J. Phys. Chem. C 2009, 113, 7223–7226
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Carbon Nanotube Enhanced Gripping in Polymer-Based Actuators Fei-Peng Du,† Chak-Yin Tang,*,‡ Xiao-Lin Xie,*,† Xing-Ping Zhou,† and Li Tan*,§ Hubei Key Laboratory of Materials Chemistry and SerVice Failure, School of Chemistry and Chemical Engineering, Huazhong UniVersity of Science and Technology, Wuhan, 430074 China, Department of Industrial and Systems Engineering, The Hong Kong Polytechnic UniVersity, Hung Hom, Hong Kong, China, and Department of Engineering Mechanics, Nebraska Center for Materials and Nanoscience, UniVersity of Nebraska, W306 Nebraska Hall, Lincoln, Nebraska 68588-0526 ReceiVed: August 29, 2008; ReVised Manuscript ReceiVed: February 10, 2009
Ionic polymer membrane cast from the mixture of poly(sodium 4-styrenesulfonate-co-acrylic acid) (PSA) and polyvinyl alcohol (PVA) was used as an electromechanical actuator. Surface modified multiwalled carbon nanotubes (MWNTs) were used as an efficient element to enhance the gripping of this actuator. In particular, a thin layer of PSA with a thickness of 12 nm is uniformly grafted on the surfaces of MWNTs. The watersoluble PSA-g-MWNTs can be homogeneously dispersed in the PSA/PVA membrane with a loading ratio of up to 20 wt %. Such a uniform dispersion generated many unique properties in this composite membrane, including enhanced toughness, relatively constant ionic-exchange capacity, and prominent structure integrity after water uptake. We argue that all these novel material properties are due to a removal of the interface mismatch between the MWNTs and the polymer membrane. As a result, a much-enhanced gripping is resulted from electrical stimuli, an indication of a promoted electromechanical coupling. When the loading of PSAg-MWNTs reaches more than 10 wt %, the small oscillation in the mechanical output of the actuator vanishes. Introduction Electromechanical actuators have the ability to transform electrical signal to mechanical work; or in other words, they can deform. Particularly, when the actuator consists of ionic polymers, outstanding actuation can be realized under a low DC voltage (1-5 V) with a relatively prompt response (several seconds). In addition to their light weight, fracture tolerance, and excellent processability, these polymer-based actuators have caught strong attentions in the field of material science and chemistry.1-3 While artificial muscles, fish-robots, catheter systems, and micropumps have been demonstrated based upon this concept,4 the main drawback for polymer-based actuator is its weak electromechanical coupling, giving rise to a nontrivial mechanical relaxation, or a soft gripping, under a continuous electrical stimulus. The discovery of carbon nanotubes (CNTs)5 and related nanotechnology,6 on the other hand, really inspired scientists to look for new means to improve the soft actuation in ionic polymers. To highlight a few, in 1999, Baughman et al.7 discovered that thin sheets of single-walled carbon nanotube (SWNT) have quantum-chemical and double-layer electrostatic effects, triggering an electrochemical-based actuation. And Hughes et al.8 found the multiwalled carbon nanotube (MWNT) a good electromechanical actuator in 2005. However, a much reduced entanglement between nanotubes and the resulting weak van der Waals interaction brought in unwanted creep and brittleness. Compounding carbon nanotubes (CNTs) with polymers, such as Nafion, epoxy, polyvinyl alcohol (PVA), cellulose, and polyaniline could optimize actuation to a certain degree.9-14 For example, SWNT/epoxy actuator has a higher elastic modulus * To whom correspondence should be addressed. E-mail: xlxie@ mail.hust.edu.cn (X.-L.X.);
[email protected] (C.-Y.T.); ltan4@ unlnotes.unl.edu (L.T.). † Huazhong University of Science and Technology. ‡ The Hong Kong Polytechnic University. § University of Nebraska.
and strength compared to the above SWNT actuator.11 However, a further improvement requests overcoming the issue of surface mismatch between the polymer matrix and CNTs. A weak interfacial adhesion suggests a poor electromechanical coupling. For instance, actuation in the composite of Nafion/SWNT showed no significant enhancement.9 To address this interface issue, we focused our efforts on the surface of CNTs before compounding, in the hope of enhancing the gripping in the final polymer-based actuators. We blend poly(sodium 4-styrenesulfonate-co-acrylic acid) (PSA) and PVA together as the polymer membrane and MWNTs as the strengthen element. Surfaces of these MWNTs are modified by PSA. The proofof-concept actuation is carried out by biasing the membrane of PSA/PVA (with or without PSA-modified MWNTs) and the deflection is recorded under a continuous electrical stimulus. Experimental Section Reagents and Materials. MWNTs were purchased from Chengdu Organic Chemicals Co., Ltd. (China). PVA was obtained from Kuraray (Japan) (the degree of polymerization is 2400 and degree of hydrolysis is 98∼99%). All other chemicals were purchased from Shanghai Reagents Co. and used as received. Synthesis of PSA and PSA-Grafted MWNTs. Ionic polymer, PSA, was synthesized in an aqueous mixture of sodium 4-styrenesulfonate (S) and acrylic acid (A) (WS/WA ) 1:1). Ammonium persulfate is used as the initiator (1.0 wt % of the total amount of S and A). The reaction is carried out at a temperature of 85 °C, and the final consumption efficiency of S and A is 90%. The molecular weight (Mw) and polydispersity index (PDI) of the final product (PSA) are 3.26 × 104 g/mol and 2.87, respectively. PSA-grafted MWNTs (designated as PSA-g-MWNTs) was synthesized following our earlier work.15 The surface coverage of PSA over MWNTs is 82.3 wt %. The Mw and PDI are 1.09 × 104 g/mol and 2.35, respectively.
10.1021/jp807707m CCC: $40.75 2009 American Chemical Society Published on Web 04/01/2009
7224 J. Phys. Chem. C, Vol. 113, No. 17, 2009
Du et al.
Figure 1. TEM image of pristine MWNTs (a), PSA-g-MWNTs (b), and photograph of aqueous PSA-g-MWNTs solution (4 mg/mL) stored at room temperature for six months (c).
Preparation of PSA/PVA Membrane for Actuation. PSAg-MWNTs with different weight fractions (0, 5, 10, and 20 wt %) was dispersed in an aqueous mixture of 10 wt % PSA and 20 wt % PVA. The mixture is vigorously stirred at the room temperature for 24 h. Thin membranes of 0.1 mm in thickness were then cast over the surface of poly(ethylene tetraphthalate) (PET) film. After dried at room temperature, the membrane was peeled off, followed by a thermal treatment at 100 °C for 60 min, and was further cross-linked in 5 wt % glutaraldehydeHCl/acetone solution at 40 °C for 20 min. The resulting composite membranes were cut and trimmed into rectangular stripes of 5 mm in width and 40 mm in length. Before actuation, both surfaces of the strips were sputtered with Au. And the membrane thus obtained was immersed in an aqueous solution of NaCl (1.0 M) for 12 h before the actuation test. Characterization. The microstructure of PSA-g-MWNTs was examined by JEOL 2010 transmission electron microscopy (TEM); the morphology of the PSA-g-MWNT-reinforced PSA/ PVA membrane was characterized by a Phillips XL30 scanning electron microscopy (SEM); the conductivity of the membrane was probed by a Keithley 6512 programmable electrometer (Four-point probe method). Mechanical properties were measured using a CMT-4104 testing machine with a crosshead speed of 5 mm/min at room temperature. Ion-exchange capacity (IEC) and water uptake were measured using the methods as reported by literature.16 Actuation was tested as follows: the membrane was fixed at one end with the other end immersed in 50 mL of NaCl solution (1.0 M). An external DC voltage was then applied to both electrodes on surfaces of the membrane, and the bending displacement was recorded using a CCD camera (Sony Cybershot DSC-T5).
Figure 2. SEM images of the cryogenic-fractured PSA-g-MWNTsreinforced PSA/PVA membranes with the PSA-g-MWNTs loading content of (a) 5 wt %, (b) 10 wt %, and (c) 20 wt %.
Results and Discussion Morphology of PSA-g-MWNTs and the Membrane. The nanostructures of pristine MWNTs and PSA-g-MWNTs were characterized by TEM. As shown in Figure 1, the PSA is uniformly covered on MWNTs surfaces with a thickness of 12 nm. Since PSA is anionic in nature, the resulted PSA-g-MWNTs are repulsive to each other, preventing the aggregation between MWNTs and leading to a stable nanotube suspension at a high concentration of 4 mg/mL after six months (Figure 1c). Such a uniform suspension helped the mixing of MWNTs inside a PSA/ PVA membrane and the SEM images of the cryogenic-fractured membrane clearly supported this. PSA-g-MWNTs with different loadings are used to reinforce PSA/PVA membranes as shown in Figure 2. The modified MWNTs are homogeneously dispersed in the PSA/PVA membrane even at a 20 wt % loading content. It is worthwhile to mention that the formed pores on the membrane surface are attributed to the pullout of PSA-gMWNTs during the cryogenic fracture, and the porosity of the membrane increases with the PSA-g-MWNT loading content.
Carbon Nanotube Enhanced Gripping
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TABLE 1: Mechanical Properties, Conductivity, IEC, and Water Uptake of PSA-g-MWNTs-Reinforced PSA/PVA Membrane with Different Loading Content of PSA-g-MWNTs PSA-g-MWNTs loading (wt %) -1
electric conductivity (S cm ) tensile strength (MPa) Young’s modulus (MPa) elongation at break (%) IEC (meq g-1) water uptake (g g-1)
0
5 -13
7.53 × 10 14.4 288.7 14.63 1.21 2.10
Properties of the Membrane. As expected, the electric conductivity of the modified membrane increased several orders of magnitude with an increased loading of MWNTs, as shown in Table 1, because of the high electric conductivity and homogeneous dispersion of CNTs in the matrix. The mechanical properties, or the stress-strain curves, of the modified membrane are shown in Figure 3. Corresponding tensile strength, Young’s modulus, and elongation-at-break are all listed in Table 1. It has been known that the inclusion of CNTs into a polymer matrix will lead to a reduction in toughness of the composite.17,18 Because of a homogeneous dispersion of MWNTs in PSA/PVA membrane, our results suggested otherwise. Not only has PSAg-MWNTs strengthened the membrane from 14.4 to 32.7 MPa, but it also enhanced the toughness of the PSA/PVA blend by 34.8%. Other than the aforementioned toughness reinforcement, removal of the interface mismatch between the MWNTs and PSA/PVA membrane is further demonstrated by a relative constant value of the ion-exchange capacity (IEC). For instance, the IEC of the pristine PSA/PVA blend membrane is 1.21 meq g-1. While this number is much higher than the 0.81 meq g-1 of Nafion membrane,16 because of the rich amount of ionic and hydroxyl groups on chains of PSA and PVA, the IEC value only increased slightly after the inclusion of PSA-g-MWNTs. Since ion exchange in PSA/PVA membrane is caused by protons and cations hopping between pendent, acidic and hydroxyl groups on either PSA or PVA backbone, i.e., PSA/PVA membrane is an ionic conductor. PSA-g-MWNTs does improve the electric conductivity by its excellent electron transport ability. However, the incorporation of PSA-g-MWNTs does not provide more protons or ions for increasing ion-exchange capacity due to the existence of sulfonic and carboxylic groups of 82.3 wt % PSA on the surface of PSA-g-MWNTs. The benefit to match the interfaces between MWNTs and the PSA/PVA
Figure 3. Tensile stress-displacement curves for PSA-g-MWNTsreinforced PSA/PVA membranes.
10 -9
4.91 × 10 20.1 415.7 14.39 1.53 0.57
20 -8
5.09 × 10 28.1 500.1 18.71 1.36 0.59
1.44 × 10-4 32.7 620.5 19.72 1.66 0.57
membrane becomes more prominent by a much reduced water uptake, an indication of better structure integrity after water inclusion. For example, the water uptake in PSA-g-MWNTsreinforced PSA/PVA membrane decreased significantly from a 2.10 g g-1 in pristine membrane to merely a quarter of such a value. It is reasonable to argue that the water uptake is proportional to the volume expansion or flexibility of the membrane. Clearly, the reinforced membrane loses its flexibility to freely expand by physically rejecting more water permeation after the structure is strengthened by the entangled networks of PSA-g-MWNTs. And this enhancement could be used to support the closer interactions between the MWNTs and the matrix membrane after the PSA intermediating. However, it is rather interesting to observe that the increased loading of MWNTs inside the polymer membrane kept the water uptake in the membrane relatively constant (Table 1), suggesting a firm network formed even at a small loading of MWNTs. Nonetheless, the water uptake in these PVA/PSA membrane is still much higher than those in Nafion membrane (0.16 g g-1).16
Figure 4. Time-dependent displacement of the actuator with different loading content of PSA-g-MWNTs at an applied square-wave electric potential of (() 1.5 V with a frequency of 0.25 Hz: (a) without PSAg-MWNTs (black curve), with 10 wt % PSA-g-MWNTs (red curve), and with 20 wt % PSA-g-MWNTs (blue curve) under one cycle stimulation; (b) with 20 wt % PSA-g-MWNTs under repeated stimulation.
7226 J. Phys. Chem. C, Vol. 113, No. 17, 2009 Actuation of PSA/PVA Membrane. Figure 4 shows the time-dependent displacement or actuation of the PSA/PVA membranes with or without the loading of PSA-g-MWNTs. An external bias of (1.5 V is applied upon a repeated square wave (frequency of 0.25 Hz). The maximum displacement of the PSA/ PVA membrane is 20 mm, and various loadings of PSA-gMWNTs leads to a slight but noticeable decrease in the displacement, where they are 18, 16, and 14 mm for PSA-gMWNTs loading of 5, 10, and 20 wt %, respectively. In contrast to the doubling of the Young’s modulus in membrane loaded with 20 wt % PSA-g-MWNTs, the actuation displacement only decreased by less than 30%. The elastic strain energy stored by the membrane is estimated to be increased by about 140%. This means PSA-g-MWNTs has one active function to channel more energy to the membrane under the same applied voltage. Alternatively, if the membranes with different loadings of PSAg-MWNTs are constrained from deflection, the membrane with the highest MWNTs loading is anticipated to give the strongest gripping force while the lowest loading one produces the smallest force. The effect of enhanced gripping is also evidenced by the removal of zigzag signals in Figure 4a. In this figure, the green line represents the activation voltage input. When no PSA-gMWNTs is added, the PSA/PVA actuates with certain zigzag oscillations, i.e., a suggestion of a soft gripping. The mechanical deformation always lags behind the input of the electrical stimulus. In contrast, with the loading of PSA-g-MWNTs, the membrane actuates rather smoothly, and the zigzag oscillation vanishes. Nemat-Nasser and Wu19 suggested that the oscillation is caused by the jerk force. We account that the small amplitude of oscillation may also be generated by the slipping of entangled polymer chains under an external stimulus. By incorporation of PSA-g-MWNTs that strengthens the degree of entanglement of polymer chains in the membrane, the oscillation vanishes. Figure 4b shows the repeated actuation of the PSA-g-MWNTsreinforced PSA/PVA membrane. A steady performance under the repeated square wave input is clearly visible. Generally, input electric energy is consumed to produce mechanical motion in an electromechanical actuator. Owing to the incorporation of PSA-g-MWNTs in PSA/PVA membrane, sufficient energy is converted for the actuation (i.e., doing mechanical work). The disappearance of the zigzag oscillation probably implies a more electromechanical coupling. We expect an optimum loading for MWNTs in these ionic polymer-based actuators. However, when the loading amount of PSA-gMWNTs is higher than 20 wt %, the membrane is unable to process because of the high solution viscosity of the PSA-gMWNTs-reinforced PSA/PVA blend. It is evident that homogeneous mixing of MWNTs inside the PVA/PSA membrane is vital for preparing the new ionic polymer-based actuator. The results of this study confirm that the incorporation of PSA-g-MWNTs within PSA/PVA promotes
Du et al. the mass transfer of cations and improves the electromechanical coupling of polymer-based actuators.9,10 Conclusions Ionic polymer membrane cast from the mixture of PSA and PVA was used an electromechanical actuator. MWNTs were used as an efficient element to enhance the gripping of this actuator. In contrast to other studies, a thin layer of PSA with a thickness of 12 nm is grafted on the surfaces of MWNTs. The water-soluble PSA-g-MWNTs can be homogeneously dispersed in the PSA/PVA membrane with a loading ratio up to 20 wt %. The reinforced membrane shows unique performances on multiple parameters, including enhanced mechanical toughness, relatively constant value of ion-exchange capacity, and prominent structure integrity after water uptake. As a result, enhanced gripping is resulted from the promoted electromechanical coupling. When the loading of PSA-g-MWNTs reaches more than 10 wt %, the small oscillation in the mechanical output of the actuator vanishes. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50825301, 50873040), the Research Committee of the Hong Kong Polytechnic University (G-YF06), and the NSF-CMMI 0825905. References and Notes (1) Shahinpoor, M. Smart Mater. Struct 1992, 1, 91. (2) Sadeghipourt, K.; Salomon, R.; Neogi, S. Smart Mater. Struct 1992, 1, 172. (3) Shahinpoor, M.; Bar-Cohen, Y.; Simpson, J. O.; Smith, J. Smart Mater. Struct 1998, 7, R15. (4) Shahinpoor, M.; Kim, K. J. Smart Mater. Struct 2005, 14, 197. (5) Iijima, S. Nature 1991, 354, 56. (6) Xie, X. L.; Mai, Y.-W.; Zhou, X. P. Mater. Sci. Eng. R 2005, 49, 89. (7) Baughman, R. H.; Cui, C. X.; Zakhidov, A. A.; Iqbal, Z.; Barisci, J. N.; Spinks, G. M.; Wallace, G. G.; Mazzoldi, A.; DeRossi, D.; Rinzler, A. G.; Jaschinski, O.; Roth, S.; Kertesz, M. Science 1999, 284, 1340. (8) Hughes, M.; Spink, G. M. AdV. Mater. 2005, 17, 443. (9) Landi, B. J.; Raffaelle, R. P.; Heben, M. J.; Alleman, J. L.; VanDerveer, W.; Gennett, T. Nano Lett. 2002, 2, 1329. (10) Lee, D. Y.; Park, I. S.; Lee, M. H.; Kim, K. J.; Heo, S. Sens. Actuators A 2007, 133, 117. (11) Yun, Y. H.; Shanov, V.; Schulz, M. J.; Narasimhadevara, S.; Subramaniam, S.; Hurd, D.; Boerio, F. J. Smart Mater. Struct 2005, 14, 1526. (12) Shi, J. H.; Guo, Z. X.; Zhan, B. H.; Luo, H. X.; Li, Y. F.; Zhu, D. B. J. Phys. Chem. B 2005, 109, 14789. (13) Yun, S.; Kim, J. Smart Mater. Struct 2007, 16, 1471. (14) Tahhan, M.; Truong, V. T.; Spinks, G. M.; Wallace, G. G. Smart Mater. Struct 2003, 12, 626. (15) Du, F. P.; Wu, K. B.; Yang, Y. K.; Liu, L.; Gan, T.; Xie, X. L. Nanotechnology 2008, 19, 085716. (16) Han, M. J.; Park, J. H.; Lee, J. Y.; Jho, J. Y. Macromol. Rapid Commun. 2006, 27, 219. (17) Meincke, O.; Kaempfer, D.; Weickmann, H.; Friedrich, C.; Vathauer, M. Polymer 2004, 45, 739. (18) Miyagawa, H.; Drzal, L. T. Polymer 2004, 45, 5163. (19) Nemat-Nasser, S.; Wu, Y. J. Appl. Phys. 2003, 93, 5255.
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