Investigations of Structure-Property Relationships to Enhance the Multifunctional Properties of PVDF-based Polymers

Open Access
Author:
Neese, Bret P.
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 09, 2009
Committee Members:
  • Dr Qiming Zhang, Dissertation Advisor
  • Qiming Zhang, Committee Chair
  • Qing Wang, Committee Chair
  • Michael T Lanagan, Committee Member
  • James Patrick Runt, Committee Member
Keywords:
  • polymer
  • ferroelectric
  • piezoelectric
  • electrocaloric effect
  • PVDF
  • capacitor
Abstract:
Poly(vinylidene fluoride) (PVDF)-based polymers have been some of the most widely researched semicrystalline polymers over the past several decades, due mostly to their ability to exhibit ferroelectric properties not capable in many soft materials. While much has been learned about these properties and much advancement has been made in utilizing them for many applications, we are still only beginning to understand their origins and how they can be enhanced by altering the polymer structure. In this thesis, structure-property relationships are analyzed in a variety of PVDF-based polymers with varying chemical and crystalline structures. The work consists of three parts as distinguished by the property under investigation: electromechanical effects, electrical energy storage, and the electrocaloric effect (ECE). First is the electromechanical effects, where a large converse piezoelectric effect is discovered in P(VDF-HFP) (HFP: hexafluoropropylene) copolymer. The nature of the piezoelectric property is linked to the structure change through a detailed structural analysis to provide explanation to the large and unusual electromechanical strain response. P(VDF-HFP) is further investigated for its use as an energy storage capacitor and various processing methods are utilized to alter the crystalline structure and study the effects on the energy storage characteristics. The results indicate that uniaxial stretching is beneficial in improving energy storage efficiency up to a certain draw ratio (4-5¡Ñ the original length), but as the draw ratio is increased and the polar £] crystalline phase becomes more prominent, ferroelectric losses become detrimental to the energy storage efficiency. Furthermore, the effects of biaxial stretching on the crystalline structure are examined. The data suggests that biaxial stretching of extruded films to a similar draw ratio as the uniaxially stretched blown films produces a similar composition of crystalline structure. In view of the fact that the structure-property relationships show that the polar crystalline phase contributes to energy losses in the material, electron irradiation is examined as a method of destabilizing the £]-phase in P(VDF-HFP) and P(VDF-CTFE) and reducing ferroelectric energy loss. A structural analysis confirms that the defects introduced into the crystalline structure of both polymers by the irradiation does indeed increase the composition of the nonpolar £-phase. Furthermore, analysis of electric displacement-electric field (D-E) hysteresis loops indicates improvements in energy storage efficiency as a result of irradiation. The final portion of this dissertation probes a property previously unstudied in polymeric materials, the ECE. The results show that applying an electrical field to a polar polymer may induce a large change in the dipolar ordering, and if the associated entropy changes are large, they can be explored in cooling applications. With the use of the Maxwell relation between the pyroelectric coefficient and the ECE, it was determined that a large ECE can be realized in the ferroelectric P(VDF-TrFE) (TrFE: trifluoroethylene) copolymer at temperatures above the ferroelectric-paraelectric transition (above 70¢XC), where an isothermal entropy change of more than 55 J/(kgK) and adiabatic temperature change of more than 12¢XC were observed. We further show that a similar level of ECE near room temperature can be achieved by working with the relaxor ferroelectric polymer of P(VDF-TrFE-chlorofluoroethylene). Furthermore, the difference in temperature dependence of the electrocaloric effect in P(VDF-TrFE) and P(VDF-TrFE-CFE) suggests different entropy change mechanisms. The contribution to polarization from nanopolar domains in the terpolymer at low temperature is not as effective at generating large entropy change as the paraelectric to ferroelectric transition. Moreover, the adiabatic entropy change ƒ´S is proportional to the square of the electric displacement D (ƒ´S = -1/2 ƒÒ£GD2) in both systems and the coefficient ƒÒ increases with temperature in the terpolymer as opposed to the copolymer where £] is temperature independent. This temperature dependent behavior of ƒÒ is caused by the ferroelectric relaxor nature of the polymer in which the polarization response from the nano-polar regions does not generate much entropy change. Consequently, the ECE effect in the relaxor ferroelectric terpolymer is smaller than that in the normal ferroelectric copolymer. Moreover, a study of quenched terpolymer samples as compared to annealed samples with higher crystallinity shows strong correlation between the ECE and crystallinity, suggesting that the ECE occurs predominantly in the crystalline regions of the semicrystalline polymers.