Please consider attending the following: MATERIALS ENGINEERING SEMINAR "Orientation of Polymer Films for Improvement of Dielectric Properties in High-Energy Density Capacitor Applications" By Megan Forshey Purdue MSE Ph.D. Final Exam Advisor: Professor Mukerrem Cakmak ABSTRACT For over 20 years, biaxially oriented polypropylene (BOPP) has been used in capacitors as the dielectric material. BOPP has very high breakdown strength, low electric loss, and is relatively inexpensive however, it suffers from low dielectric constant and low usage temperature. The ever growing technology market requires more robust capacitors which can be used in high temperature and pulsed power applications, and the aim of this research is to meet or exceed dielectric properties of BOPP by combining specific polymer materials in layered structures, biaxially orienting the films, and heat setting the films to further improve thermal stability. Post-processing is done on custom built machines which track real-time true stress, true strain and birefringence values, allowing for a more complete picture of mechano-optical properties generated during the stretching process. This data, along with offline characterization techniques such as X-ray scattering and DSC was coupled with dielectric property testing to help form relationships between polymer processing, morphology, and dielectric properties. A 32 Layer film consisting of 50:50 vol ratio of poly(ethylene terephthalate) (PET) and poly(vinylidene fluoride) (PVDF) was provided by PolymerPlus, Inc. Since ideal processing temperatures for PET and PVDF do not match, an initial uniaxial stretching study was conducted over a wide range of stretching temperatures to determine effect of final morphology on starting state of each polymer. For this film, we have established ideal processing temperature to be a very narrow range between 145-155o C. Uniaxial stretching at several temperatures was conducted for further morphological time slice study. Characterization confirmed PVDF crystal form transformation from α to β when films were stretched at 95o C, and presence of γ PVDF due to annealing effect when stretched at 185o C (near equilibrium melting point of PVDF). Dielectric testing experiments at room temperature show that films stretched uniaxially at 95o C have higher dielectric constant but also significantly higher dielectric loss. Films stretched at 150o C have lower dielectric constant but also much lower loss, making them a better candidate for improved dielectric film in capacitor applications. Thermal annealing is used in industry to heat set oriented polymer films for further use at high temperature, such as BOPP in capacitors. 32L PET/PVDF microlayer films were oriented and further annealed to study effect on morphology. Only uniaxial mode and very small simultaneous stretch ratios were feasible. Annealing over a range of temperatures around PVDF melting peak caused transformation of PVDF polymorphs which were studied via DSC, WAXS and FTIR. Finally, morphology was correlated with dielectric constant and loss values in both constant (room) temperature and temperature ramping experiments. For the most part, samples containing γ and/or γ'-PVDF showed improved dielectric constant but also much higher loss values in temperature ramp dielectric testing. Date: Thursday, September 12, 2019 Time: 3:00 P.M. Place: HAMP 1113