepared by stirring for 8 hours.On the basis of the above process, these suspensions can be curing into elastomer by being vacuumized and heated.
The rheological properties were carried out on a stress-controlled rotational rheometer HAAKE MARS III (Thermo Fisher scientific, America) at 25 using a 35mm parallel plate geometry with the stress of 1Pa.Hz and the intersection, the samples exhibited solid-like properties.The viscoelastic changing point moved to higher frequency, when the dispersion time were extended, and the samples exhibited solid-like properties in a greater frequency range.
A shear thinning behavior [8,10] can be observed.The curves of suspension M10M, M2H and pure silicone were substantially parallel, while the viscosity of suspension M4H, M8H and M24H decreased with the increased frequency.The viscosity of suspension prepared by 8 hours stirring increased by 2 orders of magnitude compared to the pure silicone, which illustrated the forming of MWCNTs crosslink network improved modulus and viscosity [11] .MWCNTs owning a dielectric constant below 70 in number [12,13] .Considering the balance of effect between time-cost, 8 hours should be the optimal choice just as the conclusion given by the rheology.The dielectric loss of 1 wt% elastomer appeared randomness, 0.2~0.35,which were quite different from dielectric constant, and seemed to be independent of dispersion time.

Conclusion
In the present paper, we investigate systematically how MWCNTs dispersion degree and loading affect both the rheological behavior and electrical properties.For the given mixing method, there is an optimal mixing time to get the MWCNTs dispersed well.8 hours should be the optimal mixing time in this paper.Once the loading of MWCNTs was over 2 wt%, the suspension became too thick to flow freely.Significant increase of dielectric constant was achieved as a result of exceeded mixing time and high loading of MWCNTs.The dielectric constant increased about 2.4 times while the dielectric loss increased 2000 times when the MWCNTs increased from 1 wt% to 6 wt%.The dielectric loss exponentially increased with the loading of MWCNTs.

Fig. 1
Fig. 1 Modulus and viscosity as functions of frequency for 1 wt% MWCNTs/silicone suspensions at different stirring times Figure 1 shows the curves of modulus and viscosity in function of frequency (0.01 Hz~10 Hz) for the samples with 1 wt% MWCNTs at 25 and 1 Pa.Both elastic and modulus of MWCNTs/silicone suspensions rose with the increased frequency.Once the dispersion time achieved 4 hours, the elastic modulus and loss modulus intersected at a certain frequency which were called viscoelastic changing point.In the range between 0.01

DOI: 10 Fig. 4
Fig. 4 Dielectric constant and loss as a function of stirring time for 1 wt% MWCNTs/silicone elastomer Pristine silicone elastomer owns a quite low dielectric constant, 3 in general, showed in figure 4. The dielectric constant of 1 wt% MWCNTs elastomer M10M with 10 minutes stirring time was about 20, while the elastomer M24H with 24 hours stirring time was over 200 at 1000Hz.The results were approving for most composites doped with modified

DOI: 10 Fig. 5
Fig. 5 Dielectric constant and loss as a function of MWCNTs loading for composite elastomer with 8 hours stirring timeThe dielectric constant of elastomers increased gradually with the increased MWCNTs loading, while the dielectric losses increased exponentially as shown in figure5.The dielectric constant of the material reaches 430 at last.When the loading of MWCNTs was up to 3 wt%, and the dielectric loss was over 1, which means the materials consume more electricity than storing energy.Margin effects are limited, when the content of MWCNTs reaches certain extent, but enormous costs have to be paid.It is not advisable to gain high dielectric constant by increasing MWCNTs loading, because the dielectric loss increased 4 orders of magnitude.