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Characterization of the dispersion properties of carbon nanotubes in ionic liquids by the separation behaviour in the centrifugal field
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T. Sobisch, D. Lerche
LUM Berlin/Germany www.LUM-gmbh.com
Y. Korth, C. Friedrich
Albert-Ludwigs-Universität Freiburg/Germany www.fmf.uni-freiburg.de
Scope
Carbon nanotubes are considered as nano materials with a broad application potential, e.g. as filler, in electronic devices and batteries. The relatively good dispersibility in ionic liquids is an interesting approach.
Carbon nanotubes in ionic liquids tend to form extended inter-tangled fibre networks, however, for efficient application a high degree of dispersion is required.
In this study dispersions of different multiwalled carbon nanotube products in 1-butyl-3-methyl-imidazoliniumtetrafluoroborat (BMIBF4) were characterized by multisample analytical centrifugation with high resolution photometric detection. Based on the sedimentation/consolidation behaviour pronounced differences in the degree of network formation and in the packing density were identified. The size distribution of the products was also determined by multisample analytical centrifugation.
Carbon nanotube dispersions [1]
Raw materials
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Fig. 1 Overview over materials used and below related TEM pictures
The 3 different products used differ largely in material properties and morphology (Fig. 1)
Dispersion preparation
In a first step the nanotubes were dispersed in ethanol by sonication. After adding BMIBF4 sonication was repeated and in a third step the ethanol was evaporated. Dispersions of 1 % m/m were prepared. For the centrifugal analysis dispersions were further diluted to 0.1 % m/m.
Method of multisample analytical centrifugation
Time and space resolved detection of light transmission (STEP-Technology) combined with multisample analytical centrifugation (LUMiSizer, LUM, Germany
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Fig. 2 Measurement scheme of the multisample analytical centrifuge. Local changes are detected during centrifugation. The distribution of transmission is recorded over the whole sample length at predefined time intervals.
Up to 12 different samples can be analysed at the same time at constant or variable centrifugal acceleration up to 2300 g. The shape and progression of the transmission profiles contain the information on the dispersion properties. The transmission profiles (Fig. 4) are representative for the distribution of particle concentration inside the sample (low transmission means high, high transmission means low particle concentration). Based on this, sedimentation and clarification kinetics, the dependence between packing density and pressure, as well as the velocity and particle size distribution can be obtained. Polycarbonate cells with an optical path length of 2.1 mm were used.
Results and discussion
Size distributions
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Fig. 3 Comparison of the cumulative intensity weighted distribution of the 3 products.
The particle size is obtained as equivalent particle diameter according to the hydrodynamic diameter of spherical particles. Therefore the large asymmetry of the nanotubes will have a pronounced impact on these results. Nevertheless, it can be seen that the Nanocyl product has a narrower distribution and lacks the coarse fraction present in the two other products.
Separation behaviour as function of raw material
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Fig. 4 Transmission profiles obtained during 2 hours centrifugation at 2300 g as function of the raw material used (first profile after 10 seconds in red, last profile after 2 hours in green).
As obvious from Fig. 4 the evolution of transmission profiles during centrifugation can be used as finger print to distinct the 3 products. All 3, especially Nanocyl, exhibit a different behaviour.
The transmission profiles depict the transmission as function of the position inside the sample given as distance from the centre of rotation.
In case of Nanocyl sharp vertical profiles are obtained showing that all particles move with identical velocity. The distance between the consecutive profiles is narrowing due to the resistance to compaction is increasing with the packing density obtained. This is characteristic for a flocculated particle network, in this case for a network of entangled nanotubes. For the other 2 products there is no sharp sedimentation front, instead the transmission is gradually decreasing towards the cell bottom. This relates to a polydisperse sedimentation, the particles or aggregates/agglomerates move with different velocity. It can be further noticed that for these two dispersions sediment (range of minimum transmission close to the bottom) is formed instantaneously followed by a gradual increase of transmission in the supernatant. Therefore, it can be concluded that there is a fast settling of aggregates/agglomerates followed by the separation of fines.
On a closer look for Polytech&Net one can observe a gradual compression of the instantaneously formed sediment (particle network).
According to the sediment height the order of packing density is:
Nanocyl <>
From the comparison of the transmission profiles it can be concluded that the Iolitek has the highest dispersibility, Nanocyl has a pronounced tendency to form an entangled network.
Influence of the degree of ultrasonication on separation kinetics
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Fig. 5 Separation kinetics as function of intensity of sonication for the centrifugation of 0.1 % dispersion of Nanocyl in BMIBF4 at 2300 g.
For Nanocyl the intensity of sonication during dispersion was varied, abbreviated as CE.10.42, CE.10.43, CE.65.45 (amplitude and duration: 10 % 5 min / 10 % 8 min / 65% 16 min). Fig. 5 exhibits the related separation kinetics (movement of the boundary supernatant/dispersion with time). Increasing the sonication intensity the separation slows down and the equilibrium sediment height increases (difference between position of the cell bottom and the position of the boundary towards the sediment), i.e. the packing density decreases.
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Fig. 6 Compression-dilation kinetics of the sediment formed after 2 hours centrifugation in Nanocyl dispersions as function of sonication intensity. The centrifugal acceleration was increased step-wise and then decreased.
As shown in Fig. 6 sediments formed in case of Nanocyl (compacted particle network) exhibit elastic behaviour. The packing density calculated from sediment volume and particle concentration decreases with intensity of sonication.
References
[1] Y. Korth, PhD thesis Rheologische und morphologische Eigenschaften von Fasernetzwerken
Freiburg i. Breisgau, Univ., Diss., 2010
[2] D. Lerche, Dispersion stability and particle characterisation by sedimentation kinetics in a centrifugal field
J. Dispersion Sci. Technol. 23, 5, 699 ‑ 709 (2002)
[3] G.G. Badolato, F. Aguilar, H.P. Schuchmann, T. Sobisch and D. Lerche, Evaluation of long term stability of model emulsions by multisample analytical centrifugation,
Progr. Colloid Polym. Sci. 134 66–73 (2008)
[4] T. Sobisch and D. Lerche, Thickener performance traced by multisample analytical centrifugation
Colloids & Surfaces A 331 114-118 (2008)
