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Improved dielectric properties of polyvinylidene fluoride nanocomposite embedded with poly(vinyl pyrrolidone) coated gold nanoparticles Anju Toor, Hongyun So, and Albert P. Pisano ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13900 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 28, 2017

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ACS Applied Materials & Interfaces

Improved dielectric properties of polyvinylidene fluoride nanocomposite embedded with poly(vinyl pyrrolidone) coated gold nanoparticles Anju Toor,1,* Hongyun So,1,* and Albert P. Pisano2 1

Department of Mechanical Engineering, University of California, Berkeley, USA

2

Department of Mechanical & Aerospace Engineering, University of California, San Diego, USA

ABSTRACT

–

A

novel

nanocomposite

dielectric

is

developed

by

embedding

polyvinylpyrrolidone (PVP) encapsulated Au nanoparticles in the polyvinylidene fluoride (PVDF) polymer matrix. The surface functionalization of Au nanoparticles with PVP facilitates favorable interaction between the particle and polymer phase, enhancing nanoparticle dispersion. To study the effect of entropic interactions on particle dispersion, nanocomposites with two different particle sizes (5 nm and 20 nm in diameter) are synthesized and characterized. A uniform particle distribution was observed for nanocomposite films consisting of 5 nm Au particles in contrast to the film with 20 nm particles. The frequency dependent dielectric permittivity and the loss tangent is studied for the nanocomposite films. These results show the effectiveness of PVP ligand in controlling the agglomeration of Au particles in the PVDF matrix. Moreover, the study shows the effect of particle concentration on their spatial distribution in the polymer matrix and the dielectric properties of nanocomposite films.

KEYWORDS: Nanoparticle, Nanocomposite, Dielectric, Polymer, Particle dispersion

ACS Paragon Plus 1 Environment


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1

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INTRODUCTION Nanocomposite materials are widely used in the areas of photonics, electronics, catalysis,

energy storage and biotechnology.1–7 Particularly, polymer composite materials with high permittivity, high breakdown strength, low dielectric loss and good processability are of great interest because of their various applications in energy storage, capacitors, and actuators.8–10 Previously, ceramic dielectrics based on highly polarizable materials such as calcium copper titanates11 have been employed for pulse power applications. Although ceramic dielectric materials possess high dielectric constants, their breakdown strength is low.12 Further, it is hard to manufacture ceramic-based capacitors whereas polymers can be easily processed into large area films. Polymers offer high breakdown strength, ease of processing, and low dielectric loss but their dielectric constant is low. Typically, high permittivity fillers such as conductive and ferroelectric fillers (barium titanate (BaTiO3), lead zirconate titanate (PZT)) are embedded in polymers to enhance their permittivity.13–15 Ideally, the enhancement in dielectric constant should be attained without any comprise in dielectric loss or/and breakdown strength, which entails a uniform dispersion of nanoparticles in the polymer matrices without particle agglomeration and phase separation. However, the dispersion of nanoparticles in polymer matrices is usually challenging because nanoparticles tend to aggregate or phase-separate. Previously, conductive particles mainly in the form of aggregates were used as fillers in polymers.16–20 Although extremely high k values were reported for composites near percolation, the dielectric loss was significant. A similar trend in the dielectric loss was reported for composite materials containing ferroelectric materials such as BaTiO3 and PZT.13–15

High

dielectric loss and low breakdown strength can be attributed to the poor dispersion of


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nanoparticles in polymer matrices. Recently, results have been reported for BaTiO3 based nanocomposites, indicating the enhancement in dielectric permittivity with low dielectric loss values (0.03−0.15),21–24 however these systems suffer from limited energy storage capacity due to the low saturation voltage of the fillers used.25 In contrast, conductive filler based composites does not suffer from such disadvantages and can enhance the dielectric permittivity significantly.26–28 However, the particle surface still needs to be passivated with suitable ligands 29–31

to minimize dielectric loss. Therefore, particle dispersion without agglomeration is

imperative for achieving an optimal enhancement in dielectric properties regardless of the choice of filler material. In general, there are two factors that govern the dispersion of particles in the polymer matrix, the repulsive interactions between the nanoparticles and the wettability of the particles by the matrix polymer chains. For repulsion in polymer composites, functional small molecules or polymers, also called as ligands are attached to the nanoparticles surface. In nanoparticlepolymer composite systems, although enthalpic interactions (related to the nanoparticle ligands) are usually dominant,32,33 entropic interactions (i.e. nanoparticle size relative to the radius of gyration of polymer) can play a critical role in controlling particle dispersion.34 Lee et al.35 studied the effect of entropic interactions in a polymer composite consisting of polystyrene functionalized cadmium selenide (CdSe) particles dispersed in the polystyrene matrix. Grafting of polystyrene chains on the particle surface minimizes the enthalpic interactions, and hence, the entropy interactions would dominate the behavior of the polymer composite system. They observed that nanoparticles migrated to the cracks, and this segregation behavior was attributed to the entropic effects.



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In particle-polymer composites, the polymer chains undergo stretching around the solid nanoparticles, causing a loss in the conformational entropy which further depends on the particle radius.36 For large particles which are entropically disfavored, in the absence of specific interactions, particles are expelled from the bulk of the polymer.37 This characteristic significantly affects the global distribution of nanoparticles in the polymer matrix. For a multilayer system where a layer of poly(methyl methacrylate) (PMMA) containing poly(ethylene oxide) coated CdSe particles is in contact with a brittle silicon oxide layer, the nanoparticles with size comparable to the radius of gyration (Rg) of host PMMA were found to migrate in the crack in silicon oxide layer.38 This crack was induced upon heating the multilayer system above the glass transition temperature of the composite. In the case of polymer-grafted nanoparticles, wetting will be favored entropically if either the particles size is smaller than Rg of host polymer33 or the size of the grafted polymer is comparable to the size of host polymer chains.39,40 When the size of the grafted polymer is much smaller than the host polymer, the increase in the entropy due to the mixing of the grafted polymer and host polymer will be less than the loss in entropy of host polymer during interpenetration in the grafted polymer chains.34 Consequentially, this disparity in entropy makes the grafted polymer chains separate from the host matrix, thus resulting in particle aggregation 34,41

. Here, we report the structural and dielectric characteristics of a nanocomposite material

consisting of PVP-coated gold (Au) nanoparticles embedded in PVDF polymer matrix. The coating material is carefully tailored so as to aid in the dispersion of Au nanoparticles while providing local electrical resistance to allow increased volume fractions of particles in the polymer scaffold. Since PVP is miscible with PVDF, the particle dispersion is enthalpically


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favored. Furthermore, PVDF has high breakdown strength (>700 MV/m)42 and the overall dielectric permittivity of nanocomposites can be increased owing to nanoscale conductive fillers,17,18 thus serving as a high energy density dielectric material and providing several potential applications. Although the synthesis of PVDF nanocomposite with 15 nm diameter Au nanoparticles has previously been reported,43 further study on the particle agglomeration is desirable to understand the dispersion characteristics with different nanoparticles sizes. Lopes et al.44 also reported PVDF composites with 6 nm and 27 nm diameter silver nanoparticles, indicating that composites have agglomerated nanoparticles at 0.073 vol % of 27 nm silver nanoparticles. 2

EXPERIMENTAL METHODS

Materials PVP encapsulated Au nanoparticles were purchased from Nanocomposix (San Diego, USA). All the solvents and PVDF pellets were bought from Sigma Aldrich, USA, and used without further purification. Preparation of nanocomposite films Synthesis methods of PVDF nanocomposites with PVP-coated Au nanoparticles can be found in our previous report.45 In short, nanoparticle powder was dispersed in N, NDimethylformamide (DMF) to prepare a homogeneous particle suspension. Separately, the polymer solution was prepared by adding PVDF pellets to DMF, followed by heating at 100oC under continuous stirring for an hour. Nanocomposite suspension was prepared by directly mixing the particle suspension with polymer solution under sonication for an hour at room



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Figure 1. Schematic of the PVP-coated Au / PVDF nanocomposite capacitor device.

temperature. The resulting nanocomposite suspension was spin-coated onto silicon (Si) substrate to prepare nanocomposite films. Capacitor device fabrication To measure the dielectric properties of nanocomposite films, parallel plate capacitors were fabricated on Si substrates. A 100 nm thick silicon oxide (SiO2) layer was deposited using plasma-enhanced chemical vapor deposition method to electrically isolate the substrate. An Aluminum layer was sputtered (200 nm) which acts as the bottom electrode. Aluminum (Al) is widely used as an electrode material due to its high conductivity and ease of deposition. Hence, Al was the preferred choice. For top electrode, Au was used instead of Al due to the surface oxidation problem with Al. Nanocomposite films were spin coated on the bottom electrode from the nanocomposite suspension. Capacitor devices were designed by evaporating a 100 nm thick,


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top Au electrode (area: 3.14 mm2) using a shadow mask. A schematic diagram of the capacitor device is shown in Fig. 1. Characterization The morphology of the nanocomposite films was examined using a scanning electron microscope (SEM, FEI Quanta 3D FEG) and a transmission electron microscope (TEM, FEI Tecnai 12). Nanoparticle suspensions were prepared by dispersing a predetermined amount of nanoparticle powder in DMF solvent. Resulting nanoparticle solution was characterized using TEM to analyze particle dispersion. Samples were prepared by dropping the nanoparticle suspension on a copper grid covered with carbon film. The crystal phase was analyzed using the Grazing-incidence wide-angle X-ray scattering. Experiments were carried out at Beamline 7.3.3 of the Advanced Light Source at Lawrence Berkeley National Laboratory. The dielectric properties of synthesized nanocomposite were measured using a semiconductor parameter analyzer (Agilent B1500A). 3

RESULTS AND DISCUSSION

TEM micrographs of the 5 nm and 20 nm Au nanoparticles are shown in Figure 2 (a) and (b), respectively. It shows that the nanoparticles are monodispersed, and hence, no particle agglomeration occurred during the solution preparation. Nanocomposite films were synthesized by spin coating the nanocomposite suspension followed by drying in an oven for 24 hours. It was observed that the quality of the polymer film obtained was highly sensitive to the drying temperature. Films dried at room temperature exhibited highly porous morphology in comparison to the films obtained at temperatures ≥ 100oC (See Figure S1 in the Supporting Information for details). Figure 3 (a) shows a SEM image of the top surface of the



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Figure 2. TEM image of PVP-coated Au nanoparticles 5 nm (a) and 20 nm (b). It was observed that nanoparticles were monodispersed.

nanocomposite film dried at 100oC, showing a dense film without pinholes or voids. It is important to minimize the voids as these air trapped pockets are susceptible to electric spark breakdown and will result in an increase in leakage density. The dispersion of Au nanoparticles in the PVDF polymer matrix was studied using TEM. Nanocomposite film cross-sections (40–80 nm thick) were obtained using a Reichert microtome. Figures 3 (b) and (c) show the electron micrographs of the nanocomposite film with 1 wt% and 5 wt% nanoparticle content, respectively. Figure 3(d) shows a high magnification view of the 5 wt% nanocomposite film. No dominant aggregation of the Au nanoparticles at the air/film interface is observed. These images qualitatively suggest that a uniform dispersion of nanoparticles with no agglomeration has been achieved. The particle distribution suggests that the Au nanoparticles are effectively insulated by the polymer. Since nanoparticle agglomeration results in high dielectric loss and reduced electrical breakdown strength, such particle


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Figure 3. (a) SEM image (top surface view) of the PVP-coated Au nanoparticle/PVDF nanocomposite film with 1 wt% particle content. (b) Cross-sectional TEM image of nanocomposite film with Au nanoparticle (5 nm) concentration of 1 wt %. (c) and (d) Au nanoparticle-polymer composite film with 5 wt % nanoparticle content at different magnification levels.

distribution is imperative to obtain optimal enhancement in the dielectric properties. Au nanoparticle distribution in the nanocomposite film with 12.5 wt% particle content is illustrated in Figure 4. Electron micrographs are captured from the different regions of the nanocomposite film. Figures 4 (a) and (b) represent the distribution of the nanoparticles in the



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interior regions i.e. away from the air/film and film/substrate interface. It can be concluded that the particles are uniformly distributed in the PVDF matrix, and no dominant aggregation of the Au nanoparticles is observed. Figure 4 (c) shows the particle dispersion in the nanocomposite

Figure 4. TEM images of nanocomposite film with Au nanoparticle concentration of 12.5 wt% at different magnifications. (a) and (b) are taken from the bulk of the polymer, and (c) and (d) are captured from the air/film interface region.


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film near the air/film interface. TEM image suggests that the relative concentration of nanoparticles near the air/film interface is higher than the interior regions of the nanocomposite film. Although the uniform distribution of nanoparticles is desirable throughout the nanocomposite film (even near the air/film interface), a magnified view in Figure 4 (d) shows that the Au nanoparticles are well-dispersed in the PVDF polymer matrix with no agglomeration. It is noteworthy that single Au particles are embedded in the PVDF polymer matrix, with an average size of ~5 nm. Typically, nanoparticle addition in polymer leads to particle agglomeration, which is undesirable in energy storage because agglomerates can form percolation pathways for current leakage, resulting in increased energy loss. Figure 5 show the electron micrographs of a nanocomposite film containing 20 nm Au particles at a particle concentration of 5 wt%. TEM images are captured from the various regions of the nanocomposite film. Figures 5 (a) and (b) show that the nanoparticles are being expelled from the bulk of the polymer towards the film/air interface. Certain regions of the film have nanoparticles embedded in the polymer, as shown in Figure 5 (c). However, the particle dispersion is not uniform. Figure 5 (d) shows a high magnification electron micrograph of the region marked by the white box in Figure 5 (c). From the TEM analysis, it can be concluded that the distribution of 20 nm Au particles in the PVDF host matrix is inhomogeneous. The dielectric properties of the nanocomposite film containing 20 nm particles are shown in Figure S2 in the Supporting Information. Since the nanoparticles are not embedded in the polymer but rather segregated towards the edge of the film, the overall enhancement obtained in the dielectric permittivity is not significant. In addition, current-voltage characteristics of the nanocomposite film containing 20 nm Au nanoparticles show a low leakage current of 8.35 ± 2.3 nA in comparison to the 0.14 ± 0.03 µA at a voltage of 1 V measured for the nanocomposite films



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containing 5 nm Au nanoparticles. (see Figure S3 in the Supporting Information). This is in agreement with the TEM analysis showing an exclusion of the nanoparticles from the bulk of the polymer matrix for the 20 nm Au particles. Therefore, 20 nm particle size was not further considered for the Au nanoparticle-PVDF nanocomposite material in this study. For further analysis, nanocomposite films embedded with 5 nm Au particles were used.

Figure 5. TEM images of nanocomposite film containing 20 nm Au nanoparticles. Figures (a) and (b) show that nanoparticles are excluded from the host polymer matrix to the film/air interface, with few particles embedded in the polymer matrix as shown in Figure (c). A high magnification view of the area highlighted by a white box in (c) is shown in (d).


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Grazing-incidence wide-angle X-ray scattering (GIWAXS) was used to characterize the different phases of the PVDF polymer. The crystal structures of PVDF, 1 wt% Au/PVDF and 12.5 wt% Au/PVDF films annealed at 100°C, were characterized by GIWAXS as presented in Figure 6. For pure PVDF films, the less intense peaks at 18.6° and 20.1° attribute to (020) α and (110) α crystal planes respectively.46 A broad peak centered at 21.4° is also observed, it could be explained as (012) γ-prohibited.47 WAXD pattern of 1 wt% PVDF nanocomposite shows a broad peak centered at 21.4°, its full width at half maximum (FWHM) ranges from 20.6° to 23°. It is attributed to the (110) and (200) crystal planes of the β phase,46 although contributions from the γ phase cannot be excluded. The peak at 21.4° is typical for the γ phase, it could be explained as (012) γ-prohibited.47 Similar to the 1 wt% case, WAXD pattern of 12.5 wt% PVDF

Figure 6. Wide angle X-ray diffraction (WAXD) patterns obtained for (a) PVDF film (b) PVDF nanocomposite with 1 wt% and (c) 12.5 wt% Au nanoparticle content.



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nanocomposite shows an intense broad peak around 20.6° that corresponds the (110) and (200) crystal planes of the β phase.46 The broad peak centered at 26.5° can be attributed to either α or the γ phase.47 For PVDF, 1 wt% and 12.5 wt% nanocomposite films, a peak is observed at 16.5° that cannot be explained by any of the known crystal structures of PVDF. To systematically investigate the dielectric properties of the composite films, impedance measurements were made using the semiconductor parameter analyzer on nanocomposite capacitors. Dielectric permittivity values were estimated from the measured capacitance values using electrode area and film thickness. Figure 7 shows the variation of the dielectric properties with frequency over the range of 1 kHz to 1 MHz, for nanocomposite films with nanoparticle content up to 12.5 wt%. In general, the dielectric permittivity was observed to decrease with the

Figure 7. Frequency dependent dielectric permittivity for the nanocomposite capacitors with various Au nanoparticle concentrations.


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increase in frequency. However, the frequency dependence was weak for nanocomposite films with particle concentration upto 1 wt%. For 5 wt% and 12.5 wt% nanocomposites, the effect of frequency on dielectric permittivity was significant, exhibiting a substantial decrease in dielectric permittivity with the increase in frequency. This is attributed to the space charge polarization between the Au nanoparticles and the PVDF polymer matrix.1 The dielectric loss can be analyzed in terms of the loss tangent (tan δ), also known as the dissipation factor. Loss tangent includes the dissipation due to the resistance in the leads and plates, dielectric damping loss and conductivity loss of a conductive material. For lossless material, there is no loss and tan δ = 0. The frequency dependent response of loss tangent is shown in Figure 8. For pure PVDF film capacitors, loss tangent show frequency independent characteristics. In case of nanocomposite films, the loss tangent was observed to have weak

Figure 8. Frequency dependent loss tangent (tan δ) for the nanocomposite capacitors with varied particle loading.



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frequency dependence at low frequency values but, an increase in loss tangent was observed at higher frequencies (105-106 Hz) which might be attributed to the resonance/relaxation of the host polymer14,15,18,24,48 or lead resistance (resistance of the connecting leads)1. The dissipation due to lead resistance can be expressed as ωRsC, where ω is the measurement frequency, Rs is the series resistance due to leads and C is the capacitance. At high frequencies, when ωC is large, the dissipation due to lead resistance can dominate the total dissipation, causing an increase in the dissipation factor. This frequency-dependent trend of loss tangent can also be found in previous reports.5,29–31 Figures 9 (a) and (b) show the effect of the nanoparticle concentration on the dielectric properties of the nanocomposite materials measured at 1 kHz. The enhancement in the dielectric permittivity increases rapidly when the particle concentration is higher than 1 wt%. A dielectric permittivity of ~ 22 was observed at 12.5 wt% particle content, which is about 3 times higher than the pure PVDF homopolymer. The effect of particle concentration on dielectric loss is illustrated in Figure 9(b). The dielectric loss increased with the increase in the particle

Figure 9. Variation of (a) dielectric constant and (b) loss tangent with filler contents for Au nanoparticle nanocomposite films at 1 kHz.


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concentration in the nanocomposite, probably due to the ohmic current leakage. However, the measured dielectric loss values are quite low even at high particle concentrations. For example, the loss tangent was measured to be ~ 0.14 for 12.5 wt% nanocomposites. This implies that the Au nanofillers are effectively insulated by the surrounding polymer matrix. On further increasing the nanoparticle concentration, the homogeneity of the nanocomposite film was observed to deteriorate with the appearance of voids in the film. There is one possible explanation for this. It may be that with the increase in nanoparticle concentration to higher values, the crystallization of PVDF from solution result in morphologies with single crystals instead of spherulites. The morphology of polymers obtained by crystallization from solution depends on the polymer concentration. From single crystals at very low concentrations to supramolecular structures such as spherulites at high concentrations, different morphologies can be observed.49 These PVDF spherulites coalesce resulting in a homogeneous dense, smooth film. 4

CONCLUSIONS

A novel hybrid composite material consisting of PVP modified Au nanoparticles (5 nm diameter) and PVDF is developed. This article presents a first step towards the study of sub 10 nm, surface functionalized, metal particle based polymer nanocomposite dielectrics. Previously, works have been published with particles of size > 20 nm, with little or no emphasis on nanoparticle dispersion. The electron microscopy images of the nanocomposite films demonstrate no particle agglomeration up to 12.5 wt% particle concentration. Nanocomposite films without voids and phase-separation between the particles and polymer phase were successfully synthesized. The nanocomposite films show high dielectric constant and low dielectric loss, i.e., a dielectric constant value of 22 and loss tangent of 0.14 at 1 kHz was measured at a particle concentration of 12.5 wt%. These results provide evidence that enhanced



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dielectric permittivity can be obtained while maintaining the low dielectric loss for polymer composites embedded with coated metal nanoparticles. Such solid-state dielectric materials would enable energy storage solutions having both high energy storage capacity and power transfer rate. SUPPORTING INFORMATION SEM images of the PVDF film, dielectric characteristics of 20 nm Au-PVDF nanocomposite film, and current-voltage characteristics of both 20 nm and 5 nm Au-PVDF nanocomposite films. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by King Abdullah University of Science and Technology (KAUST) under award number 25478. The authors gratefully acknowledge support from the Marvell Nanolab facility at University of California, Berkeley where the capacitor device was fabricated. We would also like to thank the staff of Biomolecular Nanotechnology Center (BNC) where the nanocomposite material was synthesized and characterized.


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