Effect of High-Energy-Electron Irradiation on Nonisothermal Crystallization Kinetics in P(VDF-TrFE) 65/35 mol% Copolymer
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Effect of High-Energy-Electron Irradiation on Nonisothermal Crystallization Kinetics in P(VDFTrFE) 65/35 mol% Copolymers Zhi-Min Li and Z.-Y Cheng Materials Research and Education Center, Auburn University, Auburn, AL 36849-5341 ABSTRACT The effect of the high-energy-electron irradiation on the crystallization process in poly(vinylidene fluoride-trifluoroethyelene) [(P(VDF-TrFE)] 65/35 mol% copolymers was studied by nonisothermal crystallization using the differential scanning calorimetry (DSC) technique. The experimental data are analyzed using modified Avrami, Ozawa, and combined Avrami-Ozawa methods. It is found that the crystals grow in three dimensions in the irradiated samples. It is found that the irradiation results in a lower crystallization temperature and a lower crystallization activation energy. It is also found that the irradiated samples have a lower crystallization temperature than the unirradiated samples. All these results indicate that the crystals grown in the irradiated samples have a smaller surface energy, which corresponds to a thicker interfacial layer. INTRODUCTION Poly(vinylidene fluoride-trifluoroethylene) [(P(VDF-TrFE)] copolymers with VDF contents ranging from 50 to 80 mol% are semi-crystalline polymers that can be easily prepared to present ferroelectricity at room temperature [1]. In an effort to improve their electromechanical performance, a high-energy-electron irradiation approach has been employed [2-4]. It is found that the irradiation can convert P(VDF-TrFE) from typical ferroelectric state to relaxor ferroelectric state [5,6]. The latter exhibits a higher electrostrictive strain response [2,3]. The irradiation can induce many kinds of defects, including chain scission, isomerization, bond rearrangement, double bonds, bulky pendant groups and crosslinking [7-9]. Among these defects, crosslinking and its influence on the structure have been studied using different approaches due to the fact that the crosslinking sites are one of the principal effects of irradiation on P(VDFTrFE) [10,11]. It is believed that the high strain response obtained in the irradiated copolymer originates from electric-field-induced structure/conformation change [12,13]. That is, the crystalline regions in the irradiated copolymer are dominated by TG and T3G conformations, while the external electric field converts the conformations to all-trans conformation. In order to deepen the understanding of the effects of irradiation induced defects on the microstructure of the copolymer, we initiated the recrystallization study on irradiated P(VDFTrFE) 65/35 mol% copolymers [14,15]. The results show that the structure and properties observed in recrystallized irradiated samples are strongly different from those of corresponding irradiated samples. For example, in the copolymer irradiated with a dose of 60 Mrad, the crystalline regions in the irradiated sample are dominated by TG and T3G conformation and the sample exhibits a typical behavior of relaxor ferroelectrics [5,6], while the sample after re
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