Uniform dispersion of nanotubes in thermoplastic polymer through thermal annealing

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Uniform dispersion of nanotubes in thermoplastic polymer through thermal annealing V. Tishkova a, G. Bonnet a, F. Pont b, B. Gautier b, P.H. Cadaux b, P. Puech a, W.S. Bacsa a b

a,*

29 Jeanne Marvig, CEMES/CNRS, Universite´ de Toulouse, 31055 Toulouse, France 316 Route de Bayonne, Composite Materials & Processing – ESWCT Airbus, 31060 Toulouse, France

A R T I C L E I N F O

A B S T R A C T

Article history:

The formation of uniform dispersions from agglomerated carbon nanotubes in polymers is

Received 8 March 2012

a major challenge for their use in composites. We show that agglomerated carbon nano-

Accepted 13 October 2012

tubes on top of a thermoplastic polymer are efficiently dispersed when annealing. Anneal-

Available online 1 November 2012

ing is found to disperse carbon nanotubes surprisingly well. We image the dispersion of the nanotubes in the polymer matrix using electron microscopy and Raman mapping. The dispersion of nanotubes depends on whether the tubes are multi, double or single walled. Highly uniform dispersions are found for multi wall tubes while single and double wall carbon nanotubes show less uniform dispersions. Ó 2012 Elsevier Ltd. All rights reserved.

Carbon nanotubes (CNTs) are light weight, strong and are electrically conducting. This makes them attractive for their use in polymer composites where the high aspect ratio and small diameter reduces the filling factor. Percolation of the nanotube network is often observed in the range of 0.1– 1.0 weight%. Using thermoplastic polymers as a matrix makes it possible to combine the thermal and solvent resistant properties of the polymer with the electrical properties of CNTs. Poly(ether ether ketone) (PEEK) is a thermoplastic polymer [1] which is widely used in aerospace industry [2]. CNTs inherently agglomerate which reduces the advantage of having a high aspect ratio. While individual CNTs have record breaking properties, agglomerated CNTs do not show necessarily the same properties. CNTs can be well dispersed using surfactants [3] but surfactants increase the contact resistance between tubes. Yet the tendency for agglomeration of nanotubes is beneficial for the formation of conductive network since a lower barrier for a charge carrier results in higher conductivity. CNTs agglomerate in bundles and a complex secondary structure is formed depending on their diam-

eter, length distribution or the presence of other forms of disordered carbon. To disperse CNTs in a viscous polymer matrix remains a major challenge [4]. We deposited nanotubes on the polymer substrate and heated the polymer above the melting temperature to study their wetting properties. It has been earlier found using numerical modeling that the diffusion coefficient of polyethylene is 30% larger along the tube axis than perpendicular to the tube axis in a polymer SWNT composite above the glass transition temperature [5]. We expect that a larger diffusion of polymer molecules along the tube axis influences CNT agglomeration. The interface between the filler and the matrix plays clearly a decisive role in the overall properties of the composite [6]. We show here that thermal diffusion can lead to the formation of a surprisingly uniform surface layer with homogeneous CNT distribution. Multi wall carbon nanotubes (MWCNTs) were purchased from Arkema (Graphistrength c100), the MWCNT density is 50–150 kg/m3, with 5–15 layers and an average diameter of

* Corresponding author. E-mail address: [email protected] (W.S. Bacsa). 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.10.033

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15 nm, 0.1–10 lm in length and carbon purity of 90%. Double wall CNTs were produced by catalytic chemical vapor deposition (CCVD) method [7]. Single wall nanotubes (SWCNT) were purchased from Nanocarblab, Russia. We use PEEK as supplied by Victrex, UK (VictrexÒ PEEKTM 90P). To form a polymer film, PEEK powder was mixed with acetone and several droplets were deposited on a glass slide and the sample was backed after evaporation of the solvent at 380 °C (PEEK melting temperature 343 °C) for 10 min in argon atmosphere. Subsequently a dispersion of CNTs was prepared by sonication using acetone. The CNTs were deposited on the PEEK film by placing droplets and annealed at 380 °C for 10 min in argon atmosphere. To analyze the tube distribution the thin films were placed in an epoxy matrix and cut with Ultratome equipped with a diamond knife, in a direction perpendicular to the film interface. Fig. 1(a) shows an optical image across the surface layer where multi wall CNTs diffused into the PEEK substrate. The surface layer containing the nanotubes shows a uniform contrast in the optical image with clear interfaces with PEEK and the epoxy cap layer. Fig. 1(b) shows a Raman line scan across the surface layer in the spectral range of the Raman G and D band. The uniform intensity of the Raman lines across the diffusion layer (Fig. 1c) shows that the CNT distribution is uniform at scales of 700 nm. We have subtracted the luminescent background from the Raman spectra and plotted it separately in Fig. 1c). The luminescence background is strongly reduced in the diffusion layer. This can be explained either by the reduction of the probed volume or by the quenching of photo generated charge carriers by the presence of the CNTs. The tube distribution is uniform across the diffusion layer and the interface between the diffusion layer and PEEK is well defined. To see the dispersion at smaller scale we have used TEM. Fig. 2 shows a TEM cross section at the interface between the diffusion layer and the PEEK matrix. The MWCNTs are uniformly distributed and the interface is well defined down to scales of 100 nm, the average distance between tubes in the image plane. No large agglomerations are observed. Interestingly no concentration changes are observed in the vicinity of the diffusion layer or at scales larger that the mean distance between the tubes. The formation of a uniform diffusion layer from highly agglomerated multi wall tubes is unexpected. In general the diffusion of nanoparticles in a polymer matrix depends on size and shape of the nanoparticle. While the diameter of CNTs is comparable or smaller than the chain length of the polymer, the tube length is several orders of magnitude larger. For individual nanotubes with different mobility depending on diameter and length, one would expect a gradual reduction of the concentration with increasing distance from the surface. However, the fact that the tubes were agglomerated when the solvent evaporated on the surface of the PEEK film indicates that the tubes remain connected in a loose network after annealing acting as a single phase. The TEM images are consistent with what has been observed in the optical image (Fig. 1a). The anisotropy of CNTs has the effect that polymer diffusion along and perpendicular the tube axis is not the same. Brownian motion of CNTs and in particular, the higher diffusion of the polymer molecules along the tube axis appears to

Fig. 1 – (a) Optical cross section image of diffusion layer of MWCNTs in PEEK, (b) Raman line scan image across the diffusion layer, (c) Raman intensity of G and D band and luminescent background signal (excitation wavelength: 780 nm).

have the effect to break up the tube agglomerations. CNTs are conductors or small gap semiconductors while the polymer matrix is insulating. As a result, CNTs are excellent absorbers of electromagnetic radiation. We propose the following mechanism. The heat gradient at the polymer nanotube interface implies increased diffusion of the polymer molecules in the neighborhood of the tubes. In addition, the large anisotropy of the tubes and preferential diffusion along the tube axis has the effect that a diffusion path is formed along the tubes. The high infrared absorption properties of nanotubes

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Fig. 2 – TEM image of the interface between the diffusion layer and the PEEK matrix. The MWCNT distribution is uniform at scales down to 500 nm. compared to the polymer matrix can also lead to selective heating. Regions with higher concentrations of nanotubes absorb more photons and get heated to a higher temperature. The higher temperature increases the diffusion of the polymer molecules on the surface of the nanotube and we suggest that this leads to a more uniform dispersion of the tubes. The higher temperature of the tubes also increases their Brownian motion which contributes to separating the tubes. The properties of macroscopic amounts of CNTs depend on the diameter distribution, number of layers, tube lengths and the presence of other forms of carbon such as disordered forms of graphene layers. We therefore carried out similar diffusion experiments using single wall tubes (SW) and double wall tubes (DW). DWs grown by the CCVD method form long bundles and are more stable compared to single wall tubes. While the length of the tubes is favorable for percolation, long tubes are more difficult to disperse. Fig. 2(a) shows an optical cross section image and a corresponding Raman image when using DWs. The contrast of the film and the interface is found to be less well defined for DWs than for MWs. But again the Raman intensity of the G and D bands does not decrease with thickness. We observe, however fluctuations in the intensity due to areas with a higher concentration of tubes. The TEM image (Fig. 3) shows that the DWs are concentrated in distinct adjacent regions and are not uniformly distributed in the diffusion layer. This can be associated to the secondary entanglement of tube bundles which is still present when the nanotubes penetrate into the polymer matrix. The longer length of the DWs compared to MW has the effect that DWs remain entangled and as a result no clear interface is formed for the diffusion layer at scales of 200 nm. When using SW tubes, the formation of a well defined interface is seen between the diffusion layer and the polymer substrate (Fig. 4). A second interface with less contrast can also be seen. The intensity of Raman G band across the diffusion layer is, however not uniform. While a good part of the tubes is well dispersed some of the agglomerations are still present. This might be due to their length or the fact that small diameter

Fig. 3 – Optical image (a) and Raman line scan perpendicular to polymer interface (b) of DWCNTs in PEEK (excitation wavelength: 780 nm).

Fig. 4 – TEM image of DWCNTs in the diffusion layer. The nanotubes are concentrated in distict adjacent regions due to extensive tube bundling. tubes can be semiconducting absorbing less electromagnetic radiation. We find thermal diffusion of agglomerated multi-wall CNTs lead to a highly uniform diffusion layer where the tubes are homogeneously distributed at scales of 500 nm. This is

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explained by suggesting that a higher diffusion rate of the polymer molecules along the tube axis and higher heat absorption of the nanotubes favors molecular diffusion in the proximity of the tubes. Double wall CNTs form less homogeneous diffusion films and form sponge like agglomerations of tubes. Single wall tubes form a uniform diffusion film which contains few agglomerations of tubes.

Acknowledgements The authors acknowledge financial support from the ministry of industry (MINEFI-INMAT) and the region Midi-Pyre´ne´es.

R E F E R E N C E S

[1] Searle OB, Pfeiffer RH. VictrexÒ poly(ethersulfone) (PES) and VictrexÒ poly(etheretherketone) (PEEK). Polym Eng Sci 1985;8(25):474–6.

[2] Denault J, Dumouchel M. Consolidation process of PEEK/ carbon composite for aerospace applications. Adv Perf Mater 1998;5:83–96. [3] Sandler J, Shaffer MSP, Prasse T, Bauhofer W, Schulte K, Windle AH. Development of a dispersion process for CNTs in an epoxy matrix and the resulting electrical properties. Polymer 1999;21(40):5967–71. [4] Bangarusampath DS, Ruckda¨schel H, Altsta¨dt V, Sandler JKW, Garray D, Shaffer MSP. Rheological and electrical percolation in melt-processed poly(ether ether ketone)/ multi-wall carbon nanotube composites. Chem Phys Lett 2009;482:105–9. [5] Cadek M, Coleman NJ, Barron V, Hedicke K, Blau JW. Morphological and mechanical properties of carbon-nanotube nanotube reinforced. Appl Phys Lett 2002;5123(81):1–3. [6] Deng F, Ogasawara T, Takeda N. Tensile properties at different temperature and observation of micro deformation of carbon nanotubes–poly(ether ether ketone) composites. Compos Sci Technol 2007;67:2959–64. [7] Flahaut E, Bacsa R, Peigney A, Laurent C. Gram-scale CCVD synthesis of double-walled carbon nanotubes. Chem Commun 2003;12:1442–3.

Importance of the rheological properties of resorcinol– formaldehyde sols in the preparation of Cu-doped organic and carbon xerogel microspheres Zulamita Zapata-Benabithe Carlos Moreno-Castilla a,* a b

a,1

, Juan de Vicente b, Francisco Carrasco-Marı´n a,

Departamento de Quı´mica Inorga´nica, Universidad de Granada, 18071 Granada, Spain Departamento de Fı´sica Aplicada, Universidad de Granada, 18071 Granada, Spain

A R T I C L E I N F O

A B S T R A C T

Article history:

The rheological properties of resorcinol–formaldehyde sols at the very beginning of the

Received 31 July 2012

emulsion polymerization process have a profound impact on the final shape of the micro-

Accepted 24 October 2012

particles obtained. This is especially the case with Cu-doped organic hydrogels at reason-

Available online 1 November 2012

ably large temperatures (40 °C), when an abrupt increase in viscoelastic properties is observed within only a few minutes. Ó 2012 Elsevier Ltd. All rights reserved.

Carbon gel microspheres are of interest for adsorption, catalysis, and electrochemical energy storage [1,2]. Carbon gels are prepared by carbonizing organic gels obtained from the sol–gel polymerization of phenolic compounds with formaldehyde in the presence of a catalyst. Organic gel micro-

spheres can be formed by inverse emulsion of sol mixtures that have an adequate rheological performance. It is therefore important to determine the variation in rheological properties of the ingredients as a function of polymerization time. The objective of this study was to study the variation in

* Corresponding author. E-mail address: [email protected] (C. Moreno-Castilla). 1

Permanent address: Facultad de Ingenierı´a Quı´mica, Universidad Pontificia Bolivariana, 050031 Medellı´n, Colombia. 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.10.050