PVDF-based copolymers, terpolymers and their multi-component material systems for capacitor applications

Open Access
Author:
Chu, Baojin
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 13, 2008
Committee Members:
  • Qiming Zhang, Committee Chair
  • Leslie Eric Cross, Committee Member
  • Michael T Lanagan, Committee Member
  • Qing Wang, Committee Member
Keywords:
  • nanocomposite
  • blend
  • PVDF-based polymers
  • dielectric
  • energy density
Abstract:
Polyvinylidene fluoride (PVDF)-based copolymers, terpolymers, and their multi-component material systems were investigated for capacitor applications. Defect-modified poly(vinylidene fluoride-trifluoroethylene) polymers, P(VDF-TrFE-CFE) terpolymers (CFE: chlorofluoroethylene), exhibit high dielectric constant (about 55 at 1 kHz) and high breakdown electric field at room temperature. The measured energy density of the terpolymers is higher than 9 J/cm3. Due to their high dielectric constant, the polarization response of the terpolymers saturates at an electric field much lower than their breakdown field, which results in highly nonlinear dielectric response with electric field and lower energy density than expected. This result suggests that high dielectric constant may not be desirable to obtain high energy density and materials with lower dielectric constant than the terpolymers, in which the early polarization saturation can be avoided, can have higher energy density. Defect-modified PVDF polymers, P(VDF-CTFE) (CTFE: chlorotrifluoroethylene) and P(VDF-HFP) (HFP: hexafluoropropylene) copolymers, which have a lower dielectric constant of about 12 at room temperature, were further studied. Energy density higher than 24 J/cm3 was found in these copolymers. Blending with another polymer or addition of nanoparticles in the terpolymers was found to be an effective strategy to improve the energy density of the base polymer. The breakdown field of the terpolymer-based blends with a small amount of P(VDF-CTFE) copolymer (5 and 10 wt%) and poly(methyl methacrylate) (PMMA, less than 2.5 wt%) was improved compared with pure terpolymer without reducing the polarization response. The large amount of polymer/particle interfaces in the terpolymer/ZrO2 and terpolymer/TiO2 nanocomposites have a great effect on the dielectric response of these materials. Addition of nanoparticles in the terpolymer can reduce the energy barrier of polarization switching which raises the polarization response of the nanocomposites. The energy density of the blends and nanocomposites can be improved by increasing the breakdown field or improving the polarization response.