BaTiO3/polyvinylidene fluoride nanocomposites

Structural properties of composite of polyvinylidene fluoride and mechanically activated BaTiO3 particles V. P. Pavlović1, V. B. Pavlović2,3, B. Vlahović4,5, D. K. Božanić6, J. D. Pajović6, R. Dojčilović6, V. Djoković6 1 Faculty of Mechanical Engineering, University of Belgrade, 11000 Belgrade, Serbia 2 Faculty of Agriculture, University of Belgrade, 11000 Belgrade, Serbia 3 Institute of Technical Sciences, Serbian Academy of Sciences and Arts, Belgrade, Serbia 4 North Carolina Central University, Durham, North Carolina 27707, USA 5 NASA University Research Center for Aerospace Device Research and Education and NSF Center of Research Excellence in Science and Technology Computational Center for Fundamental and Applied Science and Education, North Carolina, USA 6 Vinča Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522, 11001, Belgrade Serbia

e-mail: [email protected]

Abstract. Nanocomposites of electroactive ceramics and ferroelectric polymers exploit favourable features of the matrix polymer and nanostructured filler to produce new functional materials for pressure and IR sensors. In this study, the influence of mechanical activation of BaTiO3 particles on structural properties of BaTiO3/polyvinylidene fluoride (PVDF) nanocomposites was investigated. The nanocomposite films were prepared by solution casting method and characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD) and Raman spectroscopy. It was found that mechanically activated filler promote formation of ferroelectric -phase during crystallization of PVDF. PACS numbers: 82.35.Np 1. Introduction Polyvinylidene fluoride (PVDF) is a low-density fluoro polymer that exhibits piezoelectric and pyroelectric properties [1]. PVDF and its co-polymers have a huge potential as dielectric materials, especially in those applications where high energy density and low loss at high repetition rates are required [2]. Generally, PVDF is semicrystalline material and it exhibits at least five crystalline phases (α-, β-, γ-, δ- and ε-phase) [3]. The monoclinic α-phase, a dominant phase obtained under usual melt crystallization, is non-polar, while the others are electroactive. Both β- and γ-phases are orthorhombic, while the former possesses all-trans chain (tttt) conformation. Although β-, γ-,δ- and ε-phases are all electroactive, only the β-phase is suitable for the most of sensor applications, since it has a stronger ferro-, piezo- and pyroelectric response due to its largest spontaneous polarization [4]. For this reason, a lot of research effort was directed to the optimization of the processing conditions that will result in PVDF films with high yield β-phase content. Ferroelectric PVDF polymer has certain advantages in sensing application since it has low density, flexibility and toughness, while at the same time it can be easily processed into technologically useful forms [5]. However, the performance of the devices based on pure β-phase PVDF may be very sensitive on external conditions. For example, in space applications, the functioning of sensors and actuators can be affected by prolonged interaction of polymer with ultraviolet-, γ- and X-radiation, high energy ions and atomic oxygen [6]. All this factors induce changes in its physical, chemical, optical, structural and morphological properties including a change

in crystallinity. The introduction of the nanostructured piezoelectric ceramics into polymer matrix offers a possible answer to this problem [7]. The obtained nanocomposite materials combine above mentioned properties of polymer matrix with excellent mechanical, thermal, and optical properties of ceramic particles. The resulting properties of the nanocomposites will depend on different parameters such as preparation methods, grain size of ceramic particles and their dispersion within the matrix [8]. It has also been shown that the geometry of the filler particles plays an important role in tailoring the materials properties, especially if the particles have a very high volume-to-surface ratio. The mechanical activation is an effective method for modification of physico-chemical properties of materials, during which formation of multifunctional nanoparticles with high specific surfaces takes place [9,10]. In the present study we investigate the effect of mechanically activated BaTiO3 particles on the structural properties of the PVDF-BaTiO3 nanocomposite. 2. Experimental procedure The BaTiO3/PVDF films were prepared by solution casting method. BaTiO 3 powder (Aldrich, p.a. 99.9%) was mechanically activated for 5 min in a planetary ball mill (Fritsch Pulverissete 5) with zirconia jar and zirconia balls (10 mm in diameter). The ball/sample mass ratio was 20:1, while the tray and vial rotation speeds were 317 and 396 rpm respectively. Polyvinylidene fluoride (SigmaAldrich, Mw~530 000) 5 wt./vol.% stock solution was prepared by dissolution of the polymer in 1:1 dimethyl formamide (DMF)-acetone mixture. The stock solution was subsequently diluted to 1 wt./vol.% by addition of 80 ml of acetone into 20 ml of 5 wt./vol.% PVDF. 0.1 g BaTiO3 nanopowders (non-activated and activated for 5 min) were dispersed in 50 ml of DMF and sonicated in Branson W450 D Digital Sonifier for 20 min at 20 % amplitude (80 W). Composites with 2 wt.% of inorganic content were prepared by mixing of 5 ml of BaTiO3 dispersion with 50 ml of the 1 wt./vol.% PVDF solution. Dispersions were cast into glass Petri dishes and dried for 24 hours at room temperature (~20 °C). After that period, the films were further dried at 100 °C for one hour. The pure PVDF films were prepared using the same procedure. The microstructure morphology of non-activated and activated BaTiO3 powders was investigated by using JEOL JSM-6390 scanning electron microscope (SEM). A fully automated Raman microscope (Horiba Jobin Yvon LabRam ARAMIS) was used for Raman spectroscopy measurements. The incident laser was a He-Ne laser at 633 nm. The data was collected over the Raman shift range of 200 to 3200 cm-1, using a count time of 5 seconds with 10 averaging cycles. The samples were measured under a microscope using a 100x objective. The X-ray diffraction patterns were obtained in Bragg-Brentano geometry, on a Rigaku Smart Lab X-ray diffractometer, using CuK1/2 filtered radiation. The diffracted intensities were collected in a step scan mode 0.02 o/12s. 3. Results and discussion Figure 1 shows SEM micrographs of as-received and mechanically activated BaTiO3 powders. During mechanical activation, the generation of a field of stress and the accumulation of the energy of the plastic deformation lead to the crystal comminution and the formation of ultrafine powder particles. The formation of BaTiO3 particles with reduced sizes can be clearly noticed in Figure 1. As it was mentioned in the Introduction, a decrease in size will affect physical properties of the particles but also the properties of the composite structures where they are embedded in. There are two significant factors that affect the ferroelectric properties of the nanoparticles themselves. One is a macroscopic effect related to the surface tension of the nanoparticle, and the other concerns the grain size effect on the microscopic interactions which induce ferroelectric instability. It has been shown that for perovskite oxides, spontaneous polarization decreases with decreasing in particle size and eventually disappears; that is, a size-driven phase transition take place. For barium titanate based materials, the ferroelectric critical size has been defined as the size at which phase transition from the ferroelectric-tetragonal to paraelectric-cubic phase occurs. Because of the current trends in miniaturization of ferroelectric components, significant research efforts were directed towards fabrication of nanostructured BaTiO3 and other ceramics i.e. towards reduction of ferroelectric critical size [11]. On the other hand, conventional methods for preparation of BaTiO3-based materials produce relatively coarse and agglomerated particles. Our previous investigations have shown that the

mechanical activation treatment can induce formation of nanocrystalline particles with a tetragonal structure and very small diameters (even of order of ~30 nm) [12]. In the present study, structural measurements also confirmed that BaTiO3 powders activated for 5 min exhibit tetragonal structure. Major phonon modes that can be assigned to tetragonal BaTiO3 were observed in Raman spectra (Figure 2a, inset). The broad peak centered at around 270 cm -1 can be assigned as A1(2TO) mode, the sharp peak at 307 cm-1 as B1+E(2LO)+E(3TO) modes, the asymmetric broad peak at 520 cm-1 as A1(3TO)+E(4TO) modes, while the broad weak peak at 720cm-1 belongs to A1(3LO)+E(4LO) modes [13-15]. It can also be noticed that Raman modes of mechanically activated BaTiO 3 become broader and gradually lose their intensity. This is a consequence of mechanical activation processes that, on one hand, lead to the reduction of particle and crystallite sizes and, on the other hand, create lattice distortions and a larger number of defects.

Figure 1 a) SEM micrographs of the non-activated BaTiO3 powders and b) the powders mechanically activated for 5 min (b). Due to differences in defect structure and surface reactivity of the particles, the non-activated and activated BaTiO3 powders will influence the structural properties of polyvinylidene fluoride matrix in different way. The results in Figure 2 suggest that the incorporation of non-activated BaTiO3 into the PVDF matrix induce an increase in α-crystalline phase content. It can be noticed that the intensities of the peaks at 410 cm-1 and 795 cm-1 (which can be attributed solely to α-phase) [1,16-18] increase relative to the intensities of the peaks at 839 cm-1 (characteristic for both β- and γ-phases) and 1430 cm-1 (characteristic for α-, β- and γ-phases). In order to better illustrate this effect, in Figure 2b Raman spectra of the pure PVDF, and PVDF-BaTiO3 composites (with non-activated and activated fillers) were presented in the range from 775 to 875 cm-1. As can be seen, in the spectra of the pure PVDF film, the peak at 795 cm-1 has low intensity and it appears only as a „shoulder“, while in the spectra of the composite with non-activated filler, the same peak is sharp and clearly distinguishable.

The fact that non-activated BaTiO3 particles promote α-phase crystallization of PVDF may be further confirmed by the presence of the peaks at 610 cm-1, 874 cm-1, 1058 cm-1 and 1199 cm-1 (Figure 2a). The introduction of the activated BaTiO3 filler induces somewhat different crystallization behavior of the host PVDF matrix. Due to higher specific surfaces and smaller particles sizes, the activated BaTiO3 particles promote rather β- than α-phase crystallization. It can be seen in Figure 2b that the intensity of the sharp peak at 795 cm-1 (observed in the spectra of BaTiO3/PVDF composites with non-activated filler) is significantly reduced in the spectrum of composite with activated filler. At the same time, in the spectrum of the later composite (Figure 2a), typical α-phase peaks at 410 cm-1, 610 cm-1 and 1199 cm-1 completely disappear. On the other hand, there is an increase in intensity of the band at 839 cm-1, common to both β- and γ-phases (Figure 2b). This band is much more pronounced than the bands at 812 cm-1 (γ-phase) and at ~1430 cm-1 (attributed to all three phases, α, β and γ). A dominant formation of β-crystal phase in BaTiO3/PVDF nanocomposites with activated filler is further confirmed by an increase in intensities of Raman peaks at 442 cm-1, 1081 cm-1, 1173 cm-1 and 1284 cm-1 [11,17]. The formation of PVDF β-phase has also been noticed in the presence of other type fillers, for example organically modified clays [19].

Figure 2 a) Raman spectra of the pure PVDF and BaTiO3/PVDF composites with non-activated and activated fillers; b) Raman spectra in the range from 775 to 875 cm-1. Inset: Raman spectra of BaTiO3 non-activated and mechanically activated powders. The results of XRD measurements are in agreement with the results obtained by the Raman spectroscopy analysis. The XRD spectra show that the tetragonal crystal structure of the initial BaTiO3 particles remains unchanged after the mechanical activation (Figure 3, inset). However, due to increase in structural disorder, the activated particles exhibit lower integral intensities of XRD reflections and broadened diffraction profiles. This effect, clearly visible for BaTiO3 diffraction profiles, is, however, hidden in BaTiO3/PVDF composite spectra, due to presence of a broad diffuse response of the amorphous phase of PVDF and due to overlapping of BaTiO3 major peaks ((101),

(110)) with the PVDF α-phase peak at 32.3° (Figure 3). Concerning the effects of the filler on the crystal structure of PVDF, after the incorporation of non-activated BaTiO3, additional peak at ~27° is observed. The existence of this peak, which, according to literature, represents the superposition of  (111) and  (021) reflections, together with the appearance of a sharp peak at ~ 20.3° (which is a combination of the major  (110) line and  (110),  (200),  (110) lines) [2,20-23], suggest again that the non-activated particles promote crystallization of -phase. On the other hand, after the incorporation of mechanically activated BaTiO 3 into the PVDF matrix, peak at ~27° disappeared, while the peak at ~20.3° broadened due to higher contribution of -phase intensities to integral intensity. Former results support the conclusion from Raman analysis that -phase crystallization of PVDF is favorable process in the presence of mechanically activated BaTiO3 particles.

Figure 1 a) SEM micrographs of the non-activated BaTiO3 powders and b) the powders mechanically activated for 5 min (b).

4. Conclusions In this study, we investigated the influence of mechanical activation of BaTiO3 filler on the structural properties of BaTiO3/polyvinylidene fluoride nanocomposites. X-ray diffraction and Raman spectroscopy methods were used to analyze the structure and vibrational responses of non-activated and mechanically activated BaTiO3 powders and corresponding BaTiO3/polyvinylidene fluoride composites films. The results showed that mechanical activation induced the diminution of the particles leaving the initial tetragonal structure of the powder unchanged. Due to reduced sizes of the particles, the Raman modes became broader and gradually lose their intensity, while the integral intensity of XRD reflections decreased and diffraction profiles broadened. After non-activated BaTiO3 powder was incorporated into PVDF matrix, the vibrational and XRD spectra suggested that there is an increase in PVDF α-phase content. On the other hand, it was found that in the presence of

mechanically activated filler PVDF would predominantly crystallize in -form. In that sense, our approach has several advantages in terms of the ferroelectric properties of the obtained composites. By carefully chosen activation conditions, we managed to prepare smaller particles with preserved tetragonal (ferroelectric) crystal structure, which further promote crystallization of the matrix towards (again ferroelectric) -phase. In the forthcoming paper, we will report on the electrical and mechanical properties of these materials.

Acknowledgments This work was supported in part by the Ministry of education, science and technological development, Republic of Serbia (project nos. 172056, 45020 and 172057) and projects NSF CREST (HRD0833184) and NASA (NNX09AV07A).

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