Fabrication of buckypaper via a resin infiltration technique

6 February 2018
Zaheen Ullah Khan
Polymer nanocomposite papers produced through a novel approach exhibit high flexibility, improved thermal stability, and easy processability, and can thus be used in a variety of technical applications.

In recent decades, a great demand has arisen for the production of new flexible, lightweight, robust, and environmentally friendly carbon-based paper-like materials. Indeed, such materials now play an important role in modern technologies, and are of substantial interest for many different applications (e.g., fuel cell and electrochemical energy-storage devices, membrane and chemical filters, actuators, and sensors). For example, so-called buckypaper is a sheet of randomly entangled carbon nanotubes (CNTs), or another nanofiller network, with a porous structure. The distinctive arrangement of the nanofillers means that buckypaper exhibits a range of unique field emission, electrical conductivity, gas permeability, and mechanical properties.1 Moreover, buckypaper can be harnessed to effectively transfer, via matting, the properties of a nanofiller to a matrix in composites. Nonetheless, typical buckypaper nanocomposite preparation techniques can be problematic, e.g., they give rise to poor nanofiller dispersion (and nanoparticle agglomeration) in the matrix.

To date, buckypaper has been synthesized via several different techniques. For instance, the first buckypaper was produced from functionalized CNTs and the Triton-X 100 surfactant, with subsequent ultrasonication.2 In addition, the ‘domino pushing’ technique (and chemical vapor deposition, to grow arrays of multiwalled CNTs on a silicon substrate) has been used to fabricate highly oriented CNT buckypapers.3 Solution-deposition of CNT layers (i.e., by coating a CNT suspension on a plastic substrate of polyethersulfone and polyarylate) with the use of a conventional air-spray gun has also been demonstrated.4 Nowadays, however, buckypaper is usually synthesized through the filtration of a carbon nanofiller on a porous membrane (such as highly ordered anodic aluminum oxide, polytetrafluoroethylene, or polycarbonate5, 6). Moreover, paper-like materials can be produced by assembling graphene oxide into an interlocking-tile ordered arrangement, i.e., under a direction flow, via air/liquid interface self-assembly or simple vacuum filtration.

In this work,7 we have investigated the use of a resin infiltration technique (RIT)—see Figure 1—to fabricate polyvinyl chloride (PVC) nanocomposites containing either p-phenylenediamine (PPD) or functional graphite (F-G), as shown in Figure 2. Although the RIT, which is similar to layer-by-layer assembly, has previously been used to produce buckypaper, our work is the first time this approach has been used to synthesize polymer composites and to thus improve the thermal and mechanical properties of the resultant nanocomposites. By using this technique, the PVC matrix and the F-G fillers are covalently bonded with each other. We also use the PPD functionalization of the graphite to act as a bridge between graphite layers and to thus enhance the dispersion and intercalation of the fillers in the PVC matrix (see Figure 2).

Schematic illustration of the resin infiltration technique. PVC: Polyvinyl chloride. PPD: p-Phenylenediamine. F-G: Functional graphite.

Synthetic scheme for the F-G, PPD-functionalized F-G (PPD/F-G), and PVC/PPD/F-G composite produced in this work. Cl: Chlorine.

Specifically, we fabricated and compared—in terms of morphology, structure, crystallinity, electrical conductivity, and thermal stability—two series of PVC composite papers. To that end, we synthesized PVC/PPD/F-G and PVC/F-G composite papers, and subsequently studied their structural, morphological, thermal, hardness, wettability, and conductivity properties. We also investigated the effect of an increasing amount of F-G and PPD/F-G nanofillers on the properties of the composite papers.

Some of the results of our study are illustrated in Figures 3 and 4. For example, transmission electron microscopy and scanning electron microscopy (SEM) images we obtained for the composites indicate that the graphite nanofillers were better dispersed in the PVC matrix than in the network formation of the PVC/PPD/F-G composite paper. The SEM images in Figure 3(a) and (b) depict the PVC/PPD/F-G and PVC/F-G papers that contained 0.08% F-G. We measured a maximum degradation temperature of 517 and 484°C for these two samples, respectively.

Scanning electron images of the (a) PVC/PPD/F-G and (b) PVC/F-G composite papers containing 0.08% F-G nanofillers.

(a) Variation in the microhardness of PVC/F-G and PVC/PPD/F-G composites. (b) Electronic, or ionic, conductivity (α) of various PVC, PVC/F-G, and PVC/PPD/F-G composite papers.

Our dynamic contact angle analyses indicate that the PVC/PPD/F-G and PVC/F-G composite papers had better surface wettability than neat PVC, and the addition of a nanofiller therefore significantly increased the wettability of the composites. In addition, we show the microhardness of the PVC/PPD/F-G and PVC/F-G papers, as a function of nanofiller content, in Figure 4(a). The PVC/PPD/F-G composites have higher microhardness values than both the PVC/F-G samples and neat PVC. We found that the hardness of the PVC/PPD/F-G and PVC/F-G composites increased to a maximum of 39.67 and 32.76Hv, respectively, for a 0.1% nanofiller loading. Lastly, we illustrate the measured electrical conductivity for some of our samples, as a function of the inverse temperature, in Figure 4(b). The linear change in electrical conductivities indicates a thermally activated process. Furthermore, we measured maximum conductivity values of 8.97 × 10−3 and 5.89 × 10−3Sm−1 for the PVC/PPD/F-G and PVC/F-G paper series, respectively.

In summary, we have used a RIT to synthesize a series of buckypaper-like polymer nanocomposites (i.e., that contain a PVC matrix and graphite nanofillers) for the first time. By thoroughly studying the nanostructure, morphology, mechanical, thermal, electrical, and other characteristics of our samples, we are able to re-evaluate typical buckypaper fabrication processes, and to thus achieve higher quality buckypaper composites in the future. Overall, we find that the high length–diameter aspect ratio of the nanofillers increases the axial modulus and induces low waviness of the composites. The large sonication power and long sonication times involved also substantially affect the fabrication of the composites. We expect to see buckypaper used in a host of future applications, including medical uses, fuel cells, super capacitors and batteries, electromagnetic shielding, aeronautics, automobiles, armor plating, stealth technology, as well as filtration and water purification. In the next stage of our work, we plan to fabricate conductive paper-like materials from different polymers and fillers.


Zaheen Ullah Khan
Shanghai University


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DOI:  10.2417/spepro.006986