Polyaniline/Ethylene Vinyl Acetate Composites as Dielectric Sensor
Mostafizur Rahaman, Tapan Kumar Chaki, Dipak Khastgir Rubber Technology Centre, Indian Institute of Technology, Kharagpur 721302, India
The compressive stress (pressure) sensitivity of dielectric properties has been studied on ethylene vinyl acetate (EVA)/polyaniline (Pani) composites prepared through In-situ synthesis of polyaniline in the solution of insulating EVA matrix. It is observed that the dielectric constant and loss increase with the increase in applied pressure, that is some piezoelectric effect is observed for these composites. The dielectric properties are also found to increase with respect to time when subjected under constant pressure. It is seen that changes in dielectric constant and loss follow some exponential relationships with respect to applied pressure and time duration under constant stress, and the relaxation time for the composites can be calculated. The relaxation time decreases with the increase in concentration of Pani in a composite. However, a composite with lower Pani content exhibits relatively higher change in dielectric properties against applied pressure and time duration under compression compared to one with higher loading. Granular crew type morphology of Pani is observed through scanning electron microscopic (SEM) study. This study reveals that these EVA-Pani composites can be used as dielectric sensor. POLYM. ENG. SCI., C 2013 Society of Plastics Engineers 00:000–000, 2013. V
INTRODUCTION The demand of composite dielectric materials derived from heterogeneous systems where matrix phase is insulating polymer and the dispersed phase is conductive additives are increasingly gaining importance because of their potential applications in aerospace, electrical, and electronic fields such as shielding enclosures, capacitors, sensors, microwave absorber, etc. [1–4]. The dielectric material can be effectively used for sensing and monitor-
Correspondence to: Dipak Khastgir and Mostafizur Rahaman; e-mail:
[email protected]. ernet. in or
[email protected] Mostafizur Rahaman is currently at Chemical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran, Kingdom of Saudi Arabia Contract grant sponsor: Aeronautic Research and Development Board (ARDB), Government of India. DOI 10.1002/pen.23714 Published online in Wiley Online Library (wileyonlinelibrary.com). C 2013 Society of Plastics Engineers V
POLYMER ENGINEERING AND SCIENCE—2013
ing of different electrical signals. There are considerable literatures on the use of dielectric materials as sensors. These include cure monitoring of the composites at high temperature, measurement of soil and snow moisture in environment, measuring flow of resin in transfer moulding, detecting capsules moving through pipelines, etc. [5–10]. But the literature on the use of polymer based dielectric materials as pressure sensitive sensors are really scanty. Thus it leaves good scope for researchers to investigate in this field. It is better to use polymer/polymer based composites for different electrical and electronic applications because compared to other conventional materials like ceramics or metals, polymeric materials are lightweight, flexible in nature, and easy to prototype [11]. The dielectric properties (dielectric constant and loss) of insulating polymers are relatively low. Hence the variation of dielectric properties with respect to applied pressure is expected to be marginal. Thus common insulating polymers may not be very suitable as dielectric pressure sensitive sensors. However, intrinsically conducting polymers can also not serve this purpose because of their high rigidity and brittleness. Moreover, the dielectric properties of these polymers are very high even at high frequency and their dielectric response at low frequency becomes unreliable and not measurable because of high increase in conductivity at lower frequencies. Thus composite systems consisting of conducting polymer as filler dispersed in insulating polymer matrix can serve the purposes very well. Among the conducting polymers, Pani has been found to be very promising due to low cost of monomer, ease to synthesis by various polymerization techniques, and multifunctional applications [12]. The dielectric properties of these conducting composites depend on fillers shape, size, concentration, distribution and dispersion, and interfacial attraction between filler and polymer matrix [13]. Previously, the in situ emulsion polymerization of Pani in polymer matrices has been reported by various authors [14, 15]. Jiongxin et al. (2007) have investigated the dielectric and electrical properties of in situ synthesized Pani in insulating epoxy matrix [16].
SCHEME 1. Methodology of EVA/Pani synthesis.
EVA Copolymer (Mooney viscosity, ML114 at 100 C is 20, vinyl acetate content 28% and MFI 5 6) was purchased from NOCIL, Mumbai, India. The vacuum distillation of aniline (Aldrich, Germany) was done before use. The oxidant, ammonium peroxydisulfate (APS) (analytical grade, Merck) was used without purification. Methanol (AR) was procured from SISCO Research Laboratories Pvt. Ltd., Mumbai; HCl (LR) from S. D. Fine—Chem. Ltd., Mumbai, and toluene (Synthetic Grade) from Merck Specialties Private Limited, Mumbai.
been given in Scheme 1. In this procedure, a solution of EVA (5 gm) was made by dissolving in 200 ml toluene. Variable amount of aniline depending on composition was then added to EVA solution followed by drop wise addition of HCl in this system. To start the polymerization, an equimolar amount of oxidant, aqueous ammonium peroxydisulfate (APS) was added to this system drop wise for 30 min and the reaction was continued for 6 h with constant stirring. Finally, the reaction was quenched through addition of methanol. In this polymerization process two solvents, toluene and water were used and they were immiscible with each other; hence, the synthesis was a type of interfacial in situ polymerization. The resultant mass of EVA-Pani composite was washed several times with methanol and water, and then cast into a film on a Petridis and then subjected to evaporation. The composite film was initially dried in desiccators and finally under vacuum at 50 C for 48 h. Taking same amount of EVA in toluene, and keeping same molar ratio of aniline/HCl and aniline/APS, different weight ratios of EVA/Pani composites were synthesized. The gravimetric method was used to calculate the conversion of aniline to polyaniline (Pani) and proportion of Pani in the composite. Different composites were designated by using alpha numerical number; for example, E100P54 means composite of EVA/Pani containing 54 parts of Pani by weight per hundred parts of EVA (php) and so on, where E stands for EVA and P for polyaniline.
In Situ Synthesis of Pani in EVA Matrix
Testing and Characterization
The standard chemical oxidative method has been used to carry out in situ synthesis of Pani in insulating EVA matrix [17–21]. The methodology of this synthesis has
Circular type compression molded test specimens of the composites with diameter 13.5 mm and thickness 2.0 6 0.1 mm were prepared at 120 C for the measurement
In the present work, in situ synthesis of Pani in ethylene vinyl acetate copolymer (EVA) matrix has been carried out at room temperature. The pressure sensitivity on EVA/Pani composite has been investigated for different composites with variable Pani concentration. The change in dielectric properties against time under constant pressure has also been investigated. The results have been discussed in the light of formation and breakdown of conductive networks in the polymer matrix due to applied compressive stress. The EVA/Pani composite shows a very good piezo response through the change in dielectric properties with respect to applied pressure and time duration at fixed stress. This reveals that these composites can serve the purpose of dielectric sensors. EXPERIMENTAL Materials
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SCHEME 2. Design of experimental set-up for pressure sensitivity measurement.
of dielectric properties. The dielectric properties like capacitance and dissipation factor at frequency 103 Hz were measured using the instrument QuadTech 7600 (LCR meter). The variation of dielectric properties with respect to change in compressive stress (pressure, 0–60 kPa) and time duration (0–2 h) under fixed compression was measured by GW Instek LCR meter 819 coupled with a home made electrode. The design of experimental set-up has been presented in Scheme 2. The home made electrode set-up was consisted of two electrodes namely upper electrode and lower electrode. These electrodes were placed parallel to each other within a platen. The lower electrode was fixed with lower platen but the upper electrode was movable fitted with a piston. Both electrodes were made up of brass. Platen and movable piston are made of insulating material like thick Teflon sheet. Electrodes were connected with the LCR meter through the properly insulated coaxial wires as shown in the given Scheme 2. Different composite samples were placed in between the electrodes for the measurement of dielectric pressure sensitivity. The applied compressive stress was varied stepwise. All dielectric property measurements were taken after predetermined time on application of stress to obtain stabilized value. For the present case, the stabilization time has been taken as five minutes after application of the load. The relative dielectric properties (dielectric constant and loss) with respect to either applied pressure or time have been calculated using the following Eq. 1; Rep 5Ep =E0
(1)
where Rep 5 relative dielectric property, Ep 5 dielectric property at any pressure or time, and E0 5 dielectric property at zero pressure or time. Scanning electron microscopic (SEM) analysis of EVA/Pani composites was performed using JEOL JSM 5800 scanning electron microscope (Tokyo, Japan). Gold DOI 10.1002/pen
coating of the samples was done by vacuum gold-sputter machine before SEM study. RESULTS AND DISCUSSION Variation of Dielectric Constant and Dielectric Loss against Pani Concentration The variation of dielectric constant and loss against polyaniline concentration has been shown in Fig. 1. It is evident from the figure that both the dielectric constant and loss increases with the progressive increase in amount of Pani. Actually, the Pani particle present in polymer matrix acts as a minute capacitor. An increment in Pani content increases the number of such capacitor in the polymer matrix. This in turn increases the magnitude of dielectric constant. Dielectric loss is the product of dielectric constant and dissipation factor. Dissipation factor increases due to the movement/motion of free Pani charge carrier [6, 22]. Increase in Pani content increases the number of such mobile charge carrier. Consequently, the dielectric loss increases and exhibits higher value compared to dielectric constant. For the sake of simplicity and to avoid more complication, only two composites with different Pani concentrations have been considered for further investigations. This will also reduce the manuscript size by not producing unnecessary data. Effect of Applied Pressure on Dielectric Constant and Dielectric Loss Dielectric constant and relative dielectric constant of EVA/Pani composites with respect to variation in pressure have been shown in Figs. 2 and 3; whereas, dielectric loss and relative dielectric loss have been shown in Figs. 4 and 5, respectively. It is evident from the Figs. 2 POLYMER ENGINEERING AND SCIENCE—2013 3
FIG. 1. Dielectric constant and dielectric loss of EVA/Pani composite. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
and 4 that both the dielectric constant and dielectric loss sharply increases with the increase in applied pressure up to a certain level and then further increase in pressure has a marginal effect. However, the composite with higher Pani content exhibits lower change in dielectric constant and loss with respect to pressure (Figs. 3 and 5). Actually, the application of pressure brings about two types of changes in EVA/Pani composite; the reduction in composite thickness/volume due to applied pressure and the formation of new conducting network with the simultaneous breakdown of some existing networks. For any system, containing conductive component dispersed in insulating polymer matrix, the capacitance is directly proportional to the cross-sectional area but inversely proportional to thickness of the composite [6]. Application of pressure reduced the thickness of EVA/Pani composites. Thus, the distances between the conductive particles are decreased in the polymer matrix. As a result, the dipole–
FIG. 2. Effect of applied pressure on dielectric constant. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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FIG. 3. Effect of applied pressure on relative dielectric constant. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
dipole interactions among the conductive particles are increased. This in turn increases the capacitance and hence the dielectric properties. This is because the capacitance arises due to the formation of induced double layer by the application of electric field on the surfaces of conducting particles separated by insulating polymer layers [4]. It has been mentioned earlier that there is the formation of new conducting networks with the simultaneous breakdown of some existing conducting networks in the composite by the application of pressure. At lower pressure, the formation of new conducting networks is dominant over breakdown of existing conducting networks. This leads to the increase in capacitance and hence dielectric properties. But at sufficiently higher pressure, the two effects (formation and breakdown of networks)
FIG. 4. Effect of applied pressure on dielectric loss. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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TABLE 1.
Parameters value of dielectric constant and loss, and relative dielectric constant and loss with respect to pressure. Parameters value of dielectric constant and relative dielectric loss with respect to pressure Initial value (e0 0)
Amplitude (Ae0 )
Decay (de0 )
Composition
Absolute
Relative
Absolute
Relative
Absolute
Relative
E100P54 E100P134
12,903 6 469 3,261,600 6 56,659
8.2 6 0.30 2.7 6 0.05
211,239 6 437 22,037,780 6 50,761
27.2 6 0.31 21.7 6 0.04
56 6 3.9 39 6 2.2
56 6 3.9 39 6 2.2
Parameters value of dielectric loss and relative dielectric loss with respect to pressure Initial value (e00 0)
Amplitude (Ae00 )
Decay (de00 )
Composition
Absolute
Relative
Absolute
Relative
Absolute
Relative
E100P54 E100P134
78,388 6 3399 22,614,700 6 111197
7.4 6 0.33 2.0 6 0.01
267,360 6 3138 211,115,000 6 119,521
26.4 6 0.29 21.0 6 0.01
51 6 4.5 21 6 0.6
51 6 4.5 2160.6
almost compensate each other and hence the changes in dielectric properties are marginal. The effect of pressure is pronounced more for the composite having lower Pani content that is for high resistive composite. The polymer chains for lower filled composite are more free to move without any restriction by the application of external pressure. This results in the decrease of interparticle aggregate distance to a significant extent. This is why the capacitance and hence dielectric properties are increased. However, for higher Pani content composite, the mobility of polymer chains are restricted due to the presence of large amount of Pani. This attribute to the change in inter particle aggregate distance to lesser extent, and hence there is less change in dielectric properties. The effect of applied pressure on dielectric properties can be the best fitted as follows:
e0 ðor e0 Þ ¼ e0 0 ðor e00 0 Þ þ Ae0 ðor Ae00 Þ3e ð2P=dÞ
(2)
where e0 5 dielectric constant, e00 5 dielectric loss, e0 0 5 initial value of dielectric constant, e00 0 5 initial value of dielectric loss, Ae0 5 amplitude for dielectric constant, Ae00 5 amplitude for dielectric loss, P 5 applied pressure, and finally d 5 decay constant. The parameter values obtained according to above Eq. 2 for dielectric constant and dielectric loss have been given in Table 1. It is observed from this table that the magnitude of amplitude that is the change in dielectric constant or loss is higher for composite having higher Pani content, but the relative change is higher for composite with lower Pani content. The relative change in amplitude can be correlated with the relative change in thickness. The relative change in thickness is higher for composite with lower Pani content compared with higher one due to the presence of more number of free mobile chain. However, the decay constant can be correlated with the formation of conductive networks. Higher is the value of decay constant, higher will be the formation of conductive networks. The formation of conductive networks is more for composite with lower Pani content. Effect of Time on Dielectric Constant and Dielectric Loss under Constant Pressure
FIG. 5. Effect of applied pressure on relative dielectric loss. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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The variation of dielectric constant and loss with respect to time have been shown in Figs. 6–9. It is evident from Figs. 6 and 8 that both the dielectric constant and loss are increasing with the increase in time under constant pressure. Initially, there is sharp increment in both the dielectric constant and loss, and after a time span it becomes marginal. The composite with higher Pani content exhibits relatively less increment in both the dielectric constant and loss compared to the composite having lower Pani content (Figs. 7 and 9). These phenomena can be explained in the same way as it has been explained in the case of their behavior with applied pressure. But the difference lies in the fact that when polymer POLYMER ENGINEERING AND SCIENCE—2013 5
FIG. 6. Effect of time on dielectric constant under constant pressure. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
composite is subjected to constant pressure for prolonged period; polymer undergoes molecular relaxation due to the mobility of polymer chains and Pani particles attached to polymer chains were also subjected to spatial rearrangements simultaneously, which in turn affect both the dielectric constant and loss. The molecular relaxation or relaxation time is the time at which the dielectric properties or any other parameters become asymptotic in nature to the time axis. The relaxation time for dielectric constant and loss can be calculated by the best fit of equation whose parameters values have been given in Table 2. The equation of the best fit can be given as follows in Eq. 3; e0 ðor e00 Þ ¼ e0 0 ðor e00 0 Þ þ Ae0 ðor Ae00 Þ3e ð2X=tÞ
(3)
where e0 (or e00 ) 5 dielectric constant or loss of the composite, e0 0 (or e00 0) 5 initial value dielectric constant or loss, x 5 time in seconds, Ae0 (or Ae00 ) 5 amplitude for dielectric constant or loss, and t 5 decay constant or
FIG. 7. Effect of time on relative dielectric constant under constant pressure. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
relaxation time. It is observed from Table 2 that the parameter t, which is a measure of relaxation time, is less for the composite having higher Pani content. Actually, the mobility of polymer chains under any constant stress for the composite having less amount filler (Pani) is more compared to composite with higher filler loading. In fact, interparticle gap in composite with higher Pani content is less than that in composite with lower Pani loading and possibility of particle rearrangement due to movement of polymer chain is also less in highly filled composites. In composites with lower loading there is availability of more number of free polymer chains (for lower filled composite) those are unrestricted and unlinked with the filler aggregates, and hence takes more time to be relaxed. In the case of higher filled system, the availability of polymer chains is less, and hence their movements are restricted. So these composites are required less time to be relaxed.
TABLE 2. Parameters value of dielectric constant and loss, and relative dielectric constant and loss with respect to time. Parameters value of dielectric constant and relative dielectric constant with respect to time Initial value (e00 0)
Amplitude (Ae00 )
Relaxation time (de00 )
Composition
Absolute
Relative
Absolute
Relative
Absolute
Relative
E100P54 E100P134
19,231 6 815 1,853,700 6 28,360
4.1 6 0.17 1.9 6 0.03
212,721 6 777 2817,143 6 31,070
22.7 6 0.16 20.9 6 0.03
2693 6 463 1925 6 230
2693 6 463 1925 6 230
Parameters value of dielectric loss and relative dielectric loss with respect to time Initial value (e00 0)
Amplitude (Ae00 )
Relaxation time (de00 )
Composition
Absolute
Relative
Absolute
Relative
Absolute
Relative
E100P54 E100P134
128,859 6 3286 22,313,800 6 223,158
3.4 6 0.09 1.6 6 0.02
278,238 6 4644 27,365,420 6 309,013
22.1 6 0.12 20.5 6 0.02
1320 6 248 1359 6 181
1320 6 248 1359 6 181
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FIG. 8. Effect of time on dielectric loss under constant pressure. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Scanning Electron Microscopy (SEM) Study of EVA/Pani Composites The SEM images of neat EVA, neat Pani, and EVA/ Pani composites have been presented in Fig. 10a–d. The SEM study on cryofracture surface for neat EVA and
FIG. 9. Effect of time on relative dielectric loss under constant pressure. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
EVA/Pani composite have been carried out; whereas, for neat Pani, only powder sample has been used. To check surface morphology, the SEM study after application of pressure on EVA/Pani composite has also been performed. It is seen from Fig. 10a that neat EVA is having smooth surface which is ductile in nature. Figure 10b
FIG. 10. SEM study of a) neat EVA, b) neat Pani, c) EVA/Pani composite, and d) pressed EVA/Pani composite.
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POLYMER ENGINEERING AND SCIENCE—2013 7
shows granular type morphology of neat Pani. It is expected that this grain-shaped morphology of conductive additive polyaniline is more suitable filler for polymer composite to be used as dielectric pressure sensitive sensor compared with its fibrous/rod like/ribbon-shaped morphology. Actually, the dielectric property depends on the shape of inclusion in the host medium. With the application of pressure there is a chance of change in shape from its granular form to other forms like oval/elliptical ones. Thus the scope to develop interparticle electrical interaction through polarization is increased. This may result in the increase in dielectric response. However, with the application of pressure on fibrous/rod like/ribbon shaped fillers may under go breakdown of filler chain/aggregates leading to the reduction in their aspect size/ratio. This results in more irregular change and also decrease in dielectric response with the increase in applied pressure. It is observed from Fig. 10c that the Pani particles are aggregated in the polymer matrix mainly because in this composite (E100P134) Pani is continuous phase. There is also some change in surface morphology for pressed composite as shown in Fig. 10d. CONCLUSIONS Both dielectric constant and dielectric loss of the EVA/Pani composite increase with the increase in Pani concentration in the polymer matrix. The dielectric constant and loss have been found to increase with the increase in applied pressure initially sharply followed by slow and then marginal change for all composites under investigation. The change in dielectric constant and loss against time duration under constant compressive stress are also very similar in nature only difference lies in terms of magnitude of change in two properties. The SEM picture shows grain type morphology of Pani. These results show that these composites can be used as dielectric sensors. The relaxation time for the composite having lower Pani content has been found to be higher compared with the higher filled ones. The results show that the composite having less amount of Pani is much better for the use as dielectric sensor. It is also observed that these composites are sensitive to wide pressure range but more sensitive to lower range of pressure (0–30 kPa) where they are most efficient as dielectric sensor.
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