J Polym Res (2014) 21:587 DOI 10.1007/s10965-014-0587-0
ORIGINAL PAPER
Effect of thermal processing conditions on the structure and dielectric properties of PVDF films Vineet Tiwari & Geetika Srivastava
Received: 27 July 2014 / Accepted: 9 October 2014 # Springer Science+Business Media Dordrecht 2014
Abstract The effect of annealing and quenching temperatures on the crystallinity, β phase fraction and dielectric behavior of poly (vinylidene fluoride) (PVDF) have been studied. The crystallinity and β phase fraction of these films were evaluated using X-ray diffraction and FTIR techniques for different annealing and quenching temperatures. It is seen that the thermal processing conditions play a crucial role in determining the dominant phase in PVDF. The β phase PVDF is the most desired phase for device applications such as sensors and actuators. Hence, the thermal processing conditions are optimized for obtaining β rich PVDF films. The β rich phase of PVDF is obtained for films which are annealed at 80 °C and quenched at 20 °C. The as-synthesized films for the optimized processing conditions was studied for their dielectric behavior and was found to exhibit dielectric constant as high as ~60. Keywords Crystallinity . PVDF . Dielectric . X-ray diffraction . FTIR
Introduction With the advent of MEMS technology, the functional materials such as ferroelectric materials have gained utmost attention in the recent past years. These ferroelectric materials are found to exhibit excellent piezoelectric properties which make them suitable for technological applications such as sensors and actuators. The recent technological developments in this field have led to the replacement of piezoelectric ceramics by piezoelectric polymers for their light weight, flexibility, fracture tolerance, ease in fabrication and processing techniques V. Tiwari : G. Srivastava (*) Department of Physics and Materials Science and Engineering, Jaypee Institute of Information Technology, Noida 201307, India e-mail:
[email protected]
[1]. Poly (vinyl difluoride) (PVDF), has been widely explored among all other electroactive polymers for their superior piezoelectric and ferroelectric properties. PVDF is a semicrystalline polymer, known to exist in various crystalline phases mainly α, β and γ phases. It consists of (−CH2-CF2-) or VDF (vinylidene fluoride) monomers attached in a linear chain arrangement. Each monomer has an electric dipole moment arising due to electronegative fluorine and electropositive hydrogen atoms. The phases of PVDF are different from each other in terms of crystallographic space group, the conformation and packing of the molecules and have been investigated in great detail [2–11]. In β- phase, the dipole moments of CF2 and CH2 bonds are such that the effective dipole moment is strongest and are mostly parallel compared to the other phases and is in the direction perpendicular to the carbon backbone. Hence, βphase is responsible for the ferroelectric properties in PVDF. The ferroelectric and piezoelectric properties are governed by the content of β phase in semicrystalline PVDF polymers. Hence, β-rich phase in PVDF is desired for device applications such as sensors and actuators [12–16]. Among the crystalline forms, α phase is predominant [17]. It is seen that the β crystalline phase in PVDF can be obtained from modification of α phase by different processing conditions such as mechanical deformation [18, 19], the application of high electric field [20], crystallization under the influence of high pressure [21, 22], cooling at high rates etc. [23]. In the recent study, Sharma et al. investigated the influence of various processing conditions such as annealing, poling, mechanical rolling on the material properties of hot pressed PVDF [24]. In another study, Ye et al. studied the effect of stretching parameters on the β phase fraction, electron structure and dielectric properties of these films [25]. Hence, the processing conditions are found to control the phase fractions of crystalline phase in PVDF films [26–28]. In the present study the effect of the thermal processing conditions on the β phase fraction,
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structure and dielectric properties of PVDF films have been investigated. Thus, the important parameters which affect the performance of a PVDF film are the degree of crystallinity and the phase content of β phase. The degree of crystallinity and the β phase content influences the mechanical, thermal, ferroelectric and piezoelectric properties of PVDF polymers. In this study, the basic intention is to obtain β-rich PVDF film by optimizing the processing conditions. The PVDF films synthesized were subjected to various thermal treatments for enhancing the piezoelectric β phase. These assynthesized films were characterized by X-ray diffraction, IR spectroscopy to determine the level of crystallinity and to evaluate β phase content in the film. Further, the dielectric properties of these films were investigated. The variation in the dielectric permittivity of PVDF film synthesized with different thermal treatments has been studied and is related to the changes in the crystallinity and β phase content in the film.
Materials and methods A measured quantity (15 wt %) of PVDF granules were dissolved in dimethylsulfoxide (DMSO) (20 ml) and acetone (80 ml) solutions by continuously stirring at 40 °C for 30 min. The films were prepared using solution cast technique. The solution was cast on a petri dish and kept at 60 °C for 4 days. This solution was dried at room temperature to obtain PVDF film. The films prepared by the said process were annealed for various temperatures at 70 °C, 80 °C, 90 °C, 110 °C, 130 °C and 160 °C for five hours and later rapidly quenched at 0 °C, 20 °C and 30 °C. FTIR spectra of the films were recorded by a Perkin Elmer Spectrometer for the investigation of different phases of the synthesized films. The phases of the obtained PVDF films were also characterized by X-ray diffraction using CuKα radiation on an XRD-6000 from Shimadzu Analytical, Japan. The degree of crystallinity was evaluated by the deconvulted peaks of crystalline and amorphous phase. The dielectric measurements were carried using LCR meter, Hioki, Japan at room temperature for 1 kHz.
Results and discussions
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is present in all the as-synthesized films. This can be seen by the broadening of the peak at approximately 2 =20-21° due to the convolution of the (110) α peak around 19.9°, (101) γ peak around 20.3° and the (110,200) β peak around 20.9°. In order to quantify the content of crystalline and amorphous phases present in the films, the deconvolution of the peaks was done. The characteristic peaks used for the deconvolution of peaks associated with different phases are mentioned in Table 1 [29]. Fig. 2 shows a typical graph showing the deconvoluted peaks of different crystalline and amorphous phases in X-ray diffraction pattern of a PVDF film. The formation of α, β and γ phases in PVDF films depends on the viscosity of the solution, polarity of the solvent and the induced thermal energy. It is generally seen in the PVDF film which is synthesized by solution casting around 60 °C, the low solution viscosity leads to the cooperative rotation of CF2 groups and large change in trans-gauche conformation. This results in TGTG α- phase crystallites to align themselves to TTTGTTTG γ phase or TTTT β phase. Hence, at such low temperatures of synthesis only α and γ phases are reported. The presence of β phase in this study, even for unannealed films for different quenching temperatures as shown in Fig. 1, can be attributed to the polar nature of solvent (i.e. DMSO) used in the synthesis. The polar group of the solvent tends to rotate the CF2 group by reducing the thermal barrier and hence favoring the formation of β—phase [30]. The thermal energy causes the change in trans-gauche conformation but the higher viscosity of the solvent favors TGTG conformation [26, 30]. Hence, α phase is dominant for films synthesized under unannealed conditions as is evident from the Fig. 1 and Fig. 3. Further, as the temperature of annealing is increased from 70 °C to 110 °C, the characteristic peak of α phase around 20° transforms to γ and β phases as is evident from the peak positions around 20.3 and 20.9° respectively (Fig. 1). It can also be seen from Fig. 1 that, for films annealed at 130 °C and 160 °C, the characteristic peaks of α phase become more pronounced while the peaks corresponding to β and γ phases diminish as compared to the films annealed at lower temperatures. The formation of α rich phase is evident for films processed at higher annealing temperature and is also reflected by Fig. 3. This may be explained in terms of the low viscosity of the solvent due to high annealing temperatures which results in rapid active chain motion and the reorientation in the crystalline region. This further leads to a change in trans gauche conformation and stabilizes the α phase. The degree of crystallinity, Xc, was calculated using the equation as reported in the earlier study [30].
XRD Figure 1 shows the XRD patterns obtained for PVDF films annealed and quenched at different temperatures. The XRD patterns of the PVDF films indicate a mixture of crystalline phases α, β, and γ. It can be seen from the figure that β phase
Xc ¼
Aðcrystalline phaseÞ Aðcrystalline phaseÞ þ Aðamorphous phaseÞ
ð1Þ
Where A (crystalline phase) represents the area of the peak corresponding to the crystalline α, β and γ phases and A
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Fig 1 X-ray diffraction patterns of PVDF films annealed at different temperatures and quenched at (a) 30 °C (b) 20 °C and (c) 0 °C
(amorphous phase) denotes the area corresponding to the amorphous fraction. Similarly the phase content of α, β and γ phases can be evaluated using the XRD patterns by the given equation (2), X α;β;γ ¼
A ðα; β; γ Þ A ðα þ β þ γ Þ
ð2Þ
The quantification of the different phases in these films for different processing conditions was done considering the integrated intensity areas of deconvoluted peaks [30]. It is evident from X-ray diffraction patterns that the degree of crystallinity and phase fractions of α, β, γ phases in the polymer are controlled by the processing conditions and the thermal treatment. Fig 3a,b,c shows the variation in Xα,β,γ for
where Xα, β, γ are the degree of crystallinity or the phase content of α,β and γ phases respectively, the denominator represents the area corresponding to the peaks of α, β and γ phases and A (α, β, γ) denotes the area corresponding to the peaks of α, β and γ phases respectively. Table 1 Assigned peak positions and their planes for α, β, γ phases of PVDF Notation Used
2θ (°)
Crystalline phase
Corresponding planes (hkl)
I II III IV V
17.6° 18.4° 19.9° 20.3° 20.9°
α α α γ β
(100) (020) (110) (101) (110,200)
Fig 2 A typical deconvoluted X-RD pattern for the quantification of α, β and γ phases present in the PVDF film
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quenched at 30 °C, it can be seen from the Fig. 3 (a) that with an increase in annealing temperature, α and β phases in the film increases thereby increasing the total crystallinity of the film. When the films are quenched at low temperatures viz at 20 °C and 0 °C (Fig. 3b and c), it can be seen that there is conversion of some extent of α phase to β phase content. It is also seen that quenching of the films at 20 °C gave better results. It can be further seen that the β phase is maximum for the film processed at annealing condition of 80 °C followed by quenching at 20 °C (Fig. 3b).
FTIR
Fig 3 Variation in the % crystalline content in the PVDF films annealed at various temperatures and quenched at (a) 30 °C (b) 20 °C and (c) 0 °C
films annealed under different temperatures subsequently quenched at 30 °C, 20 °C and 0 °C respectively. It can be seen from the figure that the total crystalline content increases with the increase in annealing temperature. The maximum crystalline content in the film is 50-54 % for the films processed at different thermal conditions. In case of films
The FTIR spectra of PVDF films were recorded to complement the structural analysis by XRD. Fig 4 shows the FTIR spectra of PVDF films annealed and quenched at different temperatures. The different vibration modes of PVDF have been listed in Table 2. The vibration bands at 764, 795 cm−1 have been attributed to the α phase while 778, 795 and 834 cm−1 have been assigned to γ phase and 840 cm−1 corresponds to the β phase of PVDF films [12, 13, 15, 18, 26, 27, 30–32]. It can be seen from Fig. 4 that in all the as synthesized films under different thermal processing conditions α, β and γ phases coexist. Further, it can be seen that the phase content of α phase increased while that of β and γ phases decreased for
Fig 4 FTIR spectra of PVDF films annealed at different temperatures and quenched at (a) 30 °C (b) 20 °C and (c) 0 °C
J Polym Res (2014) 21:587 Table 2 Vibration modes of PVDF along with their assigned vibration modes
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Notation used
Wave number (cm−1)
Crystalline phase
Excited vibrations
A
764
α
B C D E F
778 795 812 834 840
γ α γ γ β
CF2 bending and scelete bending CH2 Rocking CH2 Rocking CH2 out-of-plane wagging —— CH2 Rocking
the annealing temperatures of 110 °C, 130 °C and 160 °C. For the films annealed at 70 °C, 80 °C and 90 °C (Fig. 4), the β phase is found to increase as a result of transformation of γ and α phases. These results are found to be consistent with the XRD results. In order to have an idea about the content of α, β and γ phases in the crystalline, the deconvolution of the peaks was done. Fig. 5 shows a typical graph showing the deconvoluted peaks of different phases present in the polymer. The phase content of the crystalline phases was evaluated using the procedure adopted by Osaka and Ishida [33]. It is known that the absorbancies for α, β and γ phases for a film sample of thickness t and an average monomer concentration C using IR absorption bands, assuming it follows Lambert-Beer law, can be written as Aα ¼ K α CX α t
ð3Þ
Aβ ¼ K β CX β t
ð4Þ
Aγ ¼ K γ CX γ t
ð5Þ
Fig 5 A typical deconvoluted XRD pattern for the qualitative estimation of α, β and γ phases present in the PVDF film
Where Aα, Aβ and Aγ are the absorption values at 766, 840 and 834 cm−1, associated with α, β and γ phases, Kα and Kβ are the absorption coefficients at the respective wavenumbers with values of 6.1×104 and 7.7×104 cm2/mol for α and β phases respectively. Kγ represents the absorption coefficient for γ phase and is not reported in the literature for wavenumber 834 cm−1. The relative fraction of β phase present in the film is given by [20, 34] F ðβ Þ ¼
Xβ Aβ ¼ Xα þ Xβ K β =K α Aα þ Aβ
ð6Þ
Hence, δα, δβ and δγ if defined as in equations (7–9) can be related to the α, β and γ phase content present in the sample. Hence, the following terms were evaluated for all the films [30]. δα ¼
Aα Kαt
ð7Þ
δβ ¼
Aβ Kβt
ð8Þ
δγ ¼
Aγ Kγ t
ð9Þ
Figure 6a,b,c represents δ’s of the films evaluated considering the above equations for various films annealed at different temperatures and quenched at 30, 20 and 0 °C respectively. It can be seen that the variation in the phase contents of different phases obtained from the XRD pattern and from FTIR results are in agreement with each other. β rich phase was obtained for films annealed at 80 °C and quenched at 20 °C. Fig. 7 shows the variation of phase fraction of β in the crystalline phase, F (β) as a function of annealing temperatures for different quenching rates. It was earlier reported that the lower quenching temperatures yield films of β rich phase. However, in this study, it can be seen from the figure that the films quenched at 20 °C yielded β rich phase as compared to
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Fig 7 Variation in F (β) of PVDF films synthesized under different thermal processing conditions
Fig 6 Variation in δα,β,γ of PVDF films processed under different annealing temperatures and quenched at (a) 30 °C (b) 20 °C and (c) 0 °C
the films quenched at 0 °C and 30 °C for all annealing conditions. Hence, an optimum quenching condition is required for enhancing β phase in these PVDF films. Further, it can also be seen from the figure that the annealing temperature also plays a crucial role in the formation of β phase in these films. The maximum β phase of ~70 % of the total crystalline phase is obtained for an optimized annealing and
Fig 8 Variation of dielectric constant with temperature of PVDF films annealed at different temperatures and quenched at (a) 0 °C (b) 20 °C
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quenching temperatures of 80 °C and 20 °C respectively. Sharma et al. and Ye et al. also concluded in their study that the fraction of β phase is strongly dependent on the adopted process and the sequence of the synthesis conditions as observed in our study [24, 25]. Dielectric studies Fig. 8 (a,b) shows the variation of dielectric constant as a function of temperature for films processed under different annealing conditions and quenching temperatures of 0 °C and 20 °C respectively. It can be seen from the figure that the dielectric constant is maximum for the film synthesized with an annealing temperature of 80 °C and later quenched at 20 °C. This can be attributed to the optimized processing conditions which yielded β rich PVDF films. The β phase and the crystallinity are known to be responsible for the superior ferroelectric and piezoelectric properties of PVDF films. It can be seen from the figure that for the β rich PVDF films, the dielectric constant obtained was greater compared to the films having less β phase. However, the obtained dielectric constant due to thermal treatment is found to be much higher (~60) than the recent studies [24, 25].
Conclusions A detailed structural analysis on PVDF films obtained after different treatments utilizing XRD and FTIR techniques suggest that the thermal processing conditions play a major role in the formation of β rich phase in PVDF. It is seen that the quenching of PVDF film at 20 °C yielded in maximum β phase in the as synthesized PVDF films. The annealing temperature of 80 °C was found to be optimum for PVDF film quenched at 20 °C for enhanced crystallinity and β phase content. The enriched β phase and the better crystallinity were responsible for much improved dielectric behavior of these films.
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