ARTICLE pubs.acs.org/Langmuir
Influence of Plasticizer Content on the Transition of Electromechanical Behavior of PVC Gel Actuator Mohammad Ali, Takamitsu Ueki, Daijiro Tsurumi, and Toshihiro Hirai* Smart Materials Engineering, Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan
bS Supporting Information ABSTRACT:
The actuation performance of plasticized poly(vinyl chloride) (PVC) gel actuators in an electric field depends on their chemical composition and electrical and mechanical properties. The influence of plasticizer (dibutyl adipate) content on electromechanical behavior of PVC gels was investigated by impedance spectroscopy and space charge measurement. By plasticizing the PVC, the dielectric constant and space charge density of PVC gel were drastically increased at 1:2 w/w ratio of PVC to plasticizer. To apply the results obtained from the impedance spectroscopy and space charge measurement, electrostatic adhesive forces generated between the PVC gel and the anode were measured. The electrostatic adhesive force at the anode was also dramatically increased at the same plasticizer content. All of the results indicated a transition of electromechanical behavior of PVC gel in the electric field, which was considered to originate from the orientation of polarized plasticizer molecules and dipole rotation of PVC chains. By using the electrostatic adhesive force of PVC gel derived from the electromechanical transition, a new electroactive actuator can be developed for novel applications.
’ INTRODUCTION Stimuli-responsive polymer gels have gained much attention as smart materials that can change their volume or shape on application of external stimuli such as pH, temperature, light, and magnetic or electric field.19 In particular, electroactive polymer gels are the most promising candidates to serve as actuators or artificial muscles in several applications, since an electric field is the most easy controllable stimulus. Poly(vinyl chloride) (PVC) is known to be an electrically inactive dielectric polymer and has been widely used as an electrically insulating material. However, it has become clear that plasticization of PVC gives an electrically active material with possible applications as an artificial muscle. Plasticized PVC (PVC gel), a jelly-like soft substance containing a large amount of plasticizer, has been extensively investigated as an electroactive actuator material in our laboratory because it exhibits a unique electric field response. PVC gel creeps on the anode surface when an electric field is applied and quickly recovers when the electric field is removed (see Supporting Information, Figure S1). This phenomenon is called amoeba-like pseudopodial creep deformation because of its analogy to the motion of amoebae using pseudopodia.1012 r 2011 American Chemical Society
The creep deformation induces various actuation motions such as bending, rolling, sliding, and oscillation, depending on boundary conditions such as electrode size and shape and the location of the gel or electrodes, and has enabled us to develop a focuscontrollable lens and microfinger devices.13 The creep deformation has been supposed to be achieved by the migration of injected negative charges from cathode to anode together with PVC polymer chains and plasticizer molecules. Recently, we have reported the results of investigation of the electric-field-induced local layer of PVC gels using a combination of spectroscopic and mechanical measurement. The results showed that the elastic modulus of PVC gels on the anode was smaller than on the cathode, indicating that creep deformation was caused by the migration of plasticizer-rich phase.14,15 The dielectric properties are crucial parameters in the design of dielectric actuators and in understanding their behavior in an electric field. PVC is a polar polymer and in the presence of an Received: March 14, 2011 Revised: May 5, 2011 Published: May 23, 2011 7902
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Figure 1. Setup for electrostatic adhesive force observation of PVC gel to the anode: (a) under a microscope and (b) suspended weight measurement.
electric field the dipoles attempt to become aligned with the field. This creates some degree of polarization of the PVC chains. However, the addition of plasticizer to PVC may affect the polarization in a broad frequency range, and the newly polarized state may contribute to the electrical properties of PVC gel such as dielectric constant, dielectric loss, and space charge distribution of injected charges. There have been few detailed investigations of the role of plasticizer on the electrical properties of PVC gels, even though they are critical factors to improve the overall electrical actuation performance. In the present study we carefully controlled the addition of plasticizer to the PVC and investigated the influence of plasticizer content on the electromechanical behavior of PVC gels using impedance spectroscopy and mechanical property and space charge measurement. The dielectric constant at low frequency and space charge density of PVC gel were drastically increased due to the addition of plasticizer at 1:2 w/w ratio of PVC to plasticizer. On the basis of the results obtained from space charge measurement and impedance spectroscopy, the electrostatic adhesive force between PVC gel and an electrode was measured in an applied electric field. The electrostatic adhesive force of PVC gel to the anode was also drastically increased at the same content of plasticizer. All of the results revealed a transition of electromechanical behavior of PVC gel in an electric field at a critical plasticizer content. The transition originated from the orientation of plasticizer molecules in the polarized state and dipole rotation of PVC chains.
’ EXPERIMENTAL SECTION Materials. PVC gels were prepared from commercial PVC powder (Mn = 99 000 g mol1, Mw = 233 000 g mol1, n = 3728), tetrahydrofuran (THF), and dibutyl adipate (DBA, MW = 258.38 g mol1). PVC powder was purchased from Sigma-Aldrich Co. DBA and THF were purchased from Wako Pure Chemicals Industries Ltd., Japan. The weight ratios of PVC to DBA were adjusted from 1:1 to 1:9. PVC powder was dissolved in a THF/DBA mixture, and then the solution was cast in a PTFE laboratory Petri dish. THF was evaporated at room temperature for 35 days to prepare a soft, transparent PVC gel. The thickness of the PVC gel was adjusted to be in the range 500 μm1 mm. Impedance Spectroscopy. Impedance spectroscopy measurement was used for dielectric characterization of the fabricated PVC gels. The apparatus used was a model SI-1260 impedance/gain-phase analyzer coupled with a model 1296 dielectric interface (Solartron Analytical Co., Farnborough, UK). Data were collected by a PC and analyzed with software (Zplot and Zview). Impedance measurement was performed at sweep frequencies from 100 to 106 Hz (5 points/decade) with signal
amplitude 100 mV for each sample, at room temperature. The PVC gel was placed between two parallel plate electrodes, and thickness was measured by a digital thickness meter. The lower electrode was replaced by a liquid holder electrode for DBA measurement (see Supporting Information, Figure S2). A platinum plate electrode 10 mm in diameter was used for the PVC gel and solid PVC film, but for liquid plasticizer a liquid sample holder was used. Circular disk-shaped specimens about 13 mm in diameter (larger than the electrode diameter) and 500 μm1 mm thick were used. Space Charge Measurement. The space charge density for PVC gels with varying DBA contents, and for PVC film, was measured by the pulsed electroacoustic (PEA) method (see Supporting Information, Figure S3). Circular samples with diameter 10 mm (larger than the sensor diameter) were carefully placed between the electrodes with normal pressure. The force was propagated as an acoustic wave and detected by the piezoelectric device. The distribution of the space charge density was obtained when an electric field was applied at room temperature. The applied electric field and pulsed voltages were 1000 V mm1 and 600 V, respectively. Mechanical Properties. The electrical properties and electrical actuation of the PVC gel are also affected by the elastic modulus of the gels. The elastic modulus of each sample was measured with a universal tensile testing machine (Tensilon RTC-1250A, Orientec U-4410 crosshead controller, A&D Co., Ltd., Tokyo, Japan) at room temperature, at constant rate of elongation. All samples were prepared in the form of standard dumbbell shapes (see Supporting Information, Figure S4) using a super dumbbell cutter (JIS K6252-6, ISO 37-2). The length and width of the samples were 30 and 3 mm, respectively, and the samples were elongated at cross-head speed 30 mm/min. Electrostatic Force Measurement. Electrostatic adhesiveness between PVC gel and electrode was measured with laboratory designed equipment comprising an electrical balance, jog meter, and minimotor to apply the results obtained from impedance spectroscopy and space charge distribution. An ultrahigh resistance meter (Advantest R8340A) was used to apply various electric voltages, and force diagrams were obtained by wave shot software. A square (10 10 mm2) block type anode was used. The size of the PVC gel samples (20 20 mm2) was larger than the size of the anode. Aluminum tape was used for both anode and cathode to connect with the terminal wires of the ultrahighresistance meter. To secure the PVC gel to the cathode, a spacer was used on the surface of the PVC gel. In the center of the spacer, a window slightly larger than the size of the anode was used to make contact with the gel surface (see Supporting Information, Figure S5). We also carried out direct observation of electrostatic adhesive force under a microscope and using a suspended weight (Figure 1). For direct observation under a microscope, PVC gel was placed in parallel on the surface of the cathode, with the anode at a smaller distance from the gel surface (see Supporting Information, Figure S6). Force measurements were carried out at room temperature. 7903
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Figure 2. Dielectric properties of PVC gels as a function of frequency at room temperature: (a) dielectric constant, (b) dielectric loss, and (c) dependence of dielectric constant and dielectric loss on DBA content at frequency 1 Hz.
’ RESULTS AND DISCUSSION Dielectric Properties of PVC Gels. The dielectric properties of a material can be expressed as the complex dielectric constant ε*, given by
ε ¼ ε0 jε00 The complex dielectric constant is composed of the dielectric constant, ε0 (real part), and the dielectric loss, ε00 (imaginary part). The real part is related to electrical energy converted to stored energy within a material and is associated with displacement of positive charges along the applied electric field and negative charges in the opposite direction, generating polarization of dielectric material. This energy storage process is always accompanied by a loss current which represents energy that is not stored but is dissipated within the material in the form of heat. The dielectric loss phenomenon is represented by the imaginary part or dielectric loss, ε00 .1618 Figure 2 shows dielectric properties of pure PVC film, DBA only, and PVC gels containing various weight ratios of PVC to DBA as a function of frequency at room temperature. From the viewpoint of molecular structure, both PVC and DBA are dipolar due to the existence of CCl and CdO bonds.19 The dielectric constant ε0 of pure PVC film was the lowest and was almost constant at all frequencies. This result indicates that the dipole on the PVC chain could not follow the electric field polarity changes due to the structural features of the PVC film. The dielectric constant of DBA only was slightly higher than that of pure PVC film and likewise independent of frequency. Even though DBA is a liquid with low molecular weight, the dipole on the DBA could not follow the electric field polarity changes at all frequencies. In general, the dielectric constant of PVC gel increases to a high value depending on the kind and amount of plasticizer. If the plasticizer content is increased or the frequency decreased to a sufficient extent, the dielectric constant increases.20 From the results (Figure 2a,c) of impedance spectroscopy of PVC gel, at constant plasticizer content the dielectric constant increased dramatically at low frequencies (100101 Hz) due to the free charge motion in the plasticizer molecule, and the dipole of polarized PVC can follow the electric field polarity change. The dielectric constant increased to several hundred times the values for pure PVC film and DBA when the PVC:plasticizer ratio was in the range 1:2 to 1:5. This drastic change of dielectric
constant suggests strong interaction between PVC and DBA. The DBA molecules were charged and polarized in the electric field. The polarized DBA molecules were able to follow the electric field polarity change at lower frequency, and they facilitated dipole rotation of PVC chain segments. The simultaneous polarization of the DBA and PVC chains enhanced the entire polarization of the PVC gels, and the dielectric constants of the gels drastically increased. The dielectric constant of the PVC gels decreased continuously with increasing frequency up to 100 Hz, and in the frequency range 1001000 Hz the dielectric constant was constant. The use of DBA increased the dielectric loss, ε00 , of the PVC in the low frequency range. Dielectric constant, ε0 , and dielectric loss, ε00 , increased with increased DBA content and reached a maximum value at PVC:DBA = 1:5 w/w. However, with addition of DBA up to PVC:DBA = 1:9, the dielectric constant and dielectric loss decreased and approached the value for pure DBA. The dielectric behavior of PVC gels obtained from these measurements revealed that the same polarization phenomenon would occur in the PVC gels during their electrical actuation. Space Charge Density and Mechanical Properties. Space charge density is the accumulation of interior charges within a dielectric material and was measured by the pulsed electroacoustic method.21,22 The space charge distribution affects the actuation performance of PVC gels. Figure 3 shows the space charge density of pure PVC film and PVC gels with varying DBA content. The horizontal axis in Figure 3a,b shows the points in the direction of PVC gel thickness between the anode and cathode. In the analysis of space charge distributions, the peak of charge density and width of charge band were used as the parameters to evaluate both the nature of the negative charge accumulation and the polarization phenomenon in PVC gels between electrodes in an electric field. The area of the space charge density was measured as a triangle, and the values are plotted in Figure 3c. For pure PVC film (Figure 3a) only two peaks originating from the electrodes could be recognized, and thus there was no accumulation of charges. However, for PVC gels, when charges were injected from the cathode, the charges carried by the DBA molecules migrated toward the anode and accumulated on the gel surface near the anode (Figure 3b). The accumulated charge density depended on the PVC:DBA w/w ratio. For 1:1 ratio, the negative charge accumulation was less than at other weight ratios. Charge injection by the cathode may depend on the mechanical properties of the PVC gels, in particular the elastic modulus. Figure 4 shows 7904
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Figure 3. Space charge distribution of pure PVC film (a) and PVC gels made with various DBA contents (b, c). (b) shows raw data of the negative charge accumulation near the anode, and (c) shows the calculated area of charge accumulation.
Figure 4. Mechanical properties of (a) PVC film and (b) PVC gels with varying DBA content. (b) Stressstrain curves and (c) Young’s modulus.
stressstrain and Young’s modulus curves for PVC gels with various plasticizer contents. Young’s modulus decreased nonlinearly with increasing DBA content. The stressstrain curve (Figure 4a) of PVC film showed the rigidity of the film and Young’s modulus was in the MPa range with maximum strain 4%, but for PVC gel the strain was extremely high. PVC gel with 1:1 weight ratio showed higher Young’s modulus than that of the other gels. For this reason, charge injection to the PVC film and 1:1 PVC gel might be difficult from the cathode. On further increasing the DBA content, Young’s modulus suddenly decreased. The accumulation of negative charges suddenly increased at PVC:DBA = 1:2 w/w, and further increase of DBA content caused the space charge density to gradually decrease. The intensity of the negative charge accumulation depends on the discharge rate of charges carried by the DBA molecules. The discharge rate is delayed by the dipole alignment of PVC chains, which causes a higher negative charge accumulation in the gel at higher concentration of PVC. The intensity of charge accumulation was directly dependent on the intensity of the applied electric field (see Supporting Information, Figure S7). Thus, it was found that the weight ratio of PVC to DBA in PVC gels has a great
influence on the transition of space charge density and mechanical properties. Electrostatic Adhesive Force. The electrostatic force is generated by the interaction between electrically charged particles. The negative charges accumulated on the surface of PVC gel near the anode generated the electrostatic force between PVC gel and the anode. The electrostatic adhesive force was measured to support the results obtained from space charge and dielectric measurement. As PVC gels have stickiness, the initial adhesiveness was measured before application of the electric field, with the results shown in Figure 5a. The initial adhesiveness of the PVC gels increased with increased DBA content. Figure 5b shows the total and electrostatic adhesive force of PVC gel with PVC:DBA = 1:9 (w/w) as a function of applied electric field. The electrostatic force on the anode increased linearly with the applied electric field, but there was no electrostatic force on the cathode side, except for the initial adhesiveness. This electrostatic force should be exerted on the PVC chains as well as on DBA molecules during the actuation of PVC gels in an electric field. Figure 5c shows the dependence of electrostatic adhesive force on the DBA content of the gels. The electrostatic adhesive force 7905
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Figure 5. Initial adhesiveness of PVC to the electrode without electric field (a); dependence of total and electrostatic adhesive force of PVC:DBA = 1:9 weight ratio gel on applied electric field (b); and electrostatic adhesive force of PVC gels made with various DBA contents at three different electric fields (c).
Figure 6. Microscopic images of PVC gel: (a) before application of an electric field; (b) electrostatic adhesive force between PVC gel and the anode. Imaging model of PVC gel internal structure: (c) before application of an electric field; (d) electrically induced creep deformation and structural modulation after the electromechanical transition.
on the anode increased dramatically at PVC:DBA = 1:2, and with further increase in DBA content the electrostatic force gradually decreased. The electrostatic force results were consistent with the space charge density data. The mechanism for the sudden change at PVC:DBA = 1:2 may be attributed to the release of PVC chains from a frozen state and correlates to their degree of freedom. The effect of PVC is clearly evident and the density of
the PVC chains per unit area or volume in the gel is higher for lower DBA contents. The number of free dipoles on the PVC chains increases with the addition of DBA particularly at lower DBA contents, and the density of the free dipoles, which decreases at higher DBA contents. We also carried out direct observation of the electrostatic adhesive force between PVC gel and anode. When an electric field was applied, PVC gel moved 7906
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Langmuir toward and came into contact with the anode, and after changing the electric field polarity, the gel reverted to its previous position (Supporting Information, Figure S6). These results may support the hypothesis that electrostatic force arising from the polarization of charges is the origin of the actuation or deformation of PVC gels that occurs on the anode. With this force, the PVC gel is stretched on the anode with the accumulated charges. Electrically Induced Structure of PVC Gel. According to the electromechanical properties of PVC gels discussed above, on addition of plasticizer to the gel a drastic effect on electromechanical properties was found when an electric field was applied to gels with PVC:DBA weight ratio in the range 1:2 to 1:9. This drastic change is termed an electromechanical transition, and the structure of PVC gel after the transition is represented in the imaging models of Figure 6. In the gel structure, each PVC chain is loosely connected through physical cross-linking points, and the space inside the PVC chain matrix is filled with the plasticizer.23 In PVC chains, dipoles are attached directly to the chain carbon atoms and the orientation of the PVC dipoles may be random. The orientation of plasticizer molecules may be also random due to the interaction of PVC chains with the polar parts of plasticizer molecules.24 Figure 6c depicts a dipolar PVC chain segment before application of an electric field, where the dipoles are randomly arranged. In the presence of an electric field, dipoles on the PVC chains rotate toward the anode (Figure 6d), and the distribution is the same in the whole gel whether it is near the anode or cathode as long as there are no other influences such as quadrupoles. Usually quadrupoles and higher order multipoles are not important for the characterization of dielectric materials since they must arise from the same number of molecules as the dipole effects, which are dominant in dielectrics. But in our system, quadrupoles or other factors may affect the actuation of PVC gel in an electric field, which are unclear yet. However, our recent results confirm that the density of DBA molecules is slightly higher on the anode side and hence dipoles on the PVC chains near the anode may take preference on the magnitude of rotation due to the flexibility of the PVC chains, and the rotation of the dipoles in each segment may cause an effective polarized state of PVC chains.25 Moreover, the polarized plasticizer molecules are oriented in the direction of the applied electric field. As the electrostatic force is exerted on the PVC gel, the physical cross-linking points of the PVC chains might be pulled with resulting change in the internal structure of the gel.26
’ CONCLUSIONS In summary, we studied the effect of plasticizer content on the electromechanical behavior of PVC gel plasticized with DBA. The dielectric constant of PVC gel was increased 10100-fold compared to pure (unplasticized) PVC film and DBA. A transition of dielectric properties, space charge distribution, and electrostatic force was found at PVC:DBA = 1:2 (w/w) when the DBA content of the PVC gels was carefully controlled. We also found a clear relationship between space charge density and electrostatic adhesive force of PVC gel to the anode. All of the results show the vital influence of plasticizer content on the electromechanical behavior of PVC gels. At each concentration of DBA, PVC chains and DBA molecules behave differently in terms of polarization in an electric field. On the basis of different needs, we can select the best content of plasticizer to design electrically active dielectric actuators.
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’ ASSOCIATED CONTENT
bS
Supporting Information. Schematic images of the creep deformation of PVC gel (Figure S1), impedance spectroscopy setup for measurement of dielectric properties of PVC gels (Figure S2), the principle of the measurement of space charge density by the pulsed electroacoustic (PEA) method (Figure S3), schematic of tensile strength measurement of PVC gels (Figure S4), setup for the measurement of electrostatic adhesive force between PVC gels and electrode (Figure S5), observation of attraction force by optical microscope (Figure S6), dependence of space charge on electric field (Figure S7), raw data of electrostatic force (Figure S8), and direct observation of attraction force by suspended weight (Figure S9). This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was partly supported by a Grant-in-Aid for Global COE Program by the Ministry of Education, Culture, Sports, Science and Technology (Japan). ’ REFERENCES (1) Heijl Jan, M. D.; Du Prez, F. E. Polymer 2004, 45, 6771–6778. (2) Kim, S. J.; Spinks, G. M.; Prosser, S.; Whitten, P. G.; Wallace, G. G.; Kim, S. I. Nature Mater. 2006, 5, 48–51. (3) Tsutsui, H.; Akashi, R. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 4644–4655. (4) Dong, L.; Agarwal, A. K.; Beebe, D. J.; Jiang, H. Nature 2006, 442, 551–554. (5) Moniruzzaman, M.; Fernando, G. F.; Talbot, J. D. R. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 2886–2896. (6) Tatsuma, T.; Takada, K.; Miyazaki, T. Adv. Mater. 2007, 19, 1249–1251. (7) Nersesse, N.; OR, S. W.; Gregory, P C.; Scott, K. M.; Choe, W.; Harry, B R.; Mike, W. M.; Vitalij, K P.; Alexandra, P. Appl. Phys. Lett. 2004, 84, 4801–4803. (8) Osada, Y.; Okuzaki, H.; Hori, H. Nature 1992, 355, 242–244. (9) Takada, K.; Tanaka, N.; Tatsuma, T. J. Electroanal. Chem. 2005, 585, 120–127. (10) Uddin, M. Z.; Yamaguchi, M.; Watanab, M.; Shirai, H.; Hirai, T. Chem. Lett. 2001, 30 (4), 360–361. (11) Uddin, M. Z.; Watanabe, M.; Shirai, H.; Hirai, T. J. Robotics Mechatronics 2002, 14 (2), 118–123. (12) Uddin, M. Z.; Watanabe, M.; Shirai, H.; Hirai, T. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2119–2127. (13) Hirai, T.; Ogiwara, T.; Fujii, K.; Ueki, T.; Kinoshita, K.; Takasaki, M. Adv. Mater. 2009, 21, 2886–2888. (14) Xia, H.; Takasaki, M.; Hirai, T. Sens. Actuators, A 2010, 157, 307–312. (15) Xia, H.; Hirai, T. J. Phys. Chem. B 2010, 114, 10756–10762. (16) Butkewitsch, S.; Scheinbeim, J. Appl. Surf. Sci. 2006, 252, 8277– 8286. (17) Pradhan, D. K.; Choudhary, R. N. P.; Samantaray, B. K. Int. J. Electrochem. Sci. 2008, 3, 597–608. (18) Gallone, G.; Galantini, F.; Carpi, F. Polym. Int. 2010, 59, 400–406. (19) Ramesh, S.; Leen, K. H.; Kumutha, K.; Arof, A. K. Spectrochim. Acta, Part A 2007, 66, 1237–1242. 7907
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