THE OPTICAL AND DIELECTRIC APPLICATIONS OF POLY(VINYLIDENE FLUORIDE) BASED ELECTRO-ACTIVE POLYMERS

Open Access
Author:
Chen, Qin
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 28, 2008
Committee Members:
  • Qiming Zhang, Committee Chair
  • Shizhuo Yin, Committee Chair
  • Zhiwen Liu, Committee Member
  • Christopher Rahn, Committee Member
Keywords:
  • electro-optic effect
  • long-period grating
  • capacitors
  • poly(vinylidene fluoride)
Abstract:
A family of polymers modified from poly(vinylidene fluoride) exhibit very large electro-optic and dielectric responses. For instance, dielectric constant of higher than 50 and electro-optic (E-O) effect with more than 2% of refractive index change are exhibited by poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer, P(VDF-TrFE-CFE). Another example is poly( vinylidene fluoridechlorotrifluoroethylene) copolymer, P(VDF-CTFE), which can store and release electrical energy density of higher than 25J/cm2. This thesis presents a study on the application of these polymers in tunable optical devices and high energy density capacitors. As an example of tunable optical devices, an electrically tunable long-period fiber grating (LPG) with P(VDF-TrFE-CFE) terpolymer as a second cladding has been demonstrated. A theoretical model has been developed to study the device performance. Computer simulation results indicated that the indium tin oxide (ITO) layer which acts the inner electrode for the terpolymer significantly increases the tuning range of the LPG. Furthermore, the tuning range of the LPG depends on the native refractive index of the terpolymer and maximum tuning iii range can be achieved if the native index is increased by about 0.02. A fabrication process for the tunable LPG has been developed which included dip coating of polymer on fiber. In order to increase the native refractive index of the terpolymer to achieve maximum tuning range, a nanocomposite approach has been employed where small amount of high refractive index zinc sulfide nanoparticles were added into terpolymer matrix. With a properly designed chemical synthesis route the nanocomposite exhibited increased refractive index while maintaining high transparency and relatively large E-O effect. Based on the nanocomposite a tunable LPG with tuning range of 50nm has been demonstrated, which is sufficient to cover an entire optical communication band. In energy storage capacitors, not only high energy density but also high efficiency and reliability of the polymer films are required. For P(VDF-TrFE-CFE) terpolymers, electrical breakdown is an important concern that affects both the energy density and reliability of dielectric capacitors. It has been observed that both the breakdown electric field and conduction current of the terpolymer depends on the type of metal electrode, which indicates that both processes are controlled by metal-polymer interface. Aluminum and chromium showed considerably higher breakdown field and lower current density compared to gold and silver. In addition to the effect of metal work functions, another factor that contributes to the interface effect is the formation of interfacial layer. Such layer was observed for aluminum which is relatively reactive but not for gold which is inert. In P(VDF-CTFE) copolymer, it has been found that electrical conduction contribute to a significant portion of the energy loss and hence lowers the efficiency. Detailed studies revealed that the conduction current is controlled by charge iniv jection from electrodes modified by bulk mobility. An effective method to reduce the conduction current is to block the charge injection from electrodes using more insulating materials. Poly(2,6-dimethyl-1,4-phenylene oxide) and silicon nitride have been selected as examples of polymeric and inorganic blocking layers and multilayered films containing both P(VDF-CTFE) and blocking layers have been fabricated. Significant reduction of conduction current has been observed while relatively high energy densities were maintained. It was also observed that the upper limit of energy density in such multilayered films is controlled by high field tunneling of charge carriers in the blocking layer.