Article pubs.acs.org/Macromolecules
Controlling Crystal Microstructure To Minimize Loss in Polymer Dielectrics Daniel F. Miranda,† Shihai Zhang,‡ and James Runt*,† †
Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ PolyK Technologies, LLC, 2124 Old Gatesburg Road, State College, Pennsylvania 16803, United States S Supporting Information *
ABSTRACT: A model semicrystalline polymer, poly(ethylene naphthalate) (PEN), was used to examine how morphological factors inhibit chain segment relaxations that contribute to dielectric loss. This was achieved by manipulating the extent of crystallization and the crystalline microstructure through a combination of annealing and uniaxial drawing and investigating the effects on dielectric performance. Varying crystallization conditions influenced the dynamic Tg and extent of rigid amorphous fraction formation but had only a moderate effect on loss magnitude. Film orientation, however, greatly reduced loss through strain-induced crystallization and the development of oriented amorphous mesophasic regions. Postdrawing annealing conditions were capable of further refining the crystal microstructure and, in turn, the dielectric properties. These findings demonstrate that the semicrystalline polymer morphology can have a very significant influence on amorphous chain relaxations that contribute to dielectric loss, and understanding how processing conditions affect morphology is critical to the rational design of polymer dielectrics. seemingly low tan δ of 1.0% (0.01) may still be problematic. Tan δ contributes to the dielectric component of the equivalent series resistance,30,35 which has been identified as a critical parameter in capacitor design,30,35,36 particularly due to its effect on heat generation30 and efficiency.35,37 For large capacitors such as those required for pulsed power and power conditioning, the heat generated even for tan δ ∼ 1.0% is still quite substantial, and it may lead to capacitor thermal runawayas temperature increases, ionic and electrical conductivity also increase, leading to further power dissipation and temperature rise.30,4 Therefore, it is critical to identify additional approaches for reducing dielectric loss in semicrystalline polymer dielectric films. In polymer dielectric materials it is well-established that the predominant contribution to dielectric loss arises from amorphous chain relaxations and from the dynamic Tg.38,39 Therefore, inhibiting these relaxations should be an effective method to reduce loss. Polymer crystallization restrains these relaxations by restricting motion of amorphous chain segments adjacent to crystalline regions40 and also through reducing the fraction of amorphous material present to participate in these relaxations. In some cases, the presence of a rigid interphase has been inferred, consisting of amorphous chain segments constrained by the crystal surface. This is often referred to as
1. INTRODUCTION Polymer dielectric films are key components in many highperformance capacitor devices, particularly for power conditioning and pulsed power applications.1,2 The most compelling feature of polymer dielectrics is the unique ability to self-heal which many films attain after metallization, greatly increasing the lifetime of polymer capacitors compared to those constructed using ceramic dielectrics.3 Currently, the state-ofthe-art polymer dielectric is biaxially oriented polypropylene (BOPP) due to its extremely low dissipation factor (tan δ ∼ 0.01%) and high breakdown strength.4 However, the dielectric constant of BOPP is very low (2.2),5 limiting the maximum achievable energy density. Extensive efforts have been directed toward producing dielectric polymer films with high energy density through the use of multilayer films,6,7 composites,8−14 ferroelectric polymers,14−18 and functional polyolefins.19,20 These and other approaches have been reviewed extensively.21−27 Another important issue is the low operating temperature of BOPP (maximum of 105 °C).4,28 Despite the growing need for capacitors that perform at high temperatures (>125 °C),4,5,28,29 compared to the improvement in thermal resistance of other components in power electronics, film capacitors lag far behind.29 Recent publications have attempted to develop or identify polymer film materials having stable dielectric performance at such high temperatures.5,8,13,26,30−34 Many of these films have what is considered to be low dielectric loss (tan δ ∼ 1.0%). However, for many capacitor applications, even a © XXXX American Chemical Society
Received: July 7, 2017 Revised: September 28, 2017
A
DOI: 10.1021/acs.macromol.7b01450 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
of uniaxially drawn films was determined. These microstructural features are typical of all semicrystalline polymer films, and thus these results are expected to be fairly general. In addition, crystallinity is ubiquitous in polymer dielectric films, and these findings can most likely be applied in combination with other approaches to improve film capacitors.
a rigid amorphous fraction (RAF), while the more bulklike amorphous segments, largely unaffected by the interface, are referred to collectively as the mobile amorphous fraction (MAF). The effect of RAF is especially noticeable on the segmental relaxation process (α-process, or dynamic glass transition) and is typically characterized by a high-temperature skewing of the Tg,41−43 although the extent of intermolecular cooperativity and temperature dependence remains unaltered.44 Polymer chain orientation also has a strong effect on amorphous relaxation processes. The formation of an “oriented mesophase” consisting of noncrystalline chains having strong anisotropy has been inferred from X-ray scattering on uniaxially drawn polymer films and fibers.45−53 Increasing relative fractions of oriented mesophase was observed to cause an increase in the Tg of drawn fibers.49 Film or fiber drawing can also increase the crystalline fraction through strain-induced crystallization,52,54,55 and the corresponding formation of additional crystal/amorphous interface increases the RAF.55 Considering these factors, it should be possible to further reduce dielectric loss by optimizing film microstructure through orientation and thermal treatment. Orientation has long been understood empirically to improve the dielectric performance of polypropylene films (hence, biaxially oriented PP),3,56 and its effect has been studied for other dielectric films as well.6,7,15,16 Beyond these, there has been little effort to connect semicrystalline polymer morphology with dielectric properties and thereby capacitor performance. Herein, we use poly(ethylene naphthalate) (PEN) as a model crystallizable polymer dielectric to investigate this link. PEN is an aromatic polyester, very similar to poly(ethylene terephthalate) (PET), but with naphthalene functionality in the main chain repeating units rather than a phenyl ring. This alteration results in a stiffer polymer chain having improved mechanical,57 thermal,58 and barrier properties.59 PEN has also been investigated for use as a dielectric material in multiple applications60−62 and is another candidate for high-temperature polymer dielectrics.63 Also similarly to PET, PEN can be readily quenched into the glassy state, and cold crystallizes when heated above the Tg (for amorphous PEN: ∼120 °C).64 Crystalline PEN primarily adopts a triclinic unit cell with one chain per cell (α-form),64,65 although a second isomorph can be obtained under certain high temperature conditions (β-form), which is also triclinic but has a larger cell to accommodate four chains.64,65 Previous studies have explored the effect of thermal treatment66−76 and orientation45,46,55,66,71,72,75,77−80 on the semicrystalline morphology of PEN films. The presence of RAF49,62−64,67,72 or development of an oriented mesophase on drawing45,46,81 has been inferred in a few cases for PEN as well. In the present study, PEN films were uniaxially drawn and/or thermally treated in order to manipulate the crystalline microstructure. Although there has been some prior dielectric characterization of oriented and annealed PEN films, accompanying morphological analyses have been limited.55,68,70 A deeper investigation of the changes in crystalline microstructure will enable a stronger connection to be made between morphology and dielectric loss. Using X-ray scattering techniques and differential scanning calorimetry, a morphological model of the drawn and annealed films was inferred. Broadband dielectric spectroscopy at low fields was used to carry out a basic study of the relaxation processes of the treated films. Dielectric tan δ in particular was used to evaluate the films due to its relation to ESR, a critical figure in capacitor design.35 From these characterizations, the optimal morphology
2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. PEN resin (Teonex TN8065S) was obtained from Teijin. Films were extruded from the resin using a 1 in. single screw extruder with a 6 in. flex lip film die, at a die temperature of 290 °C. The extruded films are essentially amorphous (
