Structural changes and phase behavior of electron-irradiated poly(vinylidene-trifluoroethylene) copolymers

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Abstract

The effect of high-energy electron irradiation on poly(vinylidene-trifluoroethylene) P(VDF-TrFE), copolymer of 56/44 comonomer molar ratio is studied using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermally stimulated depolarization current (TSDC). DSC results show that the energy necessary to process ferroelectric transition decreases after irradiation, which indicates that high-energy electron irradiation makes ferroelectric phase less stable, causing transition to paraelectric phase at a temperature below melting point. XRD data show that the peak at 2θ=19° gradually disappears with the increase of absorbed dose, which indicates that the transition from all-trans ferroelectric state to a trans-gauche amorphous conformation takes place. It is found that both the peak temperature and released charge carrier density decrease with the irradiation doses in TSDC spectra, demonstrating less dipoles and crystalline regions in the irradiated copolymer films during ferro-paraelectric (F–P) transition. The results of TSDC measurements are related to the morphological changes induced by electron beam (EB) irradiation as well as the presence of local polar ordering and decrease of crystallinity revealed in DSC and XRD results.

Introduction

High-energy particle irradiation is an effective method to modify the properties of poly(vinylidene fluoride) (CH2CF2)n [PVDF] and its copolymers with trifluoroethylene (CHFCF2)n [P(VDF-TrFE)] [1], [2]. The effect of high-energy irradiation on the crystal structures, ferroelectric properties, and polymer morphology of P(VDF-TrFE) has been investigated extensively [3], [4]. The irradiation-induced transformations generally lead to loss of crystallinity as a result of crosslinking or chain scission, which may also include intermediate crystallographic phase changes, indicating that potentials in the improvement of the electromechanical properties. Different types of radiation, such as gamma rays, X-rays, or electron beams (EB), were applied to modify the properties of these materials because high-energy irradiation induce various structural defects into the lattice of a ferroelectric crystal [5], [6]. For instance, Lovinger found that the electron irradiation induced ferro-paraelectric phase transition at room temperature in P(VDF-TrFE) copolymers both in intra-molecular and inter-molecular fashions [7]. Odajima et al. found the similar behavior in P(VDF-TrFE) 65/35 mol% by using γ-rays [8]. More recently, Zhang et al. found that the 2.55–3.0 MeV electron irradiated P(VDF-TrFE) 65/35 and 50/50 mol% copolymers exhibit exceptionally high electrostrictive responses [9], [10]. It has been showed that the defect structures introduced by means of high-energy irradiation break up the long-range polar regions into micro-domains. An exceptionally large longitudinal strain of about 4% and a high ability of energy storage reaching to 170 J kg−1 were observed in electron-irradiated P(VDF-TrFE) copolymers. Therefore, the electromechanical performance of the copolymer can be effectively enhanced through high-energy irradiation. Compared with traditional piezoelectric and magnetostrictive materials, which suffer low strain, low elastic energy density, and low electromechanical conversion efficiency, these electron-irradiated copolymers have many advantages in the applications of transducers, actuators and sensors [11].

Although the thermal treatment effect of P(VDF-TrFE) copolymers with different molar ratios as electrets has been studied by several authors [12], [13], [14], the thermal phenomena taking place in the phase transition and the change of crystalline structure in electron irradiated copolymers are still being studied. So it is very interesting to study the thermal relationship between phase transition and supermolecular structure for the irradiated P(VDF-TrFE) copolymers.

The purpose of this work is to study the effect of high EB irradiation doses on the stability of ferroelectric phase in the copolymer P(VDF-TrFE) 56/44 mol% using X-ray, DSC and TSDC measurements.

Section snippets

Experimental

The random copolymers of P(VDF-TrFE) (Piezotech, France) in this study, had a comonomer ratio, VDF/TrFE, of 56/44 mol% in pellet form. Thin films (about 20–30 μm in thickness) were made by solvent (dimethylformamide) casting method. A vacuum oven was used to evaporate DMF. To remove residual solvent and improve the crystallinity, films were directly annealed at 120 °C in oven for 12 h and then at 135 °C for 2 h to improve crystallinity. The electron irradiation was carried out in air condition with 3 

Results and discussion

Fig. 1 shows the DSC curves for P(VDF-TrFE) 56/44 mol% copolymer samples with various electron-irradiation doses from 0 to 1100 kGy. The unirradiated copolymer exhibits two endothermic peaks corresponding to the ferro-paraelectric (F–P) phase transition Tc and the melting temperature Tm of the crystalline region during the heating runs as shown in Fig. 1(a). The values for Tc and Tm are 64.5 and 155.2 °C, respectively. After irradiation with a dose of 300 kGy and above, both Tc and Tm shift to low

Conclusions

Structural changes and phase behavior of electron-irradiated poly(vinylidene-trifluoroethylene) copolymers 56/44 mol% copolymers have been studied in this paper. DSC results show that both temperatures and enthalpies associating with the phase transition and melting decrease, indicating a broad distribution in the crystalline sizes and reduction in the crystal ordering caused by electron irradiation. X-ray diffraction data show only one reflection peak upon 300 kGy irradiation at the lower angle,

Acknowledgements

This work was supported by the Centre for Smart Materials of The Hong Kong Polytechnic University. It was also supported in part by the National Natural Science Foundation of China under Grant Nos. 59973015 and 50125309 and the Hong Kong Research Grants Council (PolyU 5147/02E).

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