Article pubs.acs.org/JPCC
Choosing the Chemical Route for Carbon Nanotube Integration in Poly(vinylidene fluoride) A. Ansón-Casaos,*,† J. M. González-Domínguez,† A. M. Díez-Pascual,‡ M. A. Gómez-Fatou,‡ and M. T. Martínez† †
Instituto de Carboquímica ICB-CSIC, Miguel Luesma Castan 4, 50018 Zaragoza, Spain Instituto de Ciencia y Tecnología de Polímeros ICTP-CSIC, Juan de la Cierva 3, 28006 Madrid, Spain
‡
S Supporting Information *
ABSTRACT: Single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) were functionalized with surface oxygen, methyl, heptadecafluorooctyl phenyl (PFO), and poly(methyl methacrylate) (PMMA) groups by four different experimental routes. Pristine and functionalized carbon nanotubes were integrated in poly(vinylidene fluoride) (PVDF) through a mixing process in acetone followed by hot pressing. The quality of carbon nanotube integration was consistently assessed by electron microscopy observations, electrical conductivity measurements, and dynamic mechanical analysis. An optimal dispersion of the filler was achieved for pristine SWCNTs and MWCNTs, as well as for MWCNTs functionalized with PFO and MMA groups. Those composites containing well-dispersed fillers demonstrated electrical percolation thresholds lower than 1 wt % and conductivities of 10−6−10−4 S/cm, while 2 × 10−14 S/cm was measured for the PVDF matrix. The highest storage modulus, i.e., the greatest filler−matrix interaction, was achieved when MMAfunctionalized nanotubes were utilized as fillers. The storage modulus for the composite containing 3 wt % of the MMA− MWCNT filler was >60% higher than for the matrix. MMA functionalization was found to be the most favorable for the integration of carbon nanotubes, especially MWCNTs, in PVDF. crystallization from a solution or by drawing α-PVDF. The addition of pristine CNTs to PVDF through effective integration methods improves its mechanical properties and greatly increases its electrical conductivity.1−4 Similarly, MWCNTs oxidized by treatment with nitric and sulfuric acid increase the mechanical resistance of PVDF.5,6 Other functionalization strategies, such as grafting poly(methyl methacrylate) (PMMA),7,8 PVDF,9 or polysulfone9 chains to MWCNT surfaces, have been reported to improve PVDF properties. Also, a favorable interaction between methylfunctionalized SWCNTs and PVDF has been theoretically predicted utilizing molecular simulations.10 However, an experimental comparison of those different functionalization routes for the integration of CNTs in PVDF has not been published. The preparation of CNT/PVDF composites can be accomplished by three methods. The first one is melt blending in a mixer machine or an extruder, usually followed by the injection or pressing of the blend, at temperatures around 200 °C. The as-prepared CNT/PVDF composites mainly contain α-phase PVDF and can be easily shaped using the appropriate injection or pressing mold.11,12 Another advantage of melt
1. INTRODUCTION It is well-known that carbon nanotubes (CNTs) demonstrate extraordinary thermal, electrical, and mechanical properties at the nanoscale, which make them interesting for a large number of applications, including their use as fillers in polymeric materials. The synthesis of CNT/polymer composites has been widely investigated during recent years, and different synthetic approaches have been reported depending on the characteristics of the polymer and the CNTs utilized. An important issue is to achieve a homogeneous distribution of the CNTs in the matrix while interfacial interactions are created between the fillers and the polymer chains. Most common strategies for improving CNT integration are the chemical modification of CNT surfaces and physically wrapping CNTs in compatibilizing polymers or macromolecules. In this article, we apply various routes for the modification of single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) with the aim of finding the most favorable approach for their integration in poly(vinylidene fluoride) (PVDF). PVDF is being currently investigated for its outstanding electromechanical properties, dielectric properties, and biocompatibility. There are five crystalline forms of PVDF, the most important ones being the α and β phases. The α-PVDF is easily produced from the polymer melt, while the β-PVDF, which is piezoelectric and pyroelectric, can be synthesized by © 2012 American Chemical Society
Received: March 7, 2012 Revised: July 5, 2012 Published: July 9, 2012 16217
dx.doi.org/10.1021/jp302212m | J. Phys. Chem. C 2012, 116, 16217−16225
The Journal of Physical Chemistry C
Article
Figure 1. Schematic illustration of the preparation procedures for the different functionalized CNT samples: (a) MMA−CNTs; (b) MET−CNTs; (c) PFO−CNTs; (d) MMA−CNTs.
relatively high amounts at a moderately low price. The interesting point is the experimental comparison of composites containing CNTs functionalized by four different routes. The CNT dispersion and the strength of their interaction with the PVDF matrix were examined by electron microscopy observations, electrical conductivity measurements, and dynamic mechanical analysis. It was found that a good correlation exists in the results obtained by the three techniques, which allows the selection of the most suitable integration route among those studied. As the CNT/PVDF composite preparation method was kept identical in all the cases, the results can be directly interpreted in terms of the influence of the surface chemistry on CNT dispersion and adhesion to the matrix. Additionally, we think that our conclusions could be extended to other preparation methods involving real PVDF solvents such as DMAc, DMF, and NMP. β-PVDF is a very attractive material for its piezoelectric and pyroelectric properties. In this work, we studied the integration of functionalized CNTs in α-PVDF, which can be transformed into β-PVDF by poling or drawing. Moreover, α-PVDF is an interesting material by itself. For example, it is worthy of mention that CNT/α-PVDF composites demonstrate a substantial piezoresistivity and could be utilized as strain sensors.17
blending is that it does not need a solvent. However, achieving a homogeneous distribution of the CNTs in the matrix by melt blending requires quite demanding conditions and equipment. The second method for the preparation of CNT/PVDF composites is based on the addition of a CNT dispersion to a PVDF solution under intensive mechanical stirring or/and sonication.3 PVDF can only be dissolved in certain low volatility solvents such as N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), dimethyl sulfoxide, or Nmethylpyrrolidone (NMP),13 which are fortunately quite efficient in dispersing CNTs. CNT/PVDF composites are obtained, after the elimination of the solvent, in the shape of fibers,14 flakes,3,4 or films, which contain PVDF mainly in the β phase.6,15 The third method for CNT/PVDF preparation is an intermediate way between the other two, as the mixture is performed in a high volatility organic liquid that does not really dissolve PVDF but finely disperses it. CNT/PVDF composites with an improved hardness, electrical conductivity, and dielectric constant have been successfully synthesized utilizing acetone1 or ethanol16 as the mixing medium, although neither of them is a real solvent for PVDF. The advantages of the method are simplicity and the ease of mixing medium elimination, which results in composites containing PVDF in the α phase. In the present study, we applied a mixing method in acetone for the preparation of CNT/PVDF composites with pristine and functionalized CNTs. Acetone is considered a good swelling agent for PVDF.13 We utilized commercial SWCNTs and MWCNTs that are easily available and can be purchased in
2. EXPERIMENTAL SECTION 2.1. Materials. SWCNTs (AP-SWNT grade) were purchased from Carbon Solutions Inc., Riverside, CA. This SWCNT powder material is synthesized by the electric arc 16218
dx.doi.org/10.1021/jp302212m | J. Phys. Chem. C 2012, 116, 16217−16225
The Journal of Physical Chemistry C
Article
reactor method using Ni/Y catalyst and contains ∼30 wt % metal residue. The average diameter and length of the SWCNTs is 1.89 and 509 nm, according to atomic force microscopy measurements.18 MWCNTs (NC 7000) were provided by Nanocyl, Sambreville, Belgium. The Nanocyl 7000 series is produced by the chemical vapor deposition process and contains
