Development of Flexible Piezoelectric Poly (dimethylsiloxane

Article pubs.acs.org/IECR

Development of Flexible Piezoelectric Poly(dimethylsiloxane)− BaTiO3 Nanocomposites for Electrical Energy Harvesting Suryakanta Nayak, Tapan Kumar Chaki,* and Dipak Khastgir* Rubber Technology Centre, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India S Supporting Information *

ABSTRACT: Flexible poly(dimethylsiloxane)−BaTiO3 (PDMS-BaTiO3) nanocomposites of different compositions are prepared via room-temperature mixing for possible sensor and electrical energy-generation applications. The effect of BaTiO3 particles (multipods) on electrical properties is extensively studied, and it is found that permittivity of composites is increased significantly whereas the volume resistivity is decreased with the increase in BaTiO3 concentration. The mechanical properties of PDMS-BaTiO3 composites are also composition-dependent where both tensile strength and percent elongation at break decreases with increase in BaTiO3 particle concentration because of the nonreinforcing nature of BaTiO3 particles, as is apparent from Kraus plots. These composites have excellent piezoelectric behavior, where the dielectric properties of these composites changed substantially with the change in the applied stress. The temperature-dependent dielectric properties reveal that dielectric properties increased with a rise in temperature up to a certain limit and decreased thereafter. Filler shape, dispersion, and distribution in the matrix polymer were observed through field emission scanning electron microscopy. and mechanical properties and dimensional stability of the final composites.5 Barium titanate (BaTiO3) is a ferroelectric ceramic oxide with good piezoelectric properties and is useful for many applications.4,23−25 Generally, BaTiO3 oxide has a high dielectric constant and can attain four crystal shapes (tetragonal, cubic, rhombohedral, and orthorhombic) depending upon the environment temperature. However, the dielectric constant value depends on many factors, such as shape and size of the crystals, impurities, and processing conditions. The present study includes the use of barium titanate nanoparticles which were prepared through high-temperature solid-state reaction using Ba(OH)2·8H2O and TiO2 as the starting materials. The prepared particles are purified before being used in composite preparation. Here, polydimethylsiloxane elastomer and prepared barium titanate nanoparticles are used for developing the nanocomposites. Different mechanical and electrical properties of PDMS-BaTiO3 composites at various filler concentrations are studied. The influence of filler concentration on electrical and mechanical properties of these nanocomposites was also measured. The effect of pressure on direct current (dc) conductivity (σdc), dc current (Idc), and dielectric constant (ε′) is studied where dielectric constant was measured with respect to the frequency. The effect of temperature on dielectric properties is also studied at three different frequencies (1, 103, and 106 Hz).

1. INTRODUCTION Nanotechnology is now considered to be one of the most promising areas for technological development in the 21st century.1 Nanocomposites are hybrid materials in which the nanometer-sized dispersed phase in a suitable matrix can enhance some existing property as well as give rise to new properties.2,3 Polymer nanocomposites (polymer matrix reinforced with nanoparticles) are of great interest because addition of a nanofiller in a polymer matrix can improve mechanical, thermal, and barrier properties and can also modify other properties.4 A review of the literature reveals that mostly these nanocomposites consists of nanoclay, nanofiber, carbon nanotubes, or other inorganic oxides or particles dispersed in a suitable matrix polymer.4−10 Some of these composites have drawn interest for their possible applications as electronic materials11 for integrated decoupling capacitors,12 electronic packaging,1 acoustic emission sensors,13 and angular accelerometers.1 Polymer−ceramic nanocomposites are good dielectrics that can be used for making thin-film capacitors. Tailor-made dielectric properties can be attained through judicial selection of appropriate electroceramic material, matrix polymer, and their relative composition.14 The good distribution or dispersion of ceramic particles in a matrix polymer implies homogeneous packing, and composites with homogeneous packing are supposed to have high permittivity.14−20 Polydimethylsiloxane (PDMS) elastomer has excellent thermal, elastic, and mechanical properties. It also has good environmental stability. The composites derived from PDMS elastomer can be used in a large range of temperatures and have outstanding weather and chemical resistance properties.21,22 PDMS elastomer can be used for biological and chemical applications because of its chemical stability, mechanical compliance, biocompatibility, optical transparency, and ease of production. Generally, ceramic fillers (oxides) are incorporated in the polymer matrix to enhance the thermal, electrical, © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials Used. Commercial grade polydimethylsiloxane elastomer was used as the base matrix for the present work, which was purchased from D J Silicone in block form; shore A Received: Revised: Accepted: Published: 14982

July 1, 2014 August 14, 2014 September 5, 2014 September 5, 2014 dx.doi.org/10.1021/ie502565f | Ind. Eng. Chem. Res. 2014, 53, 14982−14992

Industrial & Engineering Chemistry Research

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Table 1. Formulations of PDMS-BaTiO3 Nanocomposites composition parts by weight per hundred parts of polymer (BaTiO3 amount in wt % is given in parentheses) ingredients

P100BT0

P100BT10

P100BT30

P100BT50

P100BT70

PDMS BaTiO3 DCP

100 0 1.5

100 10 (9.09 wt %) 1.5

100 30 (23.08 wt %) 1.5

100 50 (33.34 wt %) 1.5

100 70 (41.18 wt %) 1.5

hardness, 40; density, 1.12 g/cm3. Anatase grade titanium dioxide (TiO2) and Ba(OH)2·8H2O were procured from Merck chemicals, India. Dicumyl peroxide (MP, 80 °C; purity, 98%; Sigma-Aldrich chemical company, United States) was used for curing purposes. The solvents (ethanol and conc. HCl) were purchased from Merck chemicals, India. 2.2. Preparation of PDMS-BaTiO3 Nanocomposites. The electro-ceramic filler (BaTiO3 multipods) used for the present investigation was prepared via high-temperature solidstate reaction. The detailed procedure for the preparation and purification of BaTiO3 multipods was reported by us in an earlier contribution.25 Different PDMS-BaTiO3 nanocomposites were prepared through room-temperature mixing, where BaTiO3 and other ingredients were added into the PDMS matrix using an internal mixer (Brabender plasticorder, model PLE 330) with a shear rate of 45 rpm with a mixing time of 10 min. Different compositions by parts of filler per hundred parts of polymer by weight (php) were prepared, and the concentration of BaTiO3 particles in the composites varied from 10 to 70 php. However, for ease of understanding, “php” concentration (loading) of filler in polymer matrix has been converted to weight percentage (wt %) of filler and can be written as 10 php = 9.09 wt %, 30 php = 23.08 wt %, 50 php = 33.34 wt %, and 70 php = 41.18 wt %. The BaTiO3 particles and the cross-linking agent were mixed with the pristine PDMS elastomer in a sequence as per the formulations given in Table 1. Then these compounds were passed through a two-roll mill to make them into sheet form. The cure characteristics of different compounds were estimated by a rubber process analyzer operating at 150 °C. Different test specimens were prepared from all composites using a compression molding press at 150 °C and cured up to the optimum curing time. Different composites were designated by using an alphanumeric designation, for example, P100BT10 indicates the composition of PDMS elastomer and BaTiO3 particles containing 10 parts of barium titanate by weight per hundred parts of polymer (php), where P stands for PDMS and BT stands for barium titanate. Note: the concentration of curative (dicumyl peroxide, DCP) remains constant in all composites, so the term for DCP is not included in the alpha-numeric designation.

precision LCR meter (Quad Tech 7600) attached with a homemade sample holder having parallel plate circular electrodes. All measurements were carried out over the frequency range of 10−106 Hz. The temperature-dependent dielectric properties of nanocomposites were measured by a Navocontrol, Alpha-A high-performance frequency analyzer over the temperature range of 40−200 °C. The variation of dielectric constant (ε′) with respect to change in compressive stress (pressure, 0−370 kPa) was measured by LCR meter (GW-Instek LCR-819) coupled with a homemade sample holder with two circular electrodes. The effect of pressure on dc current (Idc) and dc conductivity (σdc) of all the composites was analyzed using a high-resistance meter (Agilent 4339B) attached with a resistivity cell (Agilent 16008B). The dielectric breakdown strength of the composites was measured using a Multiplex 11 kV BDV tester (M/s Agro Scientific Industries, Pune, India). Thermogravimetric analysis (TGA) was carried out using TA Instruments (Q 50), at a heating rate of 10 °C min−1 under nitrogen atmosphere from 40 to 700 °C. Differential scanning calorimetry (DSC) study was done using TA Instruments (Q 100), at a heating rate of 10 °C min−1 under nitrogen atmosphere from −150 to 100 °C. The morphology and distribution of BaTiO3 filler in the composites were characterized by field emission scanning electron microscopy (FESEM, Supra 40, Carl Zeiss SMT AG, Germany). Tensile and tear properties were measured using universal testing machine (model H10KS, Hounsfield) in accordance with ASTM Standard D 412 and ASTM Standard D 624, respectively. Hardness (Shore-A) was measured according to ASTM Standard D 2240 using Durometer Type A (Shore Instrument & MFG. Co. Inc., New York). The swelling study was carried out for 5 days using the chloroform solvent.

4. RESULTS AND DISCUSSION 4.1. X-ray Diffraction Analysis. X-ray diffraction (XRD) analysis has been carried out on both impure and pure BaTiO3 powder. The purified BaTiO3 powder exhibits all well-defined peaks reported for neat BaTiO3, which revealed the tetragonal crystal structure of BaTiO3 particles. The detailed XRD analysis of BaTiO3 multipods was reported by us in a previous contribution.25 4.2. Morphology of the Neat BaTiO3 Particles and PDMS-BaTiO3 Composites. Figure 1 shows the FESEM image of the prepared barium titanate (BaTiO3) particles whose shape is like that of multipods. Each BaTiO3 multipod consists of a number of 3D-bamboo leaf-shaped particles with a greater width in the middle part of the particles and sharp ends where the average particle length is ∼3.5 μm and the average particle diameter is ∼300 nm, especially in the middle portion of the particles (see inset of Figure 1). It is seen that these particles are attached with each other at the center and starlike particle clusters are formed. Cryo-fractured surfaces of different PDMS-BaTiO3 composites with various filler loadings (e.g., 9.09 wt % (10 php), 23.08

3. MEASUREMENTS AND CHARACTERIZATION The wide-angle X-ray diffraction (WAXRD) analysis was carried out by using a high-resolution X-ray diffractometer (X′Pert PRO, Philips PAN analytical B.V., Almelo, Netherlands) with a Cu Kα (λ = 0.154 nm) radiation source. The phase and crystallographic state of the samples were scanned in the 2θ range from 10 to 90° at a scan rate of 0.13° s−1 using an X′celerator mode. The dc resistivity of neat PDMS and PDMSBaTiO3 composites were measured using a high-resistance meter (Agilent 4339B) attached with a resistivity cell (Agilent 16008B). Poling of composites was done by passing high voltage (∼1 kV) through the molded sheets for 4 h. Dielectric properties of PDMS-BaTiO3 composites were measured using a 14983

dx.doi.org/10.1021/ie502565f | Ind. Eng. Chem. Res. 2014, 53, 14982−14992


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of BaTiO3 particles are found to be distributed in the polymer matrix and many such clusters are projected out from the polymer matrix, as observed in Figure 2. It is clearly observed that top surfaces of the filler particles are free of polymer coating, as apparent from Figure 2d (41.18 wt % (70 php) of BaTiO3). The distribution of filler particles in the PDMS matrix can also be understood from the energy-dispersive X-ray spectroscopy (EDX) analysis of the composite containing 23.08 wt % (30 php) of BaTiO3; the corresponding elemental compositions are presented in Table S1 of the Supporting Information. The particle dispersion can also be visualized from the area mapping of PDMS-BaTiO3 composite (contains 41.18 wt % BaTiO3) with respect to Ba, Ti, Si, and O concentration as given in Figure S1a−d of the Supporting Information. It is observed from FESEM images that the distributions of BaTiO3 particles (individual/cluster) become uniform all over the matrix polymer when the loading of filler particles in the polymer matrix is increased. 4.3. Effect of Filler Concentration on Electrical Properties. 4.3.1. DC Volume Resistivity. The dc volume resistivity of polymer composites depends on the resistivity of both matrix polymer and particulate fillers.6 The effect of the BaTiO3 loading and poling on the dc resistivity of the composite is presented in Figure 3, and it is found that the dc volume resistivity of the PDMS-BaTiO3 composites is composition-dependent. The neat PDMS matrix has a resistivity in the order of 1015 Ω cm. However, when BaTiO3 particles are added into the PDMS matrix, the dc resistivity decreases gradually with the increase in filler concentration. The regular decrease in volume resistivity with the filler loading is due to the low resistivity of BaTiO3 particles as compared to that of the PDMS matrix. Moreover, inorganic particles also contain some moisture; as a result, with the increased filler loading, dc resistivity of the composites decreased. The presence of moisture on filler surfaces helps in the ionization of ionic species in the composite system, which decreases electrical resistivity of the composites. The dc volume resistivity of these composites is found to decrease with the poling, which

Figure 1. FESEM image of neat barium titanate (BaTiO3) particles.

wt % (30 php), 33.34 wt % (50 php), and 41.18 wt % (70 php) of BaTiO3) are analyzed by field emission scanning electron microscopy (FESEM) (Figure 2). The filler particles (BaTiO3 multipods) are clearly seen from these images where particulate fillers are well-coated with the matrix polymer at low loading (10 php). It is observed that particulate fillers are properly wetted by the polymer matrix. However, as the filler concentration in the polymer matrix is increased, the wettability of filler particles with polymer matrix is reduced because of the dilution effect.6 It is observed that with the increase in the filler loading in the matrix polymer, the tendency of clustering of fillers increases. This reveals that all filler particles in a big cluster are not properly wetted by the matrix polymer. This means that filler particles are not strongly attached to the matrix polymer. In fact, the detached particulate fillers act as stress raisers (failure site) when subjected to a deforming force during the mechanical test. As a result, the strength of the polymer matrix will be reduced with the increase in particulate BaTiO3 loading as expected. Indeed, that is what is observed in the case of mechanical properties such as percent elongation at break (% EB) and tensile strength. At high filler loading, bigger clusters

Figure 2. FESEM images of the cryo-fractured surfaces of the composites (a) 9.09 wt %, (b) 23.08 wt %, (c) 33.34 wt %, and (d) 41.18 wt % of barium titanate. 14984



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dielectric constant value. The variation of impedance with frequency for composites containing various filler loadings is presented in Figure S2 of the Supporting Information. It is observed that at any particular frequency the magnitude of impedance decreases with the increase in filler (BaTiO3) concentration (Figure S2), which is also apparent from the dc resistivity plot (Figure 3). The percentage error (% error) is found to be ε″; if tan δ > 1, then ε′ < ε″. For a better capacitor, the dielectric constant (ε′) should be high but tan δ should be as low as possible. Composites containing 41.18 wt % of BaTiO3 show high dielectric loss in comparison to that of other composites over the temperature range while the dielectric constant is still more than the dielectric loss value. This means tan δ < 1. From these

σac = σdc + 2πfε″

(1)

At a fixed frequency and at different temperature, how σdc and ε″ change with temperature will determine the variation of σac versus temperature. Generally, at very high frequency, the contribution of σdc is reduced, but at low frequency, this increases. Dielectric loss (ε″) is due to different polarization processes being operative in the system. How all these 14987



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Figure 9. Arrhenius plots for (a−c) composite containing 41.18 wt % BaTiO3 at different frequencies (1 Hz, 1 kHz, and 1 MHz) and (d) composites containing 23.08 and 41.18 wt % of BaTiO3 at 1 kHz.

activation energy (Ea = 6.96 kJ/mol) is more at low frequency (1 Hz), whereas it is too small (Ea = 0.26 kJ/mol) at high frequency (1 MHz). We have also checked the activation energy of composites with different filler loading (23.08 and 41.18 wt %) at a particular frequency (1 kHz) and found the activation energy is more in the case of the composite with higher loading (Ea = 3.6 kJ/mol) than the composite with low loading (Ea = 0.65 kJ/mol). 4.3.5. Effect of Poling on Dielectric Properties. In the present biphasic composite system, the matrix polymer is nonpolar in nature, whereas dispersed phase BaTiO3 particles have induced dipoles under applied electric field. Further, the matrix and dispersed phase differs in conductivity and dielectric constant. The generation of interfacial/space charge is most likely polarization. The restricted movements of bound charges like dipoles and interfacial charge occur when an electric field is applied to the system. The application of a strong electric field during poling causes oriental organization of induced dipoles in a particular direction (Scheme 1), thus increasing dipole moment under electric field, which is reflected in increased dielectric constant after poling. In the present study, we have

processes are altered with temperature at any particular frequency will determine the variation of σac versus temperature. This composite system does not contain permanent dipoles because of the nonpolar matrix, but it has induced dipoles because of filler particles, space charge, and interfacial charge because of heterogeneous system in which the matrix and filler differ significantly with respect to dielectric constant and conductivity. The variations of ac conductivity (σac) as a function of temperature (Arrhenius plot) are shown in Figure 9a−d. The ac conductivity increases with the increase in temperature, as described in Figure S3a−c of the Supporting Information, which is also fitted to Arrhenius exponential equation (eqs 2 and 3) as shown in Figure 9a−d.27 ⎛ E ⎞ σac = σ0 exp⎜ − a ⎟ ⎝ RT ⎠

(2)

Ea RT

(3)

ln σac = ln σ0 −

where σac is the ac conductivity, σ0 the high-temperature limits of conductivity, Ea the activation energy, R the gas constant (8.314 J mol−1 K−1), and T the temperature. The slope (m) of the above Arrhenius equation is −Ea/R from which we can calculate activation energy (Ea). The conduction activation energy of the conductivity influenced by temperature is the minimum energy required to overcome the potential barrier in the composite system. The linear curves called Arrhenius plots shown as straight lines are obtained using a linear curve fitting method. We found there is an increase in ac conductivity (σac) of composite (filled with 41.18 wt % BaTiO3) with increase in the measurement frequency (Figure 9a−c). From Figure 9a−c, it is seen that

Scheme 1. Effect of External Electric Field on Polarization/ Dielectric Constant (ε′)

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applied ∼1 kV to the composites for 4 h to see the effect of poling on dielectric properties. The dielectric constant value increases with the increase in polarization, and in the present system the increase in dielectric constant after poling is more in the case of composites with higher filler content (41.18 wt %) than in the case of composites with lower loading, as is apparent from Figure 10.

marginally when the concentration of BaTiO3 filler is increased from 0 to 23.08 wt %. Beyond 23.08 wt % filler, the breakdown strength marginally decreases with filler concentration. However, the breakdown strength is still marginally higher than that of the neat PDMS matrix even at the highest loading of 41.18 wt %. The maximum breakdown strength observed in composites filled with 23.08 wt % of BaTiO3 particles could be attributed to the proper dispersion of filler particles in PDMS matrix and to better interaction between fillers and polymer matrix. The introduction of higher concentration of BaTiO3 particles in PDMS matrix inevitably introduces microvoids and defect centers, which enhances the local electric field at these spots. Beyond 23.08 wt % filler loading, further increase in local electric filed is due to more number of microvoids, resulting in decrease in the breakdown strength. When high dielectric constant filler particles are added into a neat polymer matrix, the electric field in the polymer around the filler particles is relatively larger than that in the bulk. At higher filler loading, the dilution effect occurs, which signifies that all filler particles are not properly wetted by the polymer matrix. The loosely bound filler aggregates act as electrical stress raisers and initiate electrical breakdown.30 Thus, the breakdown strength reduces with increase in filler concentration. 4.4. Effect of Composition on Mechanical Properties. The variation of tensile strength (TS) and percent elongation at break (% EB) for different PDMS-BaTiO3 composites against filler loading are presented in Figure 12a. With the increase in concentration of barium titanate (BaTiO3) particles in the PDMS-BaTiO3 composites, continuous decrease in both % EB and tensile strength are observed. This reveals that BaTiO3 particles (multipods) act as a nonreinforcing filler for PDMS matrix. The reduction in mechanical properties is due to the poor polymer−filler interaction that exists between the PDMS matrix and particulate barium titanate. Polymer−filler interaction can be estimated from the Kraus plot in accordance with eq 5.6

Figure 10. Effect of poling on dielectric constant of composites filled with 23.08 or 41.18 wt % of BaTiO3.

4.3.6. Dielectric Breakdown Strength. Dielectric breakdown is a process which refers to the sudden reduction in the resistance of an electrical insulator, when the voltage applied across it exceeds the breakdown voltage. This results in mechanical damage with electrical conduction, depending on the defect density of the solid material.5,28 Dielectric breakdown is not due to normal charge carriers. It is due to sudden release of a huge amount of strongly bound charges from the system. The breakdown strength of a material also refers to the highest strength of electrical field that can be applied to a film without losing its insulating properties.29 The dielectric strength also determines the energy density (Ue) of a dielectric material that is equal to the integral (eq 4)

Ue =

∫ E dD

Vr0 mϕ =1− Vrf (1 − ϕ)

(4)

(5)

where Vr0 is the volume fraction of elastomer phase in the swollen gel of unfilled gum elastomer vulcanizate, Vrf the volume fraction of elastomer phase in the swollen gel of filled elastomer vulcanizate, Φ the volume fraction of filler, and m the polymer−filler interaction parameter obtained from the slope of the plot of Vr0/Vrf against Φ/(1− Φ). According to the Kraus equation, the plot of Vr0/Vrf versus Φ/(1− Φ) should be a straight line. Positive slope of this plot represents weak polymer−filler interaction (valid for nonreinforcing filler), whereas negative slope represents strong polymer−filler interaction (for reinforcing filler). For the present system (Figure S4 of the Supporting Information), the positive slope for Kraus plot is observed as the value of Vr0/Vrf is continuously increasing with the filler loading. This indicates that particulate barium titanate acts as nonreinforcing filler in the PDMS matrix. The volume fraction of elastomer (Vr) was calculated by using eq 6.

where E is the electric field and D is the electric displacement (charge density). The dependence of the breakdown strength on the different weight fraction of BaTiO3 particles (multipods) is shown in Figure 11. It is observed that the dielectric strength increases

Vr =

(D − FT )ρr −1 (D − FT )ρr −1 + A 0ρs−1

(6)

where T is the weight of the test specimen, D the deswollen weight of the test specimen, F the weight fraction of the

Figure 11. Effect of filler loading on dielectric breakdown strength. 14989



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bond between the polymer matrix and filler particle, the tearing of the matrix would have taken place through a longer path, as filler particles act to deviate the tearing process, leading to an incraese in tear strength.6 4.5. Thermal Analysis. Thermogravimetric analysis (TGA) was conducted in an inert atmosphere (N2) at a heating rate of 20 °C min−1 for neat PDMS and PDMS-BaTiO3 composites. Thermal degradation behavior of the neat PDMS elastomer and its composites are given in Figure 13a,b, and the quantitative

Figure 12. (a) Variation of tensile strength and percent elongation with the filler concentration. (b) Tensile modulus at 100%, 200%, and 300% elongation against BaTiO3 loading. (c) Variation of tear strength and hardness versus filler loading.

Figure 13. (A) TGA and (B) DTG thermograms of neat PDMS and its nanocomposites.

insoluble component in the sample, A0 the weight of absorbed solvent, ρr the density of the elastomer, and ρs the density of the solvent. The variation of tensile modulus against the filler loading for different composites is given in Figure 12b. High strain modulus at both 100% and 200% elongation is found to increase slowly but steadily with filler loading, whereas very high strain, i.e., at 300% elongation, the increase is only up to 9.09 wt %; modulus decreases beyond that value. Generally, both reinforcing and nonreinforcing fillers have positive effects on the modulus, unlike tensile strength, up to a certain level of filler concentration.6 In the case of modulus at 300% elongation, the decrease beyond 9.09 wt % BaTiO3 loading is due to the particle aggregation. The hardness (low strain modulus) of these composites is progressively increased with the increase in filler concentration (Figure 12c). The continuous increase in hardness with the filler loading is due to restriction imposed on movement of polymer chains (as polymer exists above its Tg = −120 °C) by filler particles. The tear strength value is also found to decrease with the increase in filler concentration, as shown in Figure 12c. The continuous decrease in tear strength with the filler is also due to the nonreinforcing nature of filler particles as discussed in the case of decrement of tensile strength. Generally, if there is a strong

Table 2. Data Summarized from TGA and DTG Thermograms of PDMS and PDMS-BaTiO3 Composites composition pristine PDMS PDMS-BaTiO3 PDMS-BaTiO3 PDMS-BaTiO3 PDMS-BaTiO3

(9.09 wt %) (23.08 wt %) (33.34 wt %) (41.19 wt %)

Tmax (°C)

R680wt%

571 578 580 575 542

23.8 31.4 41.4 49.4 55.3

values are given in Table 2. The data given in Table 2 includes Tmax (the temperature at which maximum degradation of the composite takes place) and R680 wt % (the residue left after the decomposition at 680 °C). As seen in Table 2, there is an increase in thermal stability with the increase in barium titanate (BaTiO3) concentration in the composite, but the increase is not continuous up to the highest filler concentration. It is observed that the maximum degradation temperature (Tmax) is observed for BaTiO3 loading of 23.08 wt %, but for further increase in filler loading, thermal stability is found to decrease. So it may be concluded that the increase in thermal stability up to 23.08 wt % loading is due to 14990



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the homogeneous distribution of the particulate filler in the matrix polymer up to this loading and that filler particles are acting as heat sink, thus requiring higher thermal energy for polymer degradation. Moreover, the decreased thermal stability of composites containing filler beyond 23.08 wt % may be due to the photocatalytic activity of BaTiO3, which surpasses the thermal stability imparted by filler particles acting as heat sink. Figure 14 shows the DSC curves for the neat PDMS elastomer and PDMS-BaTiO3 nanocomposites. It is observed

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ASSOCIATED CONTENT

S Supporting Information *

FESEM area mapping (Figure S1); impedance (|Z|) versus frequency plot for PDMS-BaTiO3 composites (Figure S2); effect of temperature on ac conductivity (σac) of composites (Figure S3); relative swelling (Vr0/Vrf) versus [Φ/(1−Φ)] (Figure S4); SEM-EDX values of the PDMD-BaTiO3 (23.08 wt % BaTiO3) composite (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +91-3222283192/82. Fax: +91-3222282292. *E-mail: [email protected]. Tel.: +91-3222283192/82. Fax: +91-3222282292. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to Indian Institute of Technology Kharagpur, India for its research facility and for providing research fellowship. The authors are also thankful to Mrs. B. Mandal and Mr. G. C. Dhara, Nicco Corporation Limited, Kolkata, India for the dielectric breakdown strength measurement.

Figure 14. DSC curves for neat PDMS and PDMS-BaTiO 3 nanocomposites.

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that both neat elastomer and the composites show a melting temperature of ∼(−42 °C). The change in glass transition temperature (Tg) with the change in filler loading is marginal. This is apparent from weak polymer−filler interaction leading to decrease in mechanical properties with the increase in filler loading and also is concluded from the Kraus plot.

REFERENCES

(1) Youssef, A. M. Polymer nanocomposites as a new trend for packaging applications. Polym.-Plast. Technol. Eng. 2013, 52, 635−660. (2) Manias, E. Polymer nanocomposite technology, fundamentals of barrier. http://zeus.plmsc.psu.edu/∼manias/PDFs/TAPPI09-paper. pdf (accessed August 6, 2014). (3) Manias, E. Nanocomposites stiffer by design. Nat. Mater. 2007, 6, 9−11. (4) Nayak, S.; Chaki, T. K.; Khastgir, D. Development of poly(dimethylsiloxane)/BaTiO3 nanocomposites as dielectric material. Adv. Mater. Res. (Berlin, Ger.) 2013, 622−623, 897−900. (5) Nayak, S.; Sahoo, B.; Chaki, T. K.; Khastgir, D. Development of polyurethane-titania nanocomposites as dielectric and piezoelectric material. RSC Adv. 2013, 3, 2620−2631. (6) Nayak, S.; Rahaman, M.; Pandey, A. K.; Setua, D. K.; Chaki, T. K.; Khastgir, D. Development of poly(dimethylsiloxane)-titania nanocomposites with controlled dielectric properties: Effect of heat treatment of titania on electrical properties. J. Appl. Polym. Sci. 2013, 127, 784−796. (7) Nguyen, Q. T.; Baird, D. G. Preparation of polymer-clay nanocomposites and their properties. Adv. Polym. Technol. 2006, 25, 270−285. (8) Moniruzzaman, M.; Winey, K. I. Polymer nanocomposites containing carbon nanotubes. Macromolecules 2006, 39, 5194−5205. (9) Sahoo, N. G.; Rana, S.; Cho, J. W.; Li, L.; Chan, S. H. Polymer nanocomposites based on functionalized carbon nanotubes. Prog. Polym. Sci. 2010, 35, 837−867. (10) Beyer, F. L.; Tan, N. C. B.; Dasgupta, A.; Galvin, M. E. Polymerlayered silicate nanocomposites from model surfactants. Chem. Mater. 2002, 14, 2983−2988. (11) Karttunen, M.; Ruuskanen, P.; Pitkanen, V.; Albers, W. M. Electrically conductive metal polymer nanocomposites for electronics applications. J. Electron. Mater. 2008, 37, 951−954. (12) Ramesh, S.; Shutzberg, B. A.; Huang, C. C.; Gao, J.; Giannelis, E. P. Dielectric nanocomposites for integral thin film capacitors: Materials design, fabrication and integration issues. IEEE Trans. Adv. Packag. 2003, 26, 17−24.

5. SUMMARY AND CONCLUSIONS Flexible piezoelectric composites with variable dielectric property can be developed from judicial combination of a PDMS matrix and particulate BaTiO3 filler. Strong interfacial polarization is observed in these composites which is apparent from the high dielectric constant value in the low-frequency region. These composites show composition-dependent dielectric, piezoelectric, and mechanical properties. Such a system can be very effectively used for stress/strain sensitive sensors and electrical energy generation purposes. From temperature-dependent dielectric properties, it is found that both dielectric constant and dielectric loss increased with temperature up to a certain limit (∼150 °C), beyond which it decreased. It is found that barium titanate is a nonreinforcing filler for PDMS matrix, as is apparent from the Kraus plot. There is a minor variation in dielectric breakdown strength which is due to thorough distribution of BaTiO3 particles in the PDMS matrix which is also apparent from the FESEM images. In the present study, though the mechanical properties decrease with filler (BaTiO3 particle) loading because of its nonreinforcing behavior, the composite with 33.34 wt % BaTiO3 loading gives percent elongation at break more than 350, which satisfies the requirement for the development of dielectric elastomer. In the case of dielectric properties, the dielectric constant is constantly increasing with filler loading where at all loading dielectric loss is relatively smaller than the dielectric constant. On the basis of the above properties, the composite with 33.34 wt % BaTiO3 loading is the optimum composition. 14991



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Development of Flexible Piezoelectric Poly (dimethylsiloxane

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