Electrical, Rheological, and Mechanical Properties of Polystyrene

Mar 10, 2007 - ... lab using a template-directed method with two different electrode sizes: .... Cyril Sender , Jean Fabien Capsal , Antoine Lonjon , ...
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Ind. Eng. Chem. Res. 2007, 46, 2481-2487

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Electrical, Rheological, and Mechanical Properties of Polystyrene/Copper Nanowire Nanocomposites Bin Lin,† Genaro A. Gelves,‡ Joel A. Haber,‡ and Uttandaraman Sundararaj*,† Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta T6G 2G6, Canada, and Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta T6G 2G2, Canada

The electrical, rheological, and mechanical properties of polystyrene/copper nanowire (PS/CuNW) composites at different CuNW compositions were studied. The copper nanowires were synthesized in our lab using a template-directed method with two different electrode sizes: 90 cm2 (small scale) and 440 cm2 (large scale). The nanocomposites were prepared using solution processing. The electrical percolation threshold of the nanocomposites occurs at a CuNW concentration between 0.50 and 1.00 vol % if the nanowires were produced on the small scale, and between 1.00 and 2.00 vol % if the nanowires were synthesized on the large scale. The dynamic rheological data at 200 °C indicate that the microstructure transition starts at 0.5% with the small-scale nanowires and at 1.1% with the large-scale nanowires, where a combined network of the polymernanowires, polymer-polymer, and nanowire-nanowire connections restrains the relaxation of the polymer chain. The morphological characterization by SEM and TEM reveals that the copper nanowires are dispersed in the polymer matrix both as agglomerates and as single nanowires. Tensile tests show that the Young’s modulus increases, whereas the tensile strength and the elongation at break decrease, when the concentration of the nanowires increases. 1. Introduction One-dimensional (1-D) nanostructures have attracted much attention because of their excellent properties (e.g., electrical, magnetic, and optic) and their potential applications in nanodevices.1 Copper is the third most widely used commercial metal (after iron and aluminum), due to its availability and outstanding properties such as good strength, excellent malleability, and superior corrosion resistance. Moreover, copper has excellent electrical conductivity (second only to silver) and thermal conductivity, and therefore, 1-D copper nanowires (CuNWs) are of particular interest. Polymer reinforced with fillers is a common way to achieve enhanced properties in the production of modern plastics. Polymer nanocomposites, using nanoscale fillers in polymers, have emerged as a new and important class of materials. They usually have properties (e.g., electrical, thermal, and mechanical) that are superior to those of conventional composites. Nanoscale fillers, including clays2 and carbon nanotubes,3-5 are commonly used in preparing polymer nanocomposites; however, metal nanowires (e.g., CuNW) are less studied.6 The nanofillers have at least one characteristic length scale on the order of nanometers, which is about the size of the radius of gyration of a polymer chain, Rg (5-20 nm).7 It is expected that the nanofillers can alter the physicochemical properties of the polymer matrix much more significantly than do the macrofibers incorporated into conventional composites. Conductive nanofillers, such as carbon nanotubes and metal nanowires, are desirable because the resultant polymer nanocomposites could be used as antistatic, electrostatically dissipative, and electromagnetically shielding and absorbing materials.8 The nanowires and nanotubes have a high aspect ratio (length to diameter, ∼100-1000), which enables the formation of a * To whom correspondence should be addressed. Tel.: (780) 4921044. Fax: (780) 492-2881. E-mail: [email protected]. † Department of Chemical and Materials Engineering. ‡ Department of Chemistry.

conducting network at a much lower concentration than that of conventional fibers. For example, the electrical percolation threshold (at which the electrical volume conductivity increases suddenly by decades) for polymer composites using multiwalled carbon nanotubes is only 1-3 wt %, whereas it is 10-15 wt % using carbon fibers and 15-20 wt % using carbon black.9 Copper nanowires with high aspect ratios (50-500) have been synthesized by the template-directed method, as we have previously reported.6,10-12 Nanocomposites with enhanced electrical properties using copper nanowires have been found in our previous studies.6,12 In this paper, a detailed study of the properties of polystyrene/copper nanowire (PS/CuNW) composites will be presented. The rheological properties of PS/ CuNW nanocomposites with different CuNW contents are examined and used to detect the rheological percolation threshold. Morphological characterization is used to study the microstructures of the nanocomposites, and tensile tests are performed to determine the strength and modulus of the composites. 2. Experimental Section 2.1. Synthesis and Liberation of Copper Nanowires. Copper nanowires were produced using two different size aluminum (Al) electrodes, 5 cm × 11 cm and 10 cm × 25 cm, which are referred to as 90 cm2 (or small-scale) and 440 cm2 (or large-scale) electrodes, respectively. The Al was anodized according to the two-step method of Masuda and Fukuda13 to produce porous aluminum oxide templates (PAO). Then, copper nanowires were electrodeposited using 200 Hz, 10 Vrms sine waves between copper plates and the PAO electrode in a copper electrolyte solution. Then, the alumina was dissolved in 1 M sodium hydroxide (NaOH) solution to harvest CuNWs in gram quantities (∼0.8 g/batch for the small scale and ∼3 g/batch for the large scale).11 More detailed experimental procedures are described elsewhere.6,11 2.2. Preparation of Polymer Nanocomposites. Polystyrene (PS, Styron 666D) was kindly provided by Dow Chemical Co.

10.1021/ie061285c CCC: $37.00 © 2007 American Chemical Society Published on Web 03/10/2007

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The densities of copper and PS are 8.96 and 1.04 g/cm3 at 25 °C, respectively. In this paper, we assume that the CuNWs have the same density as bulk copper. Methylene chloride (CH2Cl2, HPLC grade) was purchased from Fisher Scientific and was used as received. PS/CuNW composites with CuNW concentrations at 0.5, 1.0, 2.0, 3.0, and 4.0 vol %, calculated from mass percentage using the densities specified above, were prepared. The concentrations of CuNW in this paper are expressed in volume percentage, unless otherwise specified. CuNWs were dispersed in a 1.2 wt % solution of polystyrene in an ultrasonic bath for 1 h. The suspension was first cast in an evaporation dish where the solvent was evaporated, and then the cast film was dried in a vacuum oven at 50 °C for 24 h to obtain a film with a thickness of around 120 µm.6,12 The film was chopped into small pieces (∼4 mm2), which were then pressed in a stainless steel mold to prepare either a rectangular sheet (L × W 7 × 6 cm and 0.5 mm thickness) for mechanical tests or a circular sheet (25.4 mm diameter and 0.8 mm thickness) for electrical volume resistivity measurements. During the preparation of the thin sheets, the small pieces of the chopped nanocomposite were separated from the steel plates by aluminum foils on each side. The samples were first pressed by, and then kept in, the heated compression plate press (Carver No. 2086) at 200 °C and 1.5 metric tons for 5 min. 2.3. Electrical Resistivity Measurements. The thin circular sheets prepared by compression molding were used for electrical resistivity tests. The electrical volume resistance was measured with a Keithley electrometer Model 6517 and an 8009 Resistivity Test Fixture equipped with ring electrodes. An alternate polarity method was used for the measurements utilizing the high resistance measurement software Keithley 6524. Immediately prior to making measurements, the surfaces of the samples were cleaned with ethanol. According to ASTM D4496 and D257, the measured volume resistance, RV, was converted to volume resistivity, FV, using

A FV ) RV t

(1)

where A is the effective area of the measuring electrode and t is the average thickness of the specimen. The samples were preconditioned for 24 h at 0% humidity before the measurements. Voltages of 10 or 100 V were used for conductive or resistive samples, respectively. The reported resistivity data was the average of at least three different samples. 2.4. Rheological Characterization. Dynamic rheological characterizations were performed in a Rheometrics RMS800 rheometer with a 25 mm parallel plate fixture at 200 °C under nitrogen atmosphere. The sample disks used for rheological measurements were the same circular sheets used initially for electrical measurements. Frequency sweeps were performed at low strains, 0.09-10%, where the materials show linear viscoelastic behavior. 2.5. Scanning Electron Microscopy (SEM). The lengths of the synthesized copper nanowires and the morphologies of the fracture surfaces of the composites were analyzed using a JEOL 6301F field emission scanning electron microscope (SEM). Before imaging, the samples were sputter-coated with chromium. All the SEM micrographs were taken at an accelerating voltage of 20 kV. For length analysis, the CuNWs were dispersed in methanol (CH3OH) using ultrasound for 60 min immediately after liberation from the PAO template.11 The resultant suspension was then spin-cast at 3000 rpm onto 2.5 cm × 2.5 cm glass slides. Through this process, the nanowires

Figure 1. TEM micrographs of (a) the small-scale synthesized CuNW at a lower magnification and (b) a single copper nanowire at a higher magnification, where the inset shows the corresponding SAED. Note the scale bars.

were well-dispersed, the samples were imaged using SEM, and the lengths of >1000 individual nanowires were measured using the Image J image analysis software program (Wayne Rasband, National Institutes of Health, USA, http://rsb.info.nih.gov/ij). 2.6. Transmission Electron Microscopy (TEM). The synthesized nanowires dispersed in methanol were sonicated for 10 min. A drop of the suspension was placed on a molybdenum grid coated with carbon film. Immediately after the methanol was evaporated, the nanowires were examined with a JEOL 2010 TEM equipped with a thin-window energy-dispersive X-ray spectrometer (EDS) at 200 kV. Small pieces of PS/CuNW composites cut from the rectangular sheet were embedded in the epoxy resin. The embedded sample was then ultramicrotomed to give sections with a thickness of around 70 nm using an Ultracut diamond knife at room temperature. The sample was cut in two directions: one was parallel to the flat surface of the pressed sheet and the other was perpendicular to the flat surface or, in other words, parallel to the cross section of the sheet. The thin sections of the composite sample were examined by TEM with a Philips Morgagni 268 microscope at an acceleration voltage of 70 kV. 2.7. Tensile Test. The tensile tests were performed in an Instron 4200 tensiometer according to ASTM D-638. The dogbone-shaped specimens with gauge lengths of 7.62 mm were prepared from the molded rectangular sheets. The experiments were done at 21 °C without preconditioning the samples. Crosshead speed was set up at 5 mm/min. Young’s modulus was calculated by linear regression of the stress data versus strain data from the initial strain to the data before the maximum strain. For each sample, the data reported are the average of four to five specimens. 3. Results and Discussion 3.1. Copper Nanowires. Figure 1shows the TEM images of the freshly synthesized copper nanowires obtained from the smaller scale process (using 5 cm × 11 cm electrodes). The diameter of the nanowires is around 25 nm. The copper nanowires appear to be smooth at the surface. The inset in Figure 1b is the selected-area electron diffraction (SAED) pattern of the single copper nanowire, which shows that the nanowire is polycrystalline. Figure 2 shows the length distribution of the copper nanowires synthesized from both the small and large electrodes. The nanowires synthesized in the smaller electrodes show a wider length distribution and 7.2% of the nanowires have lengths greater than 4 µm; however, the nanowires from the larger electrodes only have 1.2% longer than 4 µm. The average length of the nanowires is 1.78 ( 1.37 µm for the small-scale process, which is longer than that obtained from the large-scale process, 1.29 ( 0.83 µm.

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Figure 3. Effect of CuNW content on volume resistivity of PS/CuNW composites. The composites were prepared with small-scale copper nanowires and large-scale copper nanowires.

Figure 2. Length distribution of the synthesized CuNW from (a) smallscale and (b) large-scale production.

3.2. Electrical Resistivity. Figure 3 shows the effect of CuNW content on the volume resistivity of PS/CuNW. Copper nanowires synthesized in the small-scale and large-scale systems were used in the composite preparation. The copper nanowires produced using the small-scale system were stored under nitrogen immediately after synthesis and used for composite preparation within 1 week, and these nanowires are called smallscale CuNW; whereas, the nanowires produced using the largescale system were exposed to air for ∼30 min during the preparation and then stored under nitrogen atmosphere for about 4 months before the preparation of the nanocomposites, and these nanowires are called large-scale CuNWs. Pure PS has a high volume resistivity on the order of 1017 Ω‚cm. The volume resistivity starts to decrease when the CuNW content is greater than 0.25% and 0.50% for the composites using the small-scale and the large-scale nanowires, respectively. For the composites prepared with the small-scale copper nanowires, at 1.00% CuNW, the volume resistivity is decreased below 108 Ω‚cm, which is around 8-9 orders of magnitude less than the pure polymer. When the CuNW composition is greater than 1.00%, a slower decrease in resistivity is observed (Figure 3). Therefore, the electrical percolation threshold, the dramatic reduction in the volume resistivity, occurs at a concentration between 0.50 and 1.00% for the composites prepared with the small-scale copper nanowires. The electrical

percolation is between 1.00% and 2.00% for the sample with the large-scale composites (Figure 3). The lengths of the nanowires synthesized from the small and large electrodes are different. The smaller electrodes produce longer nanowires, or in other words, the aspect ratio is higher for the nanowires prepared in the smaller electrodes. Therefore, a lower percolation threshold is expected for CuNW from the small-scale production. For the PS/CuNW prepared with the large-scale nanowires, the volume resistivity data are generally 2-3 orders of magnitude higher than those of the composites made with the small-scale nanowires. Studies have shown that copper nanowires14,15 and copper nanotubes16 are easily oxidized when exposed to air. Toimil-Molares et al.14 investigated the electronic transport properties and oxidation processes of individual copper nanowires and found that there is 1 order of magnitude increase in the electrical resistivity because of the aging of the copper nanowire and the development of cuprous oxide (Cu2O). 3.3. Tensile Tests. The tensile tests were performed on the composites with the large-scale nanowires because the largescale synthesis produces enough nanowires for tensile tests. Figure 4 plots the Young’s modulus, tensile strength, and elongation at break at different nanowire concentrations. For the small-scale copper nanowires, the composite with 1.00% CuNW was tested for comparison. Generally, Young’s modulus increases as the nanowire content increases (Figure 4a). There is an about 19% increase in modulus at 2.00% CuNW and a 28% increase in modulus at 4.00% CuNW. However, as more nanowires are added to the PS matrix, the tensile strength decreases (Figure 4b) and the material becomes more brittle, which can be seen from the decreasing elongation at higher nanowire concentrations (Figure 4c). The tensile strength is lower at high concentrations of nanowires presumably because the adhesion between the nanowires and the polymer is weak. Improved adhesion may be achieved by functionalizing the nanowire and/or polymer to create a better bond between the two materials. This will be the subject of another study. At 1.00%, the composite made with small-scale nanowires shows slightly lower modulus, tensile strength, and elongation than the composite made with large-scale nanowires, but the values are within the error bars. 3.4. Rheological Properties. At 200 °C and low frequencies, PS chains are fully relaxed and exhibit fluidlike behavior. In the presence of the nanofillers, polymer relaxation will be

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Figure 5. Complex viscosity at 200 °C for PS/CuNW composites with (a) small-scale copper nanowires and (b) large-scale copper nanowires.

Figure 4. (a) Young’s modulus, (b) tensile strength, and (c) elongation at break of PS/CuNW at different CuNW contents.

restricted and a network between polymer chains and nanofiller particles may be formed, so that nanofillers linked by polymer chains have solid-like behavior.17,18 The onset of solid-like behavior suggests the formation of a rheologically percolated

network, which could be detected using the dynamic rheology measurements. Figure 5 shows the complex viscosity versus frequency at 200 °C for PS/CuNW at different CuNW compositions. The complex viscosity increases as the CuNW content increases, and at the same nanowire concentration, a higher viscosity value is found at the low frequency (0.1-1 s-1) for the composite made with the small-scale copper nanowires (Figure 5a) than is found with the composite made with the large-scale nanowires (Figure 5b). A plateau at low frequency exists when the smallscale CuNW composition is less than 0.50% (Figure 5a) or when the large-scale CuNW composition is lower than 1.00% (Figure 5b). When the CuNW content is increased to 1.00% and 2.00% for the nanocomposites with small- and large-scale nanowires, respectively, an increase in the viscosity at low frequency (0.1-1 s-1) is obvious, which is indicative of a yield stress. It has been shown that the existence of a yield stress is associated with nanofiller-polymer interactions and formation of combined network of polymer chains and nanofiller. Similar phenomena have been reported in the literature for other types of nanofiller in polymer.17-21 Figure 6 plots the storage modulus (G′) versus different frequencies at different CuNW compositions for small-scale CuNWs (Figure 6a) and for large-scale CuNWs (Figure 6b). As the CuNW concentration increases, G′ increases, especially

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Figure 7. Storage modulus at 0.1 s-1 for PS/CuNW composites at 200 °C.

Figure 6. Storage modulus at 200 °C for PS/CuNW composites with (a) small-scale copper nanowires and (b) large-scale copper nanowires.

at the low frequencies. Figure 6a shows the storage modulus of the composites with the small-scale copper nanowires, which has higher values at lower frequencies than the composite with the large-scale nanowires, as shown in Figure 6b. The lowfrequency storage modulus (G′) has been used to determine the threshold of the rheological percolation:18

G′ ∝ (m - mcG′)βm,G′

(2)

where m is the mass of the nanofiller, mcG′ is the mass concentration threshold of the rheological percolation, and βm,G′ is the critical exponent based on the reduced mass fraction. Figure 7 gives the storage modulus data at a low frequency, 0.1 s-1, versus the volume percentage of CuNW. We used the reduced volume fraction assuming that the density of the nanowires is constant, i.e.

G′ ∝ (V - VcG′)βV,G′

(3)

where V is the volume of the nanofiller, VcG′ is the volume threshold of the rheological percolation, and βV,G′ is the critical exponent based on the reduced volume fraction. According to eq 3, we find that VcG′ is 0.5% and 1.1% for the nanocomposites with the small-scale and large-scale nanowires, respectively. Therefore, the rheological percolation threshold for the composite with the small-scale nanowires is lower than that with

the large-scale nanowires, which is consistent with the electrical volume resistivity data. The nanocomposite is seen to show a transition from fluidlike behavior to solid-like behavior as the nanowire concentration increases (Figures 5-7). As more nanowires are added, the restriction of the polymer chain relaxation becomes obvious. The rheological percolation threshold is 0.5% and 1.1% for the composites with the small-scale and large-scale copper nanowires at 200 °C, respectively, which is somewhat lower than the electrical percolation of the systems. This difference is expected, since the rheological percolation threshold, unlike the electrical percolation threshold, originates from a combined nanowirepolymer network (including the polymer-polymer entanglement network, the nanowire network, and the nanowire-polymer network) instead of the nanowire network alone.17,18 Therefore, the percolation will occur when entanglement networks form and will not require the nanowires to come close enough to each other to allow for charge hopping. 3.5. Morphology Characterization. Backscattered electron micrographs of a sample cross section perpendicular to the pulling direction of a 2.00% large-scale CuNW sample after the tensile test are shown in Figure 8. Figure 8a gives the general features of the dispersion of the CuNW inside the polystyrene at low magnification. There are Cu nanowire agglomerates (big bright spots) along with well-dispersed nanowires (tiny white spots) in the polymer matrix. Figure 8b presents single nanowires that are well dispersed; these are highlighted by white rectangular boxes. Figure 9 shows some typical TEM micrographs of the PS/ CuNW with 2.00% large-scale CuNW. Figure 9a,b shows the cross-sectional view of the thin sheet. Some nanowires are well dispersed and others are agglomerated (Figure 9a). Figure 9b gives the enlarged view of the dotted white rectangular box shown in Figure 9a. Here, segregated nanowires and nanowire agglomerates are visible. Figure 9c-f shows the representative micrographs of the composite cut parallel to the flat surface of the thin sheet. Here too, along the flat surface of the thin sheet, part of the sample is well dispersed (Figure 9c,d) and part of it is full of agglomerates (Figure 9e,f). The agglomerates likely formed during solution processing, i.e., during the preparation of the thin composite film, when the copper nanowires settled to the bottom of the dish as the solvent evaporated. The coexistence of the agglomerates and single nanowires in both the flat and cross sections of the sample suggests that the distribution of the nanowires is similar throughout the sample. The copper nanowires in the micrographs appear to be stiff, and no entanglement of nanowires exists. They tend to be

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Figure 8. Backscattered SEM micrographs of the PS/CuNW composite (2% CuNW, large scale) (a) at low magnification and (b) at high magnification, where the dotted rectangular boxes are drawn to show the individual copper nanowires. Note the scale bars.

around 25 nm. The observed length is shorter than that of the originally synthesized nanowires (Figure 2) because (1) they may have broken down during the 1 h sonication of the nanocomposite preparation and (2) the TEM shows only parts of the nanowires that are visible in the 70 nm thin sections;22 we may have cut through wires during sample preparation. It is worthwhile to mention that, in the TEM micrographs (e.g., Figure 9f), some copper oxide formed at the surfaces of the nanowires. Therefore, it seems that the large-scale copper nanowires have already oxidized at the surface during their 4 month storage in the glovebox. SEM and TEM micrographs indicate that the copper nanowires inside the polystyrene matrix are still not well dispersed, because of the coexistence of the nanowire agglomerates along with well-separated individual nanowires. Complete deagglomeration of nanowires is difficult to obtain due to high van der Waals interactions. The nanocomposites were first prepared in the ultrasonic bath and then compression molded in the hot press at 200 °C. Figure 9e,f shows that there are some voids between the nanowires and the polymer matrix, probably due to the weak adhesion between the polymer and the nanowires. Due to the poor adhesion and the poor dispersion of the nanowires, the improvement in the tensile properties is small. To get better dispersed nanowires, appropriate shear and optimum mixing conditions need to be applied. Functionalizing the polymer23 and/or nanowires is another important way to improve the interfacial adhesion and to obtain well-dispersed nanowires. 4. Conclusion We studied polystyrene/copper nanowire composites at different CuNW compositions. The electrical percolation threshold of the composite is low, between 0.25% and 1.00% for the composites with small-scale nanowires and 1.00% and 2.00% for the composites with large-scale nanowires. The rheological properties of the composites indicate that a change in microstructure starts at a concentration of 0.5% and 1.1% CuNW at 200 °C for the composites with the small-scale and large-scale nanowires, respectively, where a network of polymer-nanowires, polymer-polymer, and nanowire-nanowire restrains the relaxation of the polymer chain. SEM and TEM micrographs show that the copper nanowires are dispersed as agglomerates and as single nanowires. The mechanical properties of the nanocomposites are improved compared to the pure polymer, but the improvement is relatively small. The Young’s modulus increases by 28% with addition of 4.00% CuNWs. Acknowledgment We would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for the financial support of this work. We thank Mr. Zakari T. M. Murakami for helping with the sample preparation and Mr. Baoliang Shi for helping with the tensile tests. Literature Cited

Figure 9. TEM micrographs of PS/CuNW (2% CuNW, large scale): (a, b) cross-sectional view and (c-f) flat surface view of the thin sheet. The rectangles drawn in (a), (c), and (e) indicate the enlarged area shown in (b), (d), and (f), respectively. Note the scale bars. Note the band of nanowires in (a) and (e), most likely due to the settling of CuNW during solution processing.

oriented in every direction, which helps in obtaining nanowire networks. The length of the copper nanowires observed in the TEM is in the range of 100-1000 nm, and the diameter is

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ReceiVed for reView October 7, 2006 ReVised manuscript receiVed January 22, 2007 Accepted January 26, 2007 IE061285C