The electret effect and electromechanical properties of solid and porous polymers

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The electret effect and electromechanical properties of solid and porous polymers

 

Author: Tai, Liang-shiang
Title: The electret effect and electromechanical properties of solid and porous polymers
Degree: Ph.D.
Year: 2005
Subject: Hong Kong Polytechnic University -- Dissertations
Polyethylene terephthalate
Piezoelectricity
Electrets
Polymers
Department: Dept. of Applied Physics
Pages: xviii, 225 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1809912
URI: http://theses.lib.polyu.edu.hk/handle/200/2082
Abstract: Electret effect, pseudo-piezoelectricity and electrostriction of solid and porous polymers were investigated in this study where solid polymers include polyethylene terephthalate (PET), polyvinylidenefluoride (PVDF), poly(vinylidenefluoride-trifluoroethylene) P(VDF-TrFE), polytetrafluoroethylene (PTFE) and fluoroethylenepropylene (FEP). Besides porous PTFE, new porous copolymer P(VDF-TrFE) films were also prepared and used in this work. Capacitive probe and high resolution TSDC techniques were employed for the measurement of surface potential and relaxation of thermal current respectively. Quasi-static piezoelectricity (d33) of solid and porous polymers were determined by measuring the change of induced charge density from the samples after a static load was applied in the thickness direction. By using a modified Michelson interferometer capable of resolving displacements of 3 nm, field-induced strains of the corona-charged samples were measured along the direction of the applied electric field. After corona charging, multi-layer polar PET films were inspected and it was found that the characteristics of polarization and space charges of the individual layers were quite similar to each other. Almost identical surface potentials and TSDC responses were observed for single film and multi-layer sample. New porous ferroelectric P(VDF-TrFE) 56/44 and 70/30 films were fabricated successfully by electrospinning method. These porous and highly flexible polymer films consisted of nano-sized fibril connected with micro-sized spheres and the porosity was about 80 %. Dielectric analysis revealed that these porous copolymers possessed the relaxor-like ferroelectric properties with low effective dielectric constant (庰r ~ 1.6). The quasi-static piezoelectric coefficients (d33 ~ 300 pC/N) of the porous copolymers were comparable to solid and porous PTFE. Pseudo-piezoelectric effects of double and triple layers based on porous copolymer with bulk PET were significantly higher than obtained using porous PTFE. Multi-layer structures of porous copolymer and PET possessed good charge stability when compared with similar structures of porous PTFE and PET. After corona charging, both polar and non-polar solid polymers showed a shift of the strain-field parabola usually found from uncharged samples. A theoretical model is presented which indicates that the frozen-in dipolar polarization of charged samples is responsible for the shift as well as the asymmetric electrostrictive effect.

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