The electroactive β-phase formation in Poly(vinylidene fluoride) by gold

Transcription

1 The electroactive β-phase formation in Poly(vinylidene fluoride) by gold nanoparticles doping Dipankar Mandal *,1, Karsten Henkel 2, and Dieter Schmeißer 2 1 Department of Physics, Jadavpur University, Kolkata , India 2 Applied Physics and Sensors, Brandenburg University of Technology, K.-Wachsmann-Allee 1, Cottbus, Germany ABSTRACT In this work we report on the preparation and characterization of a Poly(vinylidene fluoride) (PVDF) film doped with gold nano-particles (Au-NPs) prepared by a one step procedure. The preparation of Au-NPs without any external reduction agent is in the focus of interest when PVDF is used as a polymer stabilizer. It is found that a desirable amount of Au-NPs within the polymer matrix can enhance the electroactive β-phase in PVDF. The melting behavior of the crystalline polymorph of PVDF is examined by in-situ thermal FT-IR study. Spherulitic features in Au-NPs doped PVDF film are observed in the topographic images indicating a probable existence of spherulitic structures also in the β-phase (β-spherulite). * Corresponding author: dipankar@phys.jdvu.ac.in, Phone: x2880, Fax:

2 Keywords: PVDF; Au-NPs; β-phase; β-spherulite. 1.Introduction Poly(vinylidene fluoride) (PVDF) is widely used in a variety of electro-optical, electromechanical, and biomedical applications. It exhibits at least four crystalline phases (α-, β-, γ-, and δ-phase). Among them the α-phase is non-polar, whereas the others are electroactive [1]. However, only the β-phase (all-trans (TTTT) conformation) has a stronger ferro-, piezo-, and pyroelectric response due to its largest spontaneous polarization ( Cm per unit cell) [2]. Therefore several efforts have been taken in order to induce the β-phase into PVDF, e.g. mechanical stretching, the application of high pressure or the poling under high pressure [2]. Nevertheless, these methods normally induce undesired structural deformations or structural limitations which may not be feasible for several applications, in particular for electro-optical sensors and non volatile memories [3, 4]. Hence, an alternative methodology is required. In this work, we have induced the β-phase into PVDF by doping it with gold nano-particles (Au-NPs) via a single step procedure. 2.Experimental procedure In our preparation procedure we have chosen N,N-Dimethylformamide (DMF) for two reasons i) as the best known solvent for PVDF and ii) as a reduction agent for Au-NPs [5] in order to avoid unnecessary additives when Au-NPs is reduced from chloroauric acid (HAuCl 4 3H 2 O). First, 0.2 and 1.5 mm of HAuCl 4 3H 2 O (Sigma-Aldrich, Germany) were separately added to 6 wt % (w/v) PVDF-DMF (Sigma-Aldrich, Germany) solutions which were afterwards vigorously stirred at 60 o C for 4 h. Then the solutions were cast onto clean glass slides followed by a vacuum drying at 130 o C for one week. Finally, the dried films (thickness ~10-15 nm) were lifted off from the glass substrates and named neat PVDF, where no additives were 2

3 applied, PVDF-Au0.2 and PVDF-Au1.5, where HAuCl 4 3H 2 O in a concentration of 0.2 and 1.5 mm, respectively, was added into the initial solution. The formation of Au-NPs and their doping into the PVDF matrix were clarified by UV-vis spectra (Perkin Elmer, Lambda 25 Spectrophotometer) and Field Emission Scanning ElectronMicroscopy (FE-SEM, Supra TM 50/50 VP) studies. Fourier Transform Infrared Spectroscopy (FT-IR, Bio-Rad FTS-60A Spectrophotometer with a resolution of 4 cm -1 ) was performed in order to confirm the different crystalline polymorphs. The melting behavior of the crystalline polymorph was examined by insitu thermal FT-IR study with thermal ramp 1 ο C/min. The topographic images were recorded with a PUCOTECH atomic force microscope in non-contact mode (NC-AFM) using a silicon nitride tip. 3.Results and discussion The FE-SEM image of Au-NPs doped PVDF film is illustrated in Fig.1. It reveals that Au-NPs are distributed in different shapes (spherical, triangular, octahedron, square and rod type) and sizes. A photograph of the film is shown in the inset (a) of Fig. 1. The apparently transparent pink color of the film ensures the Au-NPs formation and its doping into the PVDF matrix. From UV-vis spectra (inset (b) of Fig.1) the characteristic plasmon bands of Au-NPs at 550 nm is evident [5, 6]. The little red shift and the broadening of the peak are due to different shapes and a wide size distribution of the Au-NPs. The FT-IR spectra of the neat PVDF, PVDF-Au0.2, and PVDF-Au1.5 samples are shown in Fig. 2a. The neat PVDF film is mainly composed of α- and γ- crystalline phases with characteristic absorption peaks at 976, 764, 615, 531 cm -1 (α) and 812, 1233 cm -1 (γ), respectively [3, 6-8]. In contrast, the spectra of the Au-NPs doped PVDF films (PVDF-Au0.2 and 3

4 PVDF-Au1.5) exhibit a new peak at 1275 and 445 cm -1, which corresponds to the existence of the β- phase [7, 8]. Consistently the 1233 cm -1 signal of the γ-phase is reduced. The complete disappearance of the characteristic bands of the α-phase and the intense signals of the β-phase in the sample PVDF-Au1.5 is a main finding of this work, whereas the PVDF-Au0.2 film consists of the α-, β- and γ- polymorphs. Due to the existence of the TTT structure in the chain conformation of both β- and γ-phases in PVDF, most of the vibrational bands of these two phases appear at the same or very similar frequencies in the FT-IR spectra. Therefore, it is noteworthy that the 841 and 510 cm -1 peaks carry a dual signature of the γ- as well as of the β- phases [7-9]. Nevertheless, our research trend based on the FT-IR results indicates that a desirable Au-NPs doping in PVDF can control the crystalline polymorph and rather improve the content of the polar β-phase of PVDF. The ν as (CH 2 ) and ν s (CH 2 ) vibrational bands are shifted towards lower energies in Au-NPs doped PVDF in comparison to neat PVDF (Fig. 2b), evidencing the interaction between the Au-NPs and PVDF. Another similar observation is also found in the 1405 cm -1 band (Fig. 2a) which is mainly due to the ν as (CC) and ω(ch 2 ) vibrational modes. Therefore it is quite sure that due to the large electronegativity of the fluorine atoms in PVDF and due to the surface charge of Au-NPs an interaction takes place in order to minimize the potential energy for a stereo-chemical conformation that eventually induces the β-phase. However, further studies are required in order to clarify the particular type of interaction, i.e. an adequate theoretical model should be developed. In order to confirm the melting temperature of the α-, β-, and γ- phases, we have performed an in-situ thermal FT-IR study of the PVDF-Au0.2 film, shown in Fig. 3a. The lower melting point of the β-phase can be clearly visualized by the presence of the α- and γ- phase characteristic bands at 175 o C, whereas the 1275 and 445 cm -1 vibrational bands (β-phase) are 4

5 completely disappeared. Therefore, it is confirmed that the β-phase has a lower melting temperature and γ-phase are thermally stable among three phases (Fig. 3b) as reported earlier [9-11]. Several spherulitic growths centered from the nucleus and their radial spreading with clear boundaries are observed in the topographic image of the PVDF-Au1.5 film, shown in Fig. 4a. These boundaries are due to the impingement of spherulites. The average diameters (maximum length from one edge to another edge) of the spherulites are around 2.5 μm varying from 1.2 μm to 4.4 μm with an error bar of ± 0.2 μm. It is widely agreed that due to the weak birefringence nature of the β- phase, usually it does not show optical anisotropy in polarizing optical microscopy images [8, 12]. In contrast, the α- and γ-phases commonly exhibit spherulites with a quite larger mean diameter of more than 20 and 10 μm respectively [8]. Therefore, the spherulitic structure of the PVDF-Au1.5 film revels that the β- phase may also have particular feature (i.e. β- spherulite). In order to confirm this observation, we have melted the PVDF-Au1.5 film at 200 o C for 10 min and quenched in air. In the FT-IR data taken afterwards only peaks attributed to the γ- phase can be observed (Fig. 4b). The associated topographic image is shown in the inset of Fig. 4b, where none of the spherulites remained after melt quenching within the shown scanning area μm 2 scanning area. Rather a melt fusing of the β- spherulites is observed as expected due to the lower melting temperature (< 200 o C) of the β-phase [7, 9, 10]. Therefore the existence of β- spherulites is confirmed in our Au-NPs doped PVDF film. 4.Conclusion In summary, the synthesis of Au-NPs is performed by a facile technique where PVDF acts as stabilizer. The desirable amount of Au-NPs doping in PVDF can induce the electroactive β- phase, which is in the main focus of attraction because of its polar nature. Therefore, the 5

6 traditional stretching method can be avoided. The spherulitic structure of the β- phase is observed in the Au-NPs doped PVDF film emphasizing a new feature of PVDF. Acknowledgment This work was partially supported by funding from the Jadavpur University (Ref. No. P- 1/1169/08) and German Research Foudation (DFG) within the priority program SPP 1157 (grant No. Schm 745/11-1-2). References [1] Lovinger AJ. Science 1983;220: [2] Furukawa T. Phase Trans 1989;18: [3] Dahiya RS, Cattin D, Adami A, Collini C, Barboni L, Valle M et al. IEEE Sens J 2011;11: [4] Henkel K, Lazareva I, Mandal D, Paloumpa I, Müller K, Koval Y et al. J Vac Sci Technol B 2009;27: [5] Pastoriza-Santos I, Liz-Merzan LM. Langmuir 2002;18: [6] Nehal CL, Hafber JH. J Mater Chem 2008,18: [7] Mandal D, Henkel K, Schmeißer D. J Phys Chem B 2011;115: [8] Ince-Gunduz BS, Alpern R, Amare D, Crawford J, Dolan B, Jones S, Kobylarz R, Reveley M, Cebe P. Polymer 2010;51: [9] Ramsundaram S, Yoon S, Kim KJ, Park C. J Poly Sci B Poly Phys 2008;46: [10] Tashiro K, Kobayashi M, Tadokoro H, Macromolecules 1981,14: [11] Osaki S, Ishida, Y. J Poly Sc Poly Phys Ed 1975;13:

7 [12] Lovinger AJ. Poly(vinylidene fluoride), in: Developments in Crystalline Polymers 1, Bassett, Ed. London: Applied Science Publishers;1981. Figure Captions Fig. 1. FE-SEM image (magnification:100 kx) of PVDF-Au1.5 (scale bar: 100 nm) film. Inset shows its (a) photograph and (b) UV-Vis spectra. Fig.2. FT-IR spectra of neat PVDF, PVDF-Au0.2, PVDF-Au1.5 films in different frequency regions; (a) 1600 to 400 cm -1 ; (b) 3100 to 2900 cm -1. Fig. 3. In-situ thermal FT-IR spectra of PVDF-Au0.2 film which distinctly contents three phases, i.e. β, α and γ (a) and plotted separately at 175 o C (b). Fig. 4. (a) Topographic image (NC-AFM: scan area μm 2 ) of the PVDF-Au1.5 film. (b) FT-IR spectra of the melt quenched PVDF-Au1.5 film. The inset shows the corresponding topographic image of a scan area of μm 2. Figures 7

8 Absorbance (a.u.) Absorbance (a.u.) Absorbance (a.u.) Absorbance (a.u.) Fig.1. (a) PVDF-Au1.5 PVDF-Au0.2 Neat PVDF (b) as (CH 2 ) s (CH 2 ) PVDF-Au1.5 PVDF-Au0.2 Neat PVDF Fig (a) (b) at 175 o C X o C 170 o C 165 o C 160 o C 140 o C 130 o C 100 o C 50 o C 30 o C

9 Absorbance (a.u.) Fig. 3. (a) (b) PVDF-Au1.5 (After mealting at 200 o C) Fig. 4. 9