Effect of Annealing Temperatures and Time on Structural Evolution and Dielectric Properties of PVDF Films

Transcription

1 Effect of Annealing Temperatures and Time on Structural Evolution and Dielectric Properties of PVDF Films Effect of Annealing Temperatures and Time on Structural Evolution and Dielectric Properties of PVDF Films Yunfei Zhu 1, Huijian Ye 2, Li Yang 2, Lixiang Jiang 1, Liang Zhen 2*, Jianguo Huang 1, Zilong Jiao 1, and Jipeng Sun 1 1 Reliability and Environmental Engineering National key Laboratory, China Academy of Space Technology, Beijing , China 2 School of Materials Science and Engineering, Harbin Institute of Technology, Harbin , China Summary In this paper, we investigate the effect of thermal annealing on the degree of crystallinity, the fraction of electroactive phase (b-phase) and the dielectric properties, such as dielectric constant and loss tangent. Through control the thermal annealing conditions, dramatic improvements have been achieved. The degree of crystallinity is greatly enhanced up to 45.2%. The relative content of electro-active phase increases remarkably by 61% and the dielectric constant is improved greatly by 17.8% and reaches its maximum value of 9.9 meanwhile the dielectric loss being as low as Introduction There are multiple polymorphic crystalline forms (such as α, β, γ, δ) of Poly(vinylidene fluoride) (PVDF). These crystal phases are different from each other due to two main factors: the relative orientations of the polymorphic molecules in crystal structures and the sequential orders of the torsion angles. Most of the previous research has been focused on the α- and β-phases, since α-crystal is thermodynamically stable at room temperature and β-phase exhibits large piezoelectric and dielectric properties. Thus β-phase is also known as the electro-active phase. The β-phase with all-trans bond (TTTT) conformation is comprised of fluoride atoms on one side and hydrogen atoms on the opposite side of the polymer chain backbone, leading to the highest net dipole moment among these four different polymorphs 1-2. There are two main factors which are critical for a crystal material to demonstrate high ferroelectric response: the net * Corresponding author. Tel: , Fax: , lzhen@hit.edu.cn Smithers Information Ltd., 2016 dipole moment of crystal structure and the identical oriented dipoles in the bulk material 3. Hence β-phase PVDF displays large piezoelectric and dielectric properties. Owing to wonderful mechanical flexibility, high dielectric strength, low cost and also processing advantages, the β-phase PVDF has wide application in piezoelectric sensors and actuators and energy conversion/storage 4-6. In previous studies it has been found that the crystalline phases of PVDF highly depend on the preparation conditions, such as uniaxial/biaxial stretching and annealing 7-9. Although a few of researches have been done on the phases in PVDF and its transformation of β-phase during annealing effect, some details of this conversion, such as the issues on those related to microstructure and electron structure, need to be studied further to explore the relationship between crystal structure and physical property in PVDF films. In our previous work, the effect of electro-active phase transformation on electron structures and dielectric properties of uniaxial stretching PVDF films was investigated 10. In this paper, we focus on the effect of thermal annealing conditions on the degree of crystallinity, relative fraction of β-phase and dielectric properties of PVDF films. The properties were characterized by using Differential Scan Calorimetry (DSC), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy technique (FTIR). The results of experiments were given and analyzed in detail. 2. Raw Material and Film Preparation The PVDF powders using in the following experiments were purchased from Shanghai 3F Co. Ltd. and used without further purification. The colour of powder is translucent white and the oxygen index is 46% non-flammable. This type of PVDF can be used between -40 C and 150 C. The number average molecular weight of PVDF in this study is 65,000~241,000. The PVDF films were obtained via simple solution casting method. The 0.5 g PVDF powders was dissolved Polymers & Polymer Composites, Vol. 24, No. 2,

2 Yunfei Zhu, Huijian Ye, Li Yang, Lixiang Jiang, Liang Zhen, Jianguo Huang, Zilong Jiao, and Jipeng Sun in N,N-dimethylacetamide (10 ml). Then stirred the mixture at 60 C for 30 minutes and ultra-sonicated for 20 minutes to remove gas bubbles in the solution, which is highly important to obtain high quality films. The casted films were fabricated on the clean glass substrates via solution casting. Finally, applied with different isothermal annealing conditions on the PVDF samples and the annealing effects were examined. In order to obtain high quality films, we tried other concentrations as well, such as 20 wt.%. Finally we find out this concentration in the manuscript is the optimum in our study. The surface morphology of PVDF film has been explored by using AFM in a pizeo-response mode, and three dimensional images of μm 2 area of samples exposed with various fluencies are obtained and depicted in Figure 1. The grain morphology in the original sample exhibits consistent micro-structural features in PVDF crystal region and the surface roughness of crystal segments is about 2.5 µm. diffraction angles of 2θ = 18.5 and 19.9 are corresponding with the (020) and (021) lattice plane in α-phase, while the diffraction angles of 2θ = 20.1 and 20.8 are corresponding with the (110) and (220) lattice plane in β-phase 11 (see Figure 3). For the original film, the intensity of the diffraction angles of α-phase (at 18.5 ) is larger and thus crystal forms are mainly in α-phase. As the annealing temperatures rising, the intensity of the diffraction angles of α-phase decreases slightly, while the intensity for β-phase increases. The results indicate that the content of α-phase crystal decreased gradually and the amount of β-phase Figure 1. The AFM image of PVDF film increased with increasing annealing temperature. The annealing process contributes to the alignment of PVDF molecular chains and results in the transfer from α-phase to β-phase crystallization. The results show that the relative intensity of diffraction angles of β-phase reaches to its maximum at 140 C. The intensity of diffraction angles of α-phase at 27.0 decreased as the increment of annealing temperature and almost disappeared at the annealing temperature of 140 C. It indicates that the optimized annealing temperature is 140 C. 3. Effect of Annealing Temperatures It can be seen from Figure 2 the melting peak temperature of the original PVDF film is 167 C and the melting enthalpy was measured around 31.7 J/g. Based on the differential scan calorimetry (DSC) curves in Figure 2, the PVDF films were taken for isothermal annealing respectively at 60 C, 100 C, 140 C and 180 C with 2 hours. Then the samples were cooled down with the furnace up to the room temperature. The effect of annealing temperature on structural and dielectric properties of the PVDF films is illustrated in the following sections. Figure 2. The DSC curve of the original PVDF film with thickness of 60 μm 3.1 Effect of Annealing Temperatures on Degree of PVDF Films Crystallinity The PVDF film is semi-crystalline polymer with multiple crystal forms, such as α-phase and β-phase. The 168 Polymers & Polymer Composites, Vol. 24, No. 2, 2016

3 Effect of Annealing Temperatures and Time on Structural Evolution and Dielectric Properties of PVDF Films Using the curve fitting method and the XRD curves shown in Figure 3, the degree of crystallinity of PVDF samples could be obtained, as is shown in Figure 4. It shows that the degree of crystallinity increases as the annealing temperatures increase from 60 C to 140 C and reaches its maximum (45.2%) at 140 C. The degree of crystallinity begins to decrease from 45.2% to 42.7%, when the temperature continually increases from 140 C to 180 C. Since the melting peak temperature of PVDF samples are 167 C, when the annealing temperature rose to 180 C, the crystallization zone became melted. PVDF crystallizes naturally into its α-modification when cooled from the melt and the degree of crystallinity decreased. with transmittance mode and Fourier transform infrared spectra were recorded from 600 to 4000 cm -1 with a resolution of 2 cm -1. According to previous researches, the transmittance peak at 764 cm -1, belonging to CF 2 bending and skeletal bending, corresponds to α-phase. The vibration band at 840 cm -1 belongs to CH 2 rocking and is assigned to β-phase 6, The relative fraction of β-phase PVDF can be calculated using Eq. (1), as follows: (2) in which, A α denotes the area of absorption of α-phase, A β denotes the area of absorption of -phase, K α = 6.1 x 10 4 cm 2 mol -1 is the absorption coefficient at the corresponding wave number for α-phase, and K β = 7.7 x 10 4 cm 2 mol -1 is the absorption coefficient for β-phase. Figure 3. The XRD curves of PVDF films with temperatures at 60, 100, 140 and 180 C 3.2 Effect of Annealing Temperatures on Phase Compositions of PVDF Films The Fourier transform infrared spectroscopy (FTIR) technique can measure many elements at one test with high inherent spectral accuracy, so it becomes one of the popular characterization tools for examining the chemical components. The phase compositions of PVDF films can be characterized by FTIR 14. The infrared spectroscopy follows the so-called Lambert-Beer law 12, which is written as, the absorption is determined by Eq. (2), Figure 4. The degree of crystallinity of PVDF films variation with annealing temperatures (1) where A denotes absorbance, I 0 denotes the incident intensities of the radiation, while I denotes the transmitted radiant flux, L the thickness of the PVDF samples, C the average monomer concentration in crystal phase, X the degree of crystallinity for each crystal phase, and K is the absorption coefficients at the corresponding wavelength. A Nicolet Avatar 360 apparatus (Thermo Scientific, US) was used Polymers & Polymer Composites, Vol. 24, No. 2,

4 Yunfei Zhu, Huijian Ye, Li Yang, Lixiang Jiang, Liang Zhen, Jianguo Huang, Zilong Jiao, and Jipeng Sun It can be seen from Figure 5 that as the increase of the annealing temperatures, the peaks of absorption spectrum for β-phase PVDF grows slightly. Figure 6 provides the relative content of β-phase, which is varying with the annealing temperatures. The content of β-phase in the original PVDF film is 49.7%, and the β-phase content rises dramatically up to 79.8% at 140 C. 3.3 Effect of Annealing Temperatures on Dielectric Property of PVDF Films Since the dipolar orientation after thermal annealing, the dielectric performance of PVDF film was improved. The dielectric properties of PVDF films were measured at room temperature over the frequency range of 100Hz to 1MHz, as shown in Figure 7. The annealing temperatures can improve the dielectric constant dramatically. When the test frequency is equal to 100 Hz, the dielectric constant of original PVDF sample is 8.4 and after annealing at 140 C, the dielectric constant rises up to 9.6, which is much higher than the other kind of dielectric polymers 13. In section 3.1 and 3.2, the degree of crystallinity and the content of β-phase both reached their maximum values at Figure 5. FTIR spectrum of PVDF variation with annealing temperatures Figure 6. The relative content of β-phase varied with annealing temperatures 140 C, which is the main reason of high dielectric constant at 140 C. The conversion of PVDF from (α-phase) to electro-active phase, results in the enhancement of ferroelectric response and finally improves dielectric properties 10. The dielectric loss is around at 100Hz, which is insensitive to the variation of the annealing temperatures, as shown in Figure 7(b). Keeping the dielectric loss at a low level is extremely important for the application of capacitors. 4. Effect of Annealing Time Based on the experimental results in section 3, the degree of crystallinity and dielectric constant are both optimal at 140 C. Therefore, the isothermal annealing was fixed at 140 C and the annealing times was varying to illustrate the effect the treatment duration on the PVDF properties and 2 hours, 6 hours and 10 hours were chosen in the experiments. 4.1 Effect of Annealing Time on Degree of Crystallinity and β-phase Content of PVDF Films When the isothermal annealing time is 2 hours, both the degree of crystallinity and the β-phase content of the PVDF samples were improved compared with the original films, see Figure 8. The crystallinity increased from 37.5% up to 44.2% with 2 hours isothermal annealing. The degree of crystallinity improved further if the annealing time chosen as 6 hours. The crystallinity began to incline after 6 hours annealing, which is probably due to the aging of the polymer chains. The β-phase contents rose remarkably from 49.7% up to 78.8% with 2 hours isothermal annealing, see Figure 8(b). However, it didn t exhibit considerable variation with increase annealing time from 2 hours to 10 hours. 170 Polymers & Polymer Composites, Vol. 24, No. 2, 2016

5 Effect of Annealing Temperatures and Time on Structural Evolution and Dielectric Properties of PVDF Films 4.2 Effect of Annealing Time on Dielectric Property of PVDF Films The dielectric constant and loss tangent were measured for the PVDF films with annealing and without annealing treatment, see Figure 9. To achieve the best dielectric constant, the optimal isothermal annealing time is 6 hours, since both the degree of crystallinity and β-phase content reach their maximum values. The loss tangent was around under 100 Hz and varied a little with the increase of the isothermal annealing time as shown in Figure 9(b). 5. Conclusions We studied the effect of annealing conditions on the degree of crystallinity, the relative content of β-phase and dielectric properties of PVDF films. After isothermal annealing at 140 C with 6 hours, the degree of crystallinity was improved from 37.5% up to 46.2%. The relative β-phase content increased remarkably from 49.8% to 80.3%, which is largely due to the rearrangement of fluoride atoms with the adjacent hydrogen atoms and induced the transformation from α- to β-phase Figure 7. Dielectric properties of PVDF samples varied with annealing temperatures (a) dielectric constant, (b) loss tangent Figure 8. Effect of annealing time on PVDF samples (a) degree of crystallinity, (b) content of β-phase Figure 9. The dielectric properties varied with annealing time (a) dielectric constant, (b) loss tangent Polymers & Polymer Composites, Vol. 24, No. 2,

6 Yunfei Zhu, Huijian Ye, Li Yang, Lixiang Jiang, Liang Zhen, Jianguo Huang, Zilong Jiao, and Jipeng Sun by isothermal annealing process 12,16. We will investigate the rearrangement of fluoride atoms in our next work in details. The dielectric constant is considerably enhanced meanwhile with low dielectric loss, which is critical for PVDF film with the application of capacitors. Acknowledgement This work was financially supported by National Defence Pre-research foundation with the grant no: 9140c c References 1. Lovinger A, 1983 Science Gregorio R and Borges D, 2008 Polymer Sajkiewicz P, Wasiak A and Goclowski Z, 1999 Eur. Polym. J Zhang Q M, Bharti Vivek, Zhao X, 1998 Science Chu B. J, Zhou X, Ren K L, Neese B, Lin M R, Wang Q, Bauer F and Zhang Q M, 2006 Science Kolev G, Aleksandrova M, Vucheva Y and Denishev K, 2014 J. Phys Bystrov V S, Paramonova E V, Dekhtyar Y, Pullar R C, Katashev A, Polyaka N, Bystrova A V, Sapronova A V, Fridkin V M, Kliem H and Kholkin A L, 2012 J. Appl. Phys Sencadas V, Gregorio R and Lanceros-Me ndez S, Macromol J 2009 Sci. B: Phys Ye Hui-Jian, Shao Wen-Zhu, Zhen Liang, 2013 J. Appl. Polym. Sci Ye Hui-Jian, Yang Li, Shao Wen- Zhu, Sun Song-Bai, Zhen Liang, 2013 RSC Adv Daniel M E and Brian L J, 2004 J. Polym. Sci. Part B: Polym. Phys Sencadas V, Gregorio R and Lanceros-Me ndez S, 2009 J. Macromol. Sci. Part B: Phys Wang Y, Zhou X, Chen Q, Chu B J and Zhang Q M, 2010 IEEE Trans. Dielectr. Electri.Insul Mohannadi B, Yousefi A, and Bellah S, 2007 Polym. Test He L, Yao L, Sun J, Wang X, Song R, He Y and Huang W, 2012 RSC Adv Satyanarayana C and Bolton K, 2012 Polymer Polymers & Polymer Composites, Vol. 24, No. 2, 2016