Polyvinylidene fluoride (PVDF) composite

Accepted Manuscript Title: Improved dielectric and touch sensing performance of Surface Modified Zinc Ferrite (ZF)/Polyvinylidene Fluoride (PVDF) composite Authors: Ipsita Chinya, Shrabanee Sen PII: DOI: Reference:

S0924-4247(17)30341-2 https://doi.org/10.1016/j.sna.2017.10.031 SNA 10397

To appear in:

Sensors and Actuators A

Received date: Revised date: Accepted date:

27-2-2017 22-9-2017 11-10-2017

Please cite this article as: Ipsita Chinya, Shrabanee Sen, Improved dielectric and touch sensing performance of Surface Modified Zinc Ferrite (ZF)/Polyvinylidene Fluoride (PVDF) composite, Sensors and Actuators: A Physical https://doi.org/10.1016/j.sna.2017.10.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Improved dielectric and touch sensing performance of Surface Modified Zinc Ferrite (ZF)/ Polyvinylidene Fluoride (PVDF) composite Ipsita Chinyaa,b and Shrabanee Senb,* a

b

Academy of Scientific and Innovative Research, CSIR-CGCRI

Sensor and Actuator Divisions, CSIR-Central Glass & Ceramic Research Institute, 196, Raja, S. C. Mullick Road, Jadavpur, Kolkata-700 032, India

Graphical abstract

Highlights 

The effect of homogeneous dispersion of ceramic nanofiller, Zinc ferrite (ZF), in the Polyvinylidene fluoride (PVDF) matrix on its polar phase transition was investigated.



The homogeneous dispersed composites were prepared after modifying the surface of ZF by two different modifiers: Sodium dodecyl sulphate (SDS) and Tetraethyl orthosilicate (TEOS).



The enhancement in the percentage of polar phase was observed in the polymer matrix after incorporation of surface modified ZF particles.



A probable mechanism was also proposed to explain this enhanced interaction between highly dispersed ceramic filler in the polymer matrix based on zeta potential study and spectroscopic analysis (FTIR and XPS).



The poling effect on surface modified composite was studied, where poled SDS modified ZF-PVDF composite offered the highest energy storage density (0.25 Jcm-3) and poled TEOS modified ZFPVDF composite exhibited the highest open circuit voltage of 2.2V with power density of 3.7µWcm3



on repeated single finger impact.

The above results demonstrated that surface modified ZF composite may be explored for application as touch sensor.

Abstract The continuous demand of fabricating self-powered body implantable devices have raised the development of new lead free polymer-ceramic composites. Herein, we have fabricated Zinc Ferrite (ZF)/ Polyvinylidene Fluoride (PVDF) composite by simple solution casting technique. The inherent difficulty of compatibility between two phases (polymer and ceramic) for homogeneous film was addressed by

addition of two different modifiers: Sodium dodecyl sulphate (SDS) and Tetraethyl orthosilicate (TEOS). Similarity in the piezo/ferro electrical properties between the externally poled unmodified nanocomposite with that of the surface modified composites were observed which were basically attributed to higher dispersion of nanoparticles in the polymer matrix and improved interfacial interaction between organic and inorganic matrix. It indicates that a self polarization effect has been induced in the surface modified composites. The mechanism for the interaction between two phases was discussed on the basis of zeta potential results and spectroscopic analysis. Further, external poled MSDS-ZF(C)-PVDF composite offered the highest energy storage density (0.25 Jcm-3), whereas poled TEOS modified ZF-PVDF composite exhibited an output open circuit voltage of 2.2V with power density of 3.7µWcm-3 under repeated single finger touch.

Introduction Piezoelectric materials have the inherent property of generation of electric potential when subjected to any type of external mechanical stimuli like pressure, elongation, vibration etc. These materials are thus considered as excellent candidates for energy generation [1,2]. The piezoelectric property can be exhibited by ceramic materials like PZT, BT, ZnO etc. and some electroactive polymers like PVDF and its copolymer [3,4]. The electroactive phases of PVDF exhibits the ability of improved efficiency and performance for energy storage and energy harvesting application. PVDF being a semi crystalline polymer exists in at least three main crystalline forms which are known as α, β and γ phase [5,6]. The α-phase is the inactive nonpolar phase while the other two phases (β and γ) are electroactive polar phases. Among the electroactive phases, β phase exhibits maximum dipole moment (2.1D) [7,8] resulting in superior piezo, ferro and pyroelectric properties [9]. Though enormous efforts have been attempted to meet the requirement of complete transformation of polar phase from commercially available PVDF pellets (in which nonpolar α

polymorph is predominant) to increase the efficiency of piezo electric behaviour. It includes mechanical stretching [10] electrospinning [11], melt under specific conditions [12], solvent casting [13] or incorporation of nanofillers such as carbon nanotubes (CNT) [14], carbon nanofibers (CNF) [15], biomolecules [16, 17], ceramics particles like BaTiO3 and PZT [18], ZnO [19], LiTaO3 [20], SiO2 [21], ZnSnO3 [22], GaN [23], Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 [24], doped and undoped ferrite [25-27], surfactant [28], metal/metal complex [29-30], clay [31], salt [32], etc. But it still remains a great challenge to develop efficient cost effective, reliable and environmentally friendly method for this transformation. Lead free, spinel structure ZF had exhibited extensive range of applications in microelectronics [33], pseudo capacitor [34], anode materials for lithium ion batteries [35], semiconductor photo catalysis [36], high-density data storage [37], ferro-fluid technology [38], magnetic resonance imaging [39] etc. However, as per the best of our knowledge the application of this material as nucleating agent during polar phase transformation of PVDF is still not explored so much though it shows remnant polarisation value of nearly 1μC /cm 2 [40], which may give an additional advantage in the energy storage behaviour of the composites. Although the fabrication of ceramic-polymer composites are easy but the homogeneous distribution of the particles in the polymer matrix is a very difficult task. The inhomogeneity in the composite may lead to the degradation of the dielectric and ferroelectric properties of the composites. Thus surface functionalization of the ceramic filler may increase the interfacial interaction between ceramic and polymer matrix. The surface encapsulated ceramic particle construct a core-shell structure. It was predicted in the hypothetical multilayer core model that building of intermediate layer which will get bound to both the inorganic and organic matrix, will enhance the interface compatibility. The use of appropriate modifier can alter the surface charge of the nanoparticle, enhanced the uniformity of distribution of the nanofillers in the ceramic matrix and increase the interfacial interaction by formation of hydrogen bonding, dipole-dipole

interaction or other non-covalent interaction. This helps polymer chain to properly orient on the surface of the nanoparticles. P. Martins et al. used three different types of surfactants namely anionic surfactant, cationic surfactant and non-ionic surfactant and compared their effects [25]. They also used citric acid as surfactant [41]. Liu et al. used other heterostructured polymer like polyvinylpyrrolidone as modifier to enhance the interfacial compatibility of the Ba(Zr 0.3Ti0.7)O3 nanofiber in PVDF matrix [42]. Alam et al. also reported the use of MWCNT for increasing the dispersibility of filler in the polymer matrix [22]. Previously, we have reported the effect of surface modification with SDS of BaTiO3 and PZT [18] and GaFeO3 [43] loaded PVDF nanocomposite for enhancement of the dielectric properties and energy storage capability. Keeping this in view, we made an attempt to enhance the electroactive phase formation of PVDF by incorporating ZF nanoparticles as nucleating agent and further increase the interfacial interaction by addition of surface modifiers (Sodium dodecyl sulphate (SDS) or Tetraethyl orthosilicate (TEOS). SDS is an anionic surfactant which constructs a soft layer on the ZF surface whereas through hydrolysis TEOS can easily form inorganic robust hard coating of silica on the ZF surface. We have selected SDS because of its bipolar structure which can interact both hydrophobic and hydrophilic interaction simultaniously with organic PVDF and inorganic ZF [43] respectively. On the other hand, silica act as intermidiate insulating layer and prevent the migration of negative charges from the ZF surface [44]. Thus ion-dipole interaction density between the ceramic filler and the polymer matrix increases. The space charge polarization decreases after silica insulation by reducing the number of residuals ions on the surface of the ZF particles and reduces the other defects of the nanocomposite [45]. The influence of modification on surface morphology and dielectric/ piezoelectric/ ferroelectric property of the new, flexible and light-weight ZF/PVDF composite were observed. We also studied the effect of external electrical poling on the unmodified composite and observed almost similar results in the piezo and

ferroelectric performance for both unmodified poled composite and poled modified composite. This revealed that surface modification of nanoparticles not only improved the interfacial interaction between the nanoparticle and the polymer matrix but also helped in enhancing in-situ self-polarization behaviour in the modified composite. Further, for better understanding of the effect of addition of surface modifiers, we proposed a probable mechanism for formation and stabilization of enhanced polar phase in PVDF based on results obtained in the zeta potential value and spectroscopic analysis. Experimental Section Materials Zinc nitrate[Zn(NO₃)₂.6H₂O, Sigma-Aldrich], Ferric nitrate [Fe(NO3)3. 9H2O, Sigma-Aldrich], Citric acid [S D Fine-Chem Limited] were used as precursor materials to synthesizes Zinc Ferrite (ZF(C)) via citrate nitrate method. PVDF pellets [M̄w275,000 by GPC, Sigma-Aldrich], N, N-Dimethyl formamide (DMF, C3H7NO, Merck) were used to prepare composite. Sodium dodecyl sulphate [SDS, NaC12H25SO4, Merck] and Tetraethyl orthosilicate [TEOS, Si(OC₂H₅)₄, Sigma-Aldrich] were used for to modify the surface of the nanoparticles. All the chemicals were used in the as- received condition without further purification. Synthesis of nanoparticles and their surface modification Zinc Ferrite (ZF(C)) nanoparticles were prepared by citrate nitrate route which has been reported by M. Maharajan et al [46]. The surface modification of ZF nanoparticles by SDS was done by treating the nanoparticle in 1 lit of 0.1milimole concentration of SDS aqueous solution. The SDS concentration was kept below its critical micelles concentration (CMC) with respect to all the particles for availability of all SDS molecules for surface modification. The solution was heated at 120˚C and continuously stirred till complete evaporation of water. The unreacted SDS was removed from the SDS modified ZF nanoparticle by washing several times with distilled water. The nanoparticles were collected by centrifugation (10,000 rpm, Remi,

PR-24). Further it was dried overnight in an oven at 120˚C to obtain the final product identified as ZF(C)MSDS. For the surface modification step by the second surfactant TEOS, required amount of TEOS was added on the homogeneous solution in ethanol of ZF(C) and stirred for 2 h. The nanoparticles were collected by centrifugation (10,000 rpm, Remi, PR-24) and identified as ZF(C)-MTEOS. Preparation of PVDF nanocomposite The nanocomposites were prepared using two-step solution casting method. First 8wt% PVDF was dissolved in DMF. Different wt% (0.25, 0.50, 1.00, 1.50 with respect to PVDF) of surface modified and unmodified nanoparticles ((ZF(C), ZF(C)-MSDS, and ZF(C)-MTEOS)) were mixed in the PVDF-DMF solution and stirred vigorously for 2 days and drop casted on clean glass slide. The solvent was evaporated at 120˚C for 5 h to fabricate freestanding, flexible, thin film of the composite. Four films of each composite were taken and hot pressed by Carver Press at 150˚C applying operating pressure 13×104N for 20 mins for electrical measurements. Characterizations X-Ray Diffraction (XRD) measurements were carried out using X’pert Pro MPD XRD system (PAN analytical) with nickel-filtered CuKα (λ=0.15404 nm, with a step size of 0.02o). The vibrational modes were analysed using Fourier Transform Infrared Spectroscopy (FT-IR) (FTIR-Spectrum 2, Perkin Elmer). The change in binding energy of the carbon atom of the composite as compared to neat PVDF was analysed by X-ray Photoelectron Spectroscopy (XPS) (PHI 5000 Versa probe II Scanning XPS microprobe manufactured by ULVAC-PHI, USA) employing monochromatic Al Kα source (1486.6eV) as excitation source with an overall energy resolution of 0.7 eV. High Resolution Transmission Electron Microscopy (HRTEM) analysis (Tecnai G2, 30ST, (FEI) instrument operating at 300 kV) was done for morphological analysis. The distribution of the ceramic filler in the PVDF matrix was examined by scanning electron microscopy (SEM Supra 35VP, Carl Zeiss, operated at an accelerating voltage of 10 kV). The presence of elements was identified using Energy

Dispersive X-ray Spectroscopy (EDS) equipped with FE-SEM and TEM chamber. To measure electrical properties, both sides of the hot pressed composites (thickness ≈ 0.28-0.30 mm) were painted with conductive silver paste and cured at 120oC. The dielectric study was executed with a precision impedance analyser (6500 B Wayne Kerr) in the frequency range of 100 Hz to 1MHz at room temperature in a closed chamber of constant (60%) humid condition. All the ferroelectric hysteresis loops were measured (by aixact system GmbH, TF Analyser 2000) at room temperature after application of varying electric field. In order to evaluate the performance of the composite as nanogenerator, copper wires were attached with both silver painted surfaces of the electrode. The touch sensing performance of the composites were measured by a digital storage oscilloscope (Agilent DSO3102A) via application of mechanical energy in the form of repeated single human finger impart. During finger tapping, the overall charge was distributed through the surface of the composite which leads to the formation of electrical potential difference. This was shown by the appearance of the positive and negative sign of the amplitude indicating the imparting and releasing of human finger. Similar study was done for neat PVDF to compare the effect of ceramic inclusions. The electrical poling was done by using an indigenous poling unit (Neo-Teletronix Pvt. Lmt). Result and discussion Characterizations of ZF and modified ZF nanoparticle X-ray diffraction patterns of the ZF(C), MSDS-ZF(C) and MTEOS-ZF(C) nanoparticles are shown in Fig.[1]. All ZF(C) powders matched well with reported values of ZnFe2O4 phase (JCPDS: PDF#821042). The result revealed that pure single phase cubic structure of ZnFe2O4 has been formed. The crystallite size (L) of the ZF nanoparticle was calculated by using Debye-Scherrer equation:

𝐿=

0.9 𝜆 𝑏𝐹𝑊𝐻𝑀 𝑐𝑜𝑠𝜃

(1)

where bFWHM is the full width at half maxima (FWHM) of the diffraction peak, λ is the X-ray wavelength, ϴ is the Bragg angle. The average crystallite size was found to be 31-34 nm.

The FTIR transmission spectra of ZF(C), MSDS-ZF(C) and MTEOS-ZF (C) in the 400–1600-cm-1 region are shown in Fig.[2]. In each spectra the peaks observed in the 418-420cm-1 range correspond to Zn-O stretching bond whereas those at 540-542 cm-1 correspond to Fe-O stretching [47, 48]. Additionally, for MSDS-ZF(C) spectra, peaks that appeared in 1235cm-1, 1200 cm-1, 1060 cm-1 and 975 cm-1 range are due to hydrophilic sulfonate head [49] and thus indicated that surface functionalization of ZF(C) has been successfully done. On the other hand, for MTEOS-ZF(C), peaks appeared in 1147 cm-1 , 1100 cm-1 for asymmetric and symmetric stretching in linear structures of Si-O-Si bond and 793 cm-1 for SiO2 bending [50]. The shifting of frequency of Si-O-Si bond confirms about silica encapsulation over the ZF(C) surface. These results confirmed the presence of SDS and surface coating on ZF(C) powders with SiO2 by shifting of the Si-O-Si peak which was further observed in bright field HRTEM image of MTEOS-ZF(C) discussed in subsequent paragraph. The HRTEM results are shown in Fig. [3-4(a) and (b)] (in each Fig, bright field image is on the upper left, on the upper right is the EDX result, lower left is the corresponding SAED pattern of the powders and the lattice fringes are on the lower right, inset Fig on the lower left is its corresponding FFT representing for each powders) for ZF(C), MTEOS-ZF(C) and MSDS-ZF(C) respectively. The particle size obtained from BF images of the pristine ZF(C) matched well with crystallite size calculated from corresponding XRD plot but they are in more agglomerated form. On the other hand, a noticeable contrast between core ZF and encapsulation layer was observed in BF and HRTEM image of modified ZF(C). The planes denoted in SAED patterns matched well with their respective XRD database. Characterization of nanocomposite The change in crystallinity and induced electroactive (β and γ) phases of PVDF were observed in the XRD pattern of the unmodified and modified (SDS and TEOS) composites (Fig. [S1a]) . The characteristics peak at 2θ=17.7o (100), 18.4o (020), 19.9o (110) and 26.5o (021) in pure PVDF film correspond to the α-crystalline phase. With gradual increase in filler content in the composite, the peaks at 2θ=17.7o and 26.5o

corresponds to α-phase have been reduced and finally disappeared. Thus the addition of filler may help in promoting the nucleation of electroactive phase in PVDF matrix. The peak at 26.5o completely disappeared in case of 1 wt % unmodified composite but for surface modified composite, only 0.5 wt% of nanoparticle loading is sufficient to diminish this peak. The presence of the peak at 2θ=20.1o corresponds to the diffraction from (002) plane indicating the stabilization of γ phase of PVDF in the composite. The presence of all the phases (α, β and γ) was also determined individually by deconvolution of the peaks shown for 1.5wt% (Fig. [S1b]) ZF-PVDF unmodified and modified composites. The enhancement in the percentage of the polar phase after surface modification indicated that both TEOS and SDS have helped in orientation of –CF2 bond in PVDF molecules in a more extended TTT order. Again, crystalline polymorphs were identified using FTIR analysis. The FTIR spectrum in Fig.[5] showed characteristic absorption bands at 408 cm-1, 531 cm-1, 614 cm-1, 764 cm-1, 796 cm-1, 854 cm-1,976 cm-1, 1146 cm-1, 1213 cm-1, 1383 cm-1 for α-phase; 510 cm1

, 840 cm-1, 1274 cm-1 for β-phase and 431 cm-1, 840 cm-1, 1233 cm-1 for γ-phase [51-52] respectively. Due

to the common TTT configuration, absorption bands of the β–phase and γ-phase of PVDF are sometimes superimposed on each other creating difficulty in phase separation [53]. From the FTIR spectra, it was clear that neat PVDF was enriched with nonpolar α-phase and small amount of polar β- and γ-phase. But with increasing filler concentration, intensity of the absorption bands for α-phase gradually reduced and bands for polar phases gradually increased i.e. nanoparticles stimulated polar phase transformation in PVDF. The content of the polar phase fraction (FP) in the nanocomposite can be calculated from the equation:

𝐹𝑝 =

𝐴𝑝 𝑘 𝐴𝑝 +( 840)𝐴𝛼

(2) [52, 54]

𝑘764

where Ap and Aα are the absorption intensity (peak height) at wavenumber 840 cm-1 and 764 cm-1 respectively and K764 and k840 are the absorption coefficients at the wave number 764 cm-1 and 840 cm-1, and the values are K764=6.1 × 104 cm2/mol and K840=7.7 × 104 cm2/mol respectively.

The polar phase fraction with respect to the nanoparticle loading for each unmodified and modified nanocomposites was plotted in Fig. [S2]. It was observed that with increase in the nanoparticle loading in the PVDF matrix, fraction of electroactive phase in the composite also increased. This increment was more prominent in the surface functionalized composites which indicates interfacial interaction between the negative surface charged nanoparticle and positive- CH2 bond of PVDF and leads to (TTTT) and (T3G+T3G-) configuration in PVDF corresponding to the polar β-phase and γ-phase more promptly [53]. Thus surface modification helped in the nucleation and stabilization of the polar phase in PVDF. Further to clarify the interfacial interaction of the MSDS-ZF(C) and MTEOS-ZF(C) with the molecular dipoles (CH2 and CF2) in the PVDF matrix high resolution XPS spectra analysis (Fig. [6]) was performed. The presence of two carbon moieties i.e. CH2 and CF2 with binding energy at 286.9 and 289.3 eV observed in the PVDF film shifted towards lower binding energy (284.9 eV and 287.3 eV respectively) which may be due to the change in the electronic environment of the CH2 and CF2 dipoles of PVDF after addition of ZF(C) nanoparticles. A considerable change in the peak area was observed after normalization of CH2 peak. The enhancement in the peak area of CF2 species (64%) for ZF(C) composite gives a clear indication of the interfacial interaction. In case of surface modified composite this reduction in peak area is more prominent indicating more interfacial interaction. On the basis of the detail analysis of FTIR and XPS spectra, we assume that there is an interaction between the MSDS-ZF(C) and MTEOS-ZF(C) with the CF2 dipoles of PVDF which have lead to the formation of extended trans (TTT) configuration in the modified composites. The probable mechanism for this phenomenon was predicted. From the zeta potential value (Table. [1]) of ZF(C) it was evident that ZF(C) surface is negatively charged. When ZF(C) powders were modified with SDS, hydrophilic SO42- group present in the outer part and alkyl hydrophilic part is attached electro statically on the negatively charged ZF surface. Again, negatively charged SDS modified surface attracts the positive charged H atom in the –CH2 bond PVDF molecule by ion-dipole interaction (Fig. [7a]) and thus there is a

tendency of the F atom of -CF2 bond PVDF molecule to be on the surface and form extended TTT conformation which corresponds to the electroactive phase of PVDF. For TEOS modification, TEOS forms Sample

Zeta potential value (mV)

ZF(C)

-29.07

MSDS-ZF(C)

-12.45

MTEOS-ZF(C)

-13.78

Table. 1 Zeta Potential of unmodified and modified ZF nanoparticle. Si(OH)4 in ethanol medium. Further, it reacted with the absorbed –OH ion in the ZF(C) surface and condensation reaction occurred, and one water molecule comes out for each reaction, forming one -O-SiO- bond in ZF(C) surface. The two nearest -O-Si-O- bond form one six member stable ring, where the lone pair of O atoms between two Si atoms of the six member ring (-Si-O-Si-) co-ordinately get attached (lone pair-dipole interaction) with the H atom of -CH2 bond of PVDF molecule (Fig.[7b]) and corresponding polar phase of PVDF may be formed where F atom of -CF2 bond PVDF molecule remained on the outer surface. Thus in both cases, there is a possibility of formation of extended TTT configuration leading to the enhancement of the electroactive phases.To investigate the effect of addition of ZF(C) particles in the PVDF matrix, FESEM analysis of unmodified and modified (SDS and TEOS) composites along with neat PVDF (in Fig. [8]) was done. More number of α- spherulitic structure (compared to the composite) are shown in neat PVDF (inset of Fig. [8](A)) and it indicates that neat PVDF is enriched with α-phase. For ZF(C)-PVDF composite agglomeration was observed (Fig. [8](A)) which may be due to the high surface energy of the ZF(C) nanoparticles (indicated by zeta potential values in Table.[1]). After surface modification by both SDS (Fig. [8](B)) and TEOS (Fig. [8](C)) homogeneous dispersion of ceramic particles in the polymer matrix was obtained which may be due to the lowering of surface energy of nanoparticles (Table.[1]). The presence of a silica layer was observed in spot EDX image analysis of TEOS modified composite (Fig. [8](D)).

The change in the value of dielectric constant (r) for 0.25, 0.50, 1.00 and 1.50 wt% of ZF(C) nanoparticle loaded unmodified and surface modified composite along with virgin PVDF are shown in Fig. [9] and variation of dielectric loss (tan δ) for virgin PVDF, 1.00 and 1.50 wt% of ZF(C)-PVDF and surface modified ZF(C)-PVDF are shown in Fig. [10], respectively. The dielectric loss values of 0.25wt% and 0.50 wt % of ZF(C)-PVDF and surface modified ZF(C)-PVDF composites are shown in Fig. [S3]. It was observed that the value of dielectric constant decreased with increasing frequency and the value of dielectric constant increased with increased loading of nanoparticles. The decrease in the value of the dielectric constant at high frequency was mainly caused by the reduction in Maxwell-Wagner-Sillars (MWS) polarization and space charge polarization. At 1 kHz, the dielectric constant of the 1.5-ZF(C)-PVDF composite showed 26±.5 which was 2.6 times higher than virgin PVDF (10±.5). The same composite when modified by SDS the value increased to 29.57±.5 and further when modified by TEOS there was an increment to 31.02±.5. In PVDF, both β and γ gives rise to a net dipole moment whereas nonpolar α-phase do not contribute to polarization. The addition of ZF(C) particles initially induced interfacial polarization which increased the relative permittivity. This was further enhanced by the addition of surface modifiers as they formed bonds with H atoms in the CH2 groups of PVDF. The variation of dielectric constant (r) at 1 kHz frequency with respect to addition of the ZF(C) particles are shown in Fig. [11]. The effective permittivity of a nanocomposite depends on the individual permittivity of fillers and polymer matrix and also on the interaction within them. The effective dielectric constant has been derived from various models which are developed based on some assumptions. The most simple and commonly used dielectric theoretical model is Lichtenecker’s [18, 54] which is also known as the logarithmic mixture rule. The formula is defined as:

log 𝜀𝑒𝑓𝑓 = 𝜑1 log 𝜀1 + 𝜑2 log 𝜀2

(3)

where eff, 1 and 2 are effective dielectric constant of composite, dielectric constant of polymer and dielectric constant of nano filler respectively. 1 and 2 are volume fractions of polymer and filler.

For lower filler loading, the effective dielectric constant is defined by the Maxwell’s equation described as

𝜀𝑒𝑓𝑓 =

𝜀1 (1−𝜑2 )/(2⁄3+𝜀2 ⁄3𝜀1 )+𝜑2 𝜀2 (1−𝜑2 )/(2⁄3+𝜀2 ⁄3𝜀1 )+𝜑2

(4)

Another conventional theoretical model was derived by Furukawa [18]. It was assumed that for 0-3 composite there is no interfacial effect in the composite and low loading of second phase (𝜑2 ≪ 1) when 𝜀2 ≫ 𝜀1

the effective dielectric constant is described as

𝜀𝑒𝑓𝑓 =

1+2𝜑2 1−𝜑2

𝜀1

(5)

The effective medium theory (EMT) model [54] has been established considering the morphology of the particles. According to this model the effective dielectric constant is given by

𝜀𝑒𝑓𝑓 = (1 +

𝜑2 (𝜀2 −𝜀1 )

) 𝜀1

𝜀1 +𝑛(1−𝜑2 )(𝜀2 −𝜀1 )

(6)

where n is the ceramic inclusion’s morphology fitting factor. Small value of n indicates that particle are nearly spherical in shape, whereas large value of n indicates non spherical shape. But the predicted theoretical value of effective dielectric constant calculated by the above mentioned models do not exactly follow the experimental value Fig.[12]. The maximum similarity was observed for the Furukuwa Model with deviation at higher frequency region. The main reason for this was that the models do not consider the phase transformation of PVDF due to interaction with nanoparticles and effect of surrounding medium. The interphase region has different dielectric properties than that of the polymer and the nanoparticles. Modifiers generally enhance the compatibility between the organic and inorganic phase of composites and hence there is an enhancement in the value of the dielectric constant. On the other hand, the ceramic filler (ZF) has higher dielectric loss as compared to the polymer matrix. Higher amount of nanoparticles formed a large number of interfacial areas which leads to the increase in conductive network [55]. Modified ZF nanoparticles show higher dielectric loss as compared to the unmodified ZF nanoparticles. This is further supported by variation of a.c. conductivitiy with angular

frequency (Fig. [S4]) where it was clear that increasing filler concentration the a.c. conductivity increased and this increment was more in case of modified composite compared to the unmodified one. As modification improved the dispersion of the filler in the matrices (confirmed by FESEM micrograph), interfacial area between filler and matrices also increased and as a consequence the area of free PVDF chains was reduced. Thus, rotation of the dipoles was restricted resulting in enhancement in the value of dielectric loss [54]. In order to evaluate the energy storage capability of the composites, the room temperature displacement electric field (D-E) loops were measured. There was an increment in polarization value of ZF(C)-PVDF composite when compared to the PVDF and the value were further enhanced after surface modification. The incorporation of ZF into PVDF matrix increases the internal charge of the nanocomposites which was required to compensate and thus the polarization domains were stabilized. Additionally, nanoparticles act as heterogeneous nucleation centre in the ferroelectric domains. Modified nanoparticles promote polarization levels by increasing interfacial interaction with polymer dipoles [18]. The maximum value 0.15 µC/cm2 was obtained for 1.5MSDS-ZF(C). The enhancement in the 1.5MSDS-ZF(C) composite may be due to the maximum homogeneous distribution of the surface modified ZF particle and increase of interface compatibility of the ceramic particles in the polymer matrices. The energy density of nanocomposites can be represented as:

U = f𝑐 U𝑐 + f𝑚 U𝑚 + gU𝑖

(7)

where fc and fm are the volume fractions of ceramic phase and polymer matrix. Uc and Um are the respective energy densities, Ui is the energy density with interface effects and g is proportional to the interfacial area. The enhancement in the g factor after surface modification has contributed to the increase in the energy density. Further, the energy density of dielectric material was calculated using the following integral: 𝐷

𝑈 = ∫𝐷 𝑚𝑎𝑥 𝐸𝑑𝐷 𝑚𝑖𝑛

(8)

where Dmax and Dmin represent the maximum and minimum electric displacement respectively. The plot of displacement polarisation as a function of electric field for ZF(C) composite at an applied voltage of 2kV is shown in Fig. [13(A)]. The highest energy density was obtained for PVDF- MSDS-1.5ZF (C)-poled (0.25J/cm3) which is nearly three times higher than 1.5ZF(C) –PVDF composite (0.08 J/cm3). The voltage generation capability of the fabricated composite was measured by the application of mechanical stress by imparting repeated human single finger touch. When pressure was applied, a potential difference was generated which was observed in the oscilloscope as open circuit voltage. There was an enhancement in the value of the output voltage for surface modified composites. The maximum voltage obtained was 1.02V with output current density of ~0.03μA/cm3. The enhancement in the value of generated voltage was due to the increase in polar phase formation. The modification of the composite helped in increasing the conduction path network by increasing the interfacial interaction between the polymer and ceramic matrix. It was observed that without applying electrical poling the modified MSDSZF(C) and MTEOS-ZF(C) (Fig. [13 (B)]) showed almost similar voltage as the poled unmodified one (ZF(C)poled). Further to enhance the amount of output voltage, electrical poling was applied to the composite which may lead to the proper alignment of the ZF nanoparticles in the polymer matrix. The maximum output open circuit voltage obtained was 2.20V with power density of 3.7µWcm-3 for poled MTEOS-ZF(C)PVDF composite. This result indicated that the fabricated composites have the potential for powering small scale electronic devices. Conclusions In conclusion, we have adopted a process to enhance the polar phase in PVDF polymer by addition of surface modified (SDS and TEOS) ZF nanoparticles. The nucleation and stabilization of the electroactive phases in the films were explained by the interaction model based on FTIR and XPS spectroscopy. Similar electrical properties were observed for surface modified and externally poled unmodified composites, indicating that modified composite exhibited a self-polarization behaviour to some extent which could

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Figure List Fig.1.XRD study of ZF(C) and Surface modified ZF(C).

Fig.2.FTIR study of ZF(C) and Surface modified ZF(C).

Fig.3.TEM study of ZF(C).

Fig.4(a).TEM study of MTEOS-ZF(C).

Fig.4(b).TEM study of MSDS-ZF(C).

Fig.5.FTIR study of virgin PVDF, ZF(C)-PVDF, surface modified ZF(C)-PVDF composite.

Fig.6.XPS study of virgin PVDF, ZF(C)-PVDF, surface modified ZF(C)-PVDF composite.

Fig.7.Suggested Interaction of surface modified Zinc Ferrite with PVDF.

Fig.8. FESEM image of virgin PVDF (inset (A)), ZF(C)-PVDF (A) and MSDS-ZF(C)-PVDF (B), MTEOS--ZF(C)PVDF (C), Zoom image of MTEOS--ZF(C)-PVDF (inset(C)), EDX of MTEOS-ZF(C)-PVDF, (D) and its image mapping (inset (D)).

Fig.9. Dielectric Constant variation with frequency of virgin PVDF, ZF(C)-PVDF, surface modified ZF(C)PVDF composite.

Fig.10. Dielectric Loss variation with frequency of virgin PVDF, ZF(C)-PVDF, surface modified ZF(C)-PVDF composite.

Fig.11. Dielectric Constant variation with filler wt% of virgin PVDF, ZF(C)-PVDF, surface modified ZF(C)PVDF composite.

Fig.12. Comparisons of different model of Dielectric Constant variation with frequency of 1.5 MSDSZF(C)-PVDF.

Fig. 13(A). Displacement polarisation with variation of electric field and (B). Output voltage with variation of time upon imparting single finger impact of 1.5wt% ZF(C)-PVDF composites (both poled and unpoled condition).

ZF(C)-PVDF and surface modified

Authors’ Biography:

Ms.IpsitaChinya received B.Sc. (Hons.) degree in Chemistry and Post B.Sc. B. Tech. degree in Polymer Science and Technology from University of Calcutta. She was awarded M. Tech. degree in Glass and Ceramic Technology and currently pursuing her doctoral research work as SRF in AcSIR-CSIR-CGCRI on polymer- ceramic

composite based

Nanogenerator.

Dr.Shrabanee Sen is a Senior Scientist in the Sensor and Actuator Division, CSIR-Central Glass and Ceramics Research Institute, Kolkata, India. She received Ph.D. in 2005 from Department of Physics, Indian Institute of Technology, Kharagpur. Her research interest includes the synthesis and characterization of ferroelectric, piezoelectric materials in the form of ceramics, thin films, and composites for applications like actuators and nanogenerators.