In the Laboratory
Preparation of Electrically Conductive Polystyrene/Carbon Nanofiber Nanocomposite Films Luyi Sun, Jonathan Y. O’Reilly, Chi-Wei Tien, and Hung-Jue Sue* Polymer Technology Center, Department of Mechanical Engineering, Texas A&M University, College Station, TX; *
[email protected] Nanoscience and nanotechnology have received much attention and significant progress has been achieved in the past decade. Many universities and national labs have established research centers on nanoscience and nanotechnology (1). Following this trend, it is important to introduce nanoscience and nanotechnology related content into undergraduate science and engineering curricula. We present a simple experiment to introduce science students to the usefulness of polymer nanocomposites for nanotechnology applications. Polymer nanocomposites are generally defined as polymer composites containing fillers with at least one of the dimensions in nanometer range, that is, less than 100 nm (2). This experiment includes preparation of polystyrene/ carbon nanofiber (PS/CNF) nanocomposite films, electrical conductivity measurement, morphology observation, and related theory for conductive polymer nanocomposites. Most nanoscience experiments published in scientific journals require specialized and expensive instruments, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), scanning tunneling microscope, atomic force microscopy, and X-ray photoelectron spectroscopy (3–6). Such instruments are usually not accessible in undergraduate teaching labs. Also, their operation, which requires advanced training, is usually too advanced for novice students. Furthermore, many published experiments utilize hazardous chemicals or air- and moisture-sensitive chemicals, which are not suitable for an undergraduate science lab environment (7). Thus, a simple and safe experiment is needed for undergraduate teaching. Here we present a simple experimental procedure to prepare conductive PS/CNF polymer nanocomposite films. The procedure and the materials used have been designed and modified for an undergraduate science lab.
1. CNF (8) is used as an alternative to carbon nanotubes (CNT). CNT is expensive, potentially toxic (9), and difficult to handle without proper training. CNF can be considered a larger size of CNT and is inexpensive and commercially available. CNF can also be dispersed in various media better and more easily. The average diameter of CNF is less than 100 nm, which is still considered a nanofiller. 2. A regular sonication bath, instead of a high-energy sonication probe, is utilized to disperse CNF. 3. No surfactant is used. Use of surfactant may complicate the experiment. 4. Direct coating of PS/CNF onto a silicon wafer is chosen to prepare PS/CNF film specimens instead of spin coating, which requires a spin coater and coating skill to obtain high quality specimens. 5. Optical microscopy (OM), rather than TEM, is performed to observe the formation of CNF networks in the nanocomposite films.
Several educational objectives are incorporated into this experiment. In the lab, the sonication process is introduced to help disperse CNF. The OM characterization can directly show the formation of a CNF network in PS. The conductivity measurement can demonstrate how non-conductive PS film is transformed into conductive film by adding a small quantity of CNF filler. In addition, based on the experimental results, the students can easily understand the percolation theory for conductivity. Background and Theoretical Concepts Traditionally, electrically conductive fillers, such as carbon black (CB), graphite, and metal particles, flakes, or fibers, have been used to prepare conductive polymer composites because of their electrical conductivity (10). However, a high conductivity usually cannot be achieved until a high loading of filler is incorporated. Usually for composites containing conductive fillers, a sharp transition in conductivity occurs when the filler content reaches a critical value. This critical value is defined as the percolation threshold in the percolation theory (11). The percolation threshold is mainly governed by the level of dispersion and the geometry that the fillers possess. As the filler content is gradually increased, a network of filler phase in the polymer is formed at the percolation threshold. When the fillers are well dispersed, the aspect ratio (length-to-diameter ratio) of the filler can significantly influence the percolation threshold value. Low aspect ratio fillers, such as CB particulates, require a much higher loading than high aspect ratio fillers, such as CNF, to form a percolation network when they are equally well dispersed. The formation of a percolation network for particle filler and fiber filler is illustrated in Figure 1. Lowering the percolation threshold of a composite can help achieve adequate conductivity at low filler loadings. Literature has shown that nanoscale dispersion of high aspect ratio fillers
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Figure 1. Formation of percolation networks: (A) particle and (B) fiber filler.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 8 August 2008 • Journal of Chemical Education
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In the Laboratory
in a polymer matrix is one of the most effective ways to reduce the percolation threshold (12, 13). CNF has a similar structure to multi-walled carbon nanotubes but is much less expensive. It is also easier to disperse and safer for students to handle owing to its relatively large dimensions compared to CNT. Thus in this study, CNF is selected as a filler to prepare electrically conductive PS/CNF nanocomposite films.
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Experiment This experiment can be performed in two 3-hour lab periods. The first lab period is for sample preparation, including weighing samples, preparing solutions, and casting thin films. Considering six or more samples are required for the entire experiment, it is recommended that groups be formed to conduct the experiments. This allows each student to get hands-on experience with sample preparation. It also helps to develop collaborative learning environments for students within a group. The samples require one hour ultrasonication before film casting. The instructors can use this time to discuss the details of polymer nanocomposites and percolation theory. It would also be a good opportunity to let students discuss their ideas and opinions of nanoscience and nanotechnology. After the film casting, it usually takes 8–12 hours to completely dry the films. The instructor can either collect the samples for the students or assign one student from each group to do so. The second session is for sample characterizations, including thickness measurements, electrical conductivity testing, and morphology observations. Detailed procedures for sample preparation and characterization can be found in the online supplement. The conductivity testing device may not be available in a typical chemistry lab, but it is commercially available from Lucas Labs (Gilroy, CA). Hazards CNF is fluffy and needs to be handled with care. Students are required to wear a mask, goggles, and gloves when transferring and weighing CNF. THF is harmful by inhalation, ingestion, and skin contact can cause dermatitis. The sample preparation and drying procedures should all be performed under a hood to avoid inhalation of fumes.
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Figure 2. SEM images of CNF at (A): low magnification and (B) high magnification.
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Carbon Nanofiber Content by Mass (%) Figure 3. Electrical conductivity vs CNF loading of the conductive PS/CNF films.
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Results and Discussion The SEM images of the CNF used in this study (from Applied Sciences Inc., Cedarville, Ohio) show that the sample consists of curved and intertwined fibers having an average diameter of ca. 80 nm (Figure 2). During the lab session, this portion of experiment can be omitted. The instructors can either perform the characterization in advance or show the micrographs provided by the manufacturer. The conductivities of dried PS/CNF nanocomposite films as a function of CNF loading are plotted in Figure 3. The conductivity increases correspondingly with an increase of CNF loading. A sharp jump on the conductivity is observed at a loading between 0.5 and 1.0 mass%. This indicates that the percolation threshold for this series of samples should be between 0.5 and 1.0 mass%. This value is much lower than the reported percolation threshold at about 10 mass% for CB filled polymer composites (14). The observed low percolation threshold is due to the good dispersion and high aspect ratio of CNF. The CNF conductive filler has a high aspect ratio and can help form an extensive conductive network within the PS matrix that increases electrical conductivity. When the mass percentage of the CNF is increased to 3.4 mass%, the conductivity of the composite film reaches 1.3 × 10‒3 S/cm. This value is approximately nine orders of magnitude higher than the conductivity of pure PS film and is above the requirement
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In the Laboratory
for static dissipative applications (15). Both the low percolation threshold and the high conductivity at 3.4 mass% indicate that a high degree of dispersion of CNF has been achieved in the PS matrix. The OM images of the dried PS/CNF nanocomposites are shown in Figure 4. It is clear that when the CNF loading is at 0.5 mass%, no network formation can be seen using OM (Figure 4A). When the CNF loading is increased to 1.0 mass%, a network has formed (Figure 4B). It is consistent with the conductivity jump from 0.5 to 1.0 mass% shown in Figure 3. This also confirms that the percolation threshold for this series of composite samples is between 0.5 and 1.0 mass%. When the loading is increased to 3.4 mass%, there is still no apparent aggregation of CNF in the PS matrix. This helps explain why the conductivity continues to increase.
Summary This experiment gives students a brief introduction to nanoscience and shows them how nanoscience and nanotechnology can change the property of a material, and thus how it might change our world and our future. This experiment is essentially a combination of one general chemistry experiment (sample preparation) and one analytical chemistry experiment (film characterization). Thus, the students should not encounter any specific difficulty during the experiment. It should be noted that after the completion of the experiment, the students became interested in participating in nanoscience and nanotechnology research. It is expected that more students will be similarly attracted to the nanoscience field after they experience the benefit of nanoscience through this simple experiment. Acknowledgments The PI would like to thank the Air Force Research Laboratory Department of the Air Force Minority Leaders Program Contract #FA8650-05-D-1912 for their financial support. J. Y. O’Reilly thanks the National Science Foundation Grant No. 0453578, Air Force Office of Scientific Research, U.S. Air Force, Department of Defense, and NASA Cooperative Agreement No. NCC1-02038 for sponsoring the Research Experiences for Undergraduates Program during Summer 2006. We also gratefully acknowledge Applied Sciences Inc. for the CNF sample and Dow Chemical Co. for the PS sample.
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Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Aug/abs1105.html Abstract and keywords 10 Nm
Figure 4. OM images of PS/CNF nanocomposite films containing different concentrations of CNF: (A) 0.5, (B) 1.0, and (C) 3.4 mass%.
Full text (PDF) with links to cited JCE articles Supplement Student handouts
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