Study of proton-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymers

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Study of proton-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymers

 

Author: Lau, Sien-ting
Title: Study of proton-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymers
Degree: Ph.D.
Year: 2004
Subject: Hong Kong Polytechnic University -- Dissertations
Copolymers -- Mechanical properties
Microelectromechanical systems
Proton magnetic resonance spectroscopy
Electron microscopy
Department: Dept. of Applied Physics
Pages: xix, 216 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1762019
URI: http://theses.lib.polyu.edu.hk/handle/200/2894
Abstract: In recent years, irradiation treatment in polymeric materials has attracted considerable interest as it can change both the structures and properties of the materials significantly and makes the materials useful for specific applications. In this project, high energy proton irradiation with a broad range of doses (20 - 250 Mrad) have been carried out to investigate the potential for modifying the properties of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymers. Three types of copolymers, with different VDF contents 56, 70 and 80 mol%, were prepared by compression moulding and then subjected to 3 MeV proton (H+) irradiation at room temperature. The effects of irradiation on both structures and properties of the copolymers have been investigated. These include the change in lattice spacing, molecular conformation, microstructure, phase transitional behaviour, relative permittivity and electric field induced strain. The irradiated copolymers show many features of relaxor ferroelectrics such as slim hysteresis loop in polarization measurement and dispersion in the frequency plot of the relative permittivity. It is believe that the high-energy protons can break up the coherent polarization domains in the normal ferroelectric copolymer into nano-sized regions. For copolymers with higher VDF content, a higher proton dose is required to convert them to relaxor ferroelectrics. Besides, the XRD data show that the proton irradiation induces a solid-state phase transformation from the ferroelectric (polar) phase to the paraelectric (nonpolar) phase in which the lattice constant increases significantly. The electric field induced phase transformation between these two phase leads to an ultrahigh electrostrictive strain response observed in the irradiated copolymers. With this outstanding electromechanical property, the irradiated copolymers can be a promising material for transducer and actuator applications. Thick irradiated 70/30 mol% copolymer films with thickness of- 35 um have been used as the active element for fabricating high frequency ultrasonic transducers. The transducers were characterized and their transmitting voltage response, bandwidth and centre frequency were measured. Thin irradiated 70/30 mol% copolymer films with a few um-thick were also prepared in this work for actuator applications in microelectromechanical systems (MEMs). A suspended silicon nitride/silicon dioxide (Si3N4/SiO2) membrane was prepared by etching the Si substrate using bulk micromachining technique. A copolymer thin film was then deposited on the membrane by the spin-coating method. After proton irradiation treatment, the performance of the copolymer based MEMs actuator was characterized.

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