Compared with chemical modification of ferroelectric poly (vinylidene fluoride) (PVDF) for electric energy storage, polymer blends, whether miscible or immiscible, represent a much easier approach to suppress the ferroelectricity of PVDF. In this study, we explored both miscible [i.e., poly (methyl methacrylate) or PMMA] and immiscible (i.e., polycarbonate or PC) blends with poly (VDF-co-hexafluoroethylene) [P(VDF-HFP)], as well as the PC/P(VDF-HFP) multilayer films. For miscible PMMA/P(VDF-HFP) blend films, the addition of PMMA significantly decreased the crystallinity of P(VDF-HFP). At a high PMMA content of ca. 40 wt%, the stretched PMMA/P(VDF-HFP) blend films started to exhibit the linear dielectric behavior with suppressed ferroelectricity. For the immiscible PC/P(VDF-HFP) blend films, a high PC content of 50 vol% was required to suppress the ferroelectricity in P(VDF-HFP). Instead, the PC/P(VDF-HFP) multilayer films started to show linear hysteresis loops when the content of P(VDF-HFP) was only 30 vol%. More importantly, the PC/P(VDF-HFP) multilayer films exhibited significantly higher breakdown strength than the blend films. This could be attributed to the perpendicular interfaces (with respect to the applied electric field), which serve as effective blocks for hot electrons injected from the metal electrodes to pass through the film. From this study, compared to conventional miscible and immiscible blends, multilayer films are promising for next generation film capacitors, aiming to achieve high temperature tolerance, high energy density, and low loss simultaneously.

Interfacial polarization due to space charges enhances electrical insulation and thus breakdown strength for multilayer polymer films.

Wide-angle X-ray scattering in conjunction with pole figure technique was used to study the texture of poly(vinylidene fluoride) (PVDF) α and γ phase crystals in nanolayered polysulfone/poly(vinylidene fluoride) films (PSF/PVDF) produced by layer-multiplying coextrusion. In all as-extruded PSF/PVDF films, the PVDF nanolayers crystallized into the α phase crystals. A large fraction of those crystals was oriented with macromolecular chains perpendicular to the PSF/PVDF interface as evidenced from the (021) pole figures. Further refinement of the texture occurs during isothermal recrystallization at 170 °C in conjunction with transformation of α to γ crystals. The γ crystals orientation was probed with the (004) pole figures showing the c-axis of PVDF γ crystals perpendicular to the PSF/PVDF interface. The thinner the PVDF layers the stronger the orientation of γ crystals. It was proven that the X-ray reflections from the (021) planes of α crystals and from the (004) planes of γ crystals are not overlapped with other reflections and can be effectively used for the texture determination of PVDF nanolayers in multilayered PSF/PVDF films.

Multilayer polymer films comprising alternating layers of polycarbonate (PC) and polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) show enhanced dielectric strength relative to single component films of either source polymer. Previous failure analysis on films subjected to breakdown under divergent field conditions revealed that multilayer films produced distinct surface treeing patterns whereas monolithic films did not. The choice of surface layer (PC or PVDF-HFP) contacted by a needle electrode influenced the nature of these treeing patterns. Additionally, damage within the film was largely localized to the interfaces between layers. To help explain these empirical results, we model the divergent field based on the geometry of our experimental setup and calculate the internal electric field distribution using the boundary integral equation method (BIEM). All fundamental charges, including: free, bound, trapped, and space charges are accounted for in the calculations, based on current and voltage data recorded during prior breakdown measurements. The calculations show that when PC is used as the surface layer in contact with the needle anode, there is significant field intensification in the top PC layer, in excess of 2000 V/μm. This is many times higher than the measured dielectric strength of monolithic PC and is at least partially due to charge injection from the needle anode. In contrast, the PVDF-HFP sub-layer in this configuration has very low field. These observations are consistent with breakdown occurring near the surface of the film, resulting in large-range surface treeing. When PVDF-HFP is the top layer, field intensification occurs deeper in the film, which is again consistent with the observed optical and FIB/SEM imaging results where less surface treeing and more internal damage is observed. The calculations suggest that the large contrast in field between adjacent layers generates a nexus for localized breakdown at the layer interfaces, again consistent with large internal voids formed by layer delamination in films subjected to divergent field breakdown.

In this study, we report layer thickness effect on the electrical insulation property of polysulfone (PSF)/poly(vinylidene fluoride) (PVDF) multilayer films having a fixed composition of PSF/PVDF = 30/70 (vol./vol.). Breakdown strength, dielectric lifetime, and electrical conductivity were studied for 32- and 256-layer films having various total film thicknesses. Among these films, those having thinner PVDF and PSF layers exhibited lower breakdown strength, shorter lifetime, and higher electrical conductivity than those having thicker layers. These experimental results were explained by Maxwell-Wagner-Sillars interfacial polarization due to contrasts in dielectric constant and electronic conductivity for PVDF and PSF, respectively. When both PVDF and PSF layers were thick (ca. > 100-200 nm), more space charges were available in PVDF and no electronic conduction was allowed for PSF. These accumulated interfacial charges could serve as effective traps for injected electrons from metal electrodes under high electric fields. As a result, reduced electrical conductivity and enhanced breakdown strength/dielectric lifetime properties were obtained. When both layers were thin (ca. < 100 nm), fewer space charges were available in PVDF and significant electronic conduction through PSF resulted in low interfacial polarization. Consequently, higher electrical conductivity, lower breakdown strength, and shorter lifetime were observed. These results provide us insights into potential physics to enhance electrical insulation property of polymer films using a multilayered structure having large dielectric constant contrast. (C) 2013 Elsevier Ltd. All rights reserved.

Polymer films with enhanced dielectric and breakdown properties are essential for the production of high energy density polymer film capacitors. By capitalizing on the synergistic effects of forced assembly nanolayer coextrusion and biaxial orientation, polymer multilayer films using poly(ethylene terephthalate) (PET) and a poly(vinylidene fluoride‐co‐tetrafluoroethylene) [P(VDF‐TFE)] copolymer were produced. These films exhibited breakdown fields, under a divergent field using needle/plane electrodes, as high as 1000 kV mm−1. The energy densities of these same materials, under a uniform electric field measured using plane/plane electrodes, were as high as 16 J cm−3. The confined morphologies of both PET and P(VDF‐TFE) were correlated to the observed breakdown properties and damage zones. On‐edge P(VDF‐TFE) crystals induced from solid‐state biaxial stretching enhanced the effective P(VDF‐TFE) layer dielectric constant and therefore increased the dielectric contrast between the PET and P(VDF‐TFE) layers. This resulted in additional charge buildup at the layer interface producing larger tree diameters and branches and ultimately increasing the breakdown and energy storage properties. In addition to energy storage and breakdown properties, the hysteresis behavior of these materials was also evaluated. By varying the morphology of the P(VDF‐TFE) layer, the low‐field dielectric loss (or ion migration behavior) could be manipulated, which in turn also changed the observed hysteresis behavior. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 882–896

ABSTRACTUnique three‐component multilayer films with ATBTATBTA configuration were fabricated using forced assembly multilayer coextrusion for novel dielectric systems. The dielectric breakdown strength, displacement–electric field hysteresis, and dielectric spectroscopy of 65‐layer polycarbonate (PC)/tie/poly(vinylidene fluoride‐co‐hexafluoropropylene) (P(VDF‐HFP)) were investigated with various tie materials. Three different tie materials, poly(methyl methacrylate) (PMMA), styrene‐co‐acrylonitrile copolymer with 30% acrylonitrile content (SAN30), and poly(ethylene terephthalate‐co‐1,4‐cycohexanedimethylene terephthalate) (PETG) were chosen owing to their various degrees of interaction with either P(VDF‐HFP) or PC. The 65‐layer PC/PMMA/P(VDF‐HFP) films exhibited a 25% enhancement in breakdown properties, 50% higher energy density, 40% smaller hysteresis loop areas, and orders of magnitude slower ion migration relative to the 33‐layer PC/P(VDF‐HFP) control. These property improvements are mainly attributed to the localized interactions at PMMA/P(VDF‐HFP) and PMMA/PC interfaces, forming interphase regions. The modified PMMA/P(VDF‐HFP) interphase region can effectively hinder the migration of impurity ions in P(VDF‐HFP), reducing their mobility within the layer. Additionally, a small fraction of PMMA can lead to slightly increased dielectric constant of the composite films owing to strong interaction between PMMA and P(VDF‐HFP). The other two systems with PETG and SAN30 as tie layers exhibited marginal improvements in dielectric properties owing to their weaker interactions with the P(VDF‐HFP) layers. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 978–991

A nanolayered polymer films approach to designing and fabricating gradient refractive index (GRIN) lenses with designer refractive index distribution profiles and an independently prescribed lens surface geometry have been demonstrated to produce a new class of optics. This approach utilized nanolayered polymer materials, constructed with polymethylmethacrylate and a styrene-co-acrylonitrile copolymer with a tailorable refractive index intermediate to bulk materials, to fabricate discrete GRIN profile materials. A process to fabricate nanolayered polymer GRIN optics from these materials through thermoforming and finishing steps is reviewed. A collection of technology-demonstrating previously reported nanolayered GRIN case studies is presented that include: (1) the optical performance of a f/# 2.25 spherical GRIN plano-convex singlet with one quarter (2) the weight of a similar BK7 lens and a bio-inspired aspheric human eye GRIN lens. Original research on the fabrication and characterization of a Luneburg inspired GRIN ball lens is presented as a developing application of the nanolayered polymer technology. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

The morphologies of two crystalline polymers, polyethylene terephthalate (PET) and poly(vinylidene fluoride-co-tetrafluoroethylene) [P(VDF-TFE)], were probed under nanolayer confinerhent using forced assembly multilayer film coextrusion. This multilayer system was used as a platform to investigate the effect of nanolayer coextrusion, biaxial stretching, and isothermal melt recrystallization on the confined morphologies of both of these polymers. To determine the effect of each of these variables independently, three sets of PET/P(VDF-TFE) multilayer films were produced, each with comparable film thickness and layer thickness. The morphology and X-ray data of the extruded PET/P(VDF-TFE) multilayer films, which were taken directly from the coextrusion process, indicate that the morphologies of both PET and P(VDF-TFE) were relatively unaffected by nanolayer confinement, even in very thin 40 nm layers. Biaxial stretching of multilayer films, produced from stretching micron thick layers down to nanolayers, facilitated the development of an on-edge P(VDF-TFE) crystal orientation in addition to an oriented PET fibrillar crystal structure. Finally, an approach of isothermal melt recrystallization was conducted on the biaxially stretched samples which revealed the formation of high aspect ratio in-plane P(VDF-TFE) crystals under nanolayer confinement while also further crystallizing the PET fibril crystals. Therefore, in the same multilayer system, three P(VDF-TFE) crystal orientations were achieved by utilizing nanolayer confinement, biaxial stretching, and isothermal melt recrystallization. Oxygen permeability was used as an additional structural probe for these confined PET and P(VDF-TFE) layer morphologies. From the transport data, it was determined that the PET layers possessed similar oxygen transport characteristics to the bulk materials, which was in good agreement with the morphology data of the PET layers in the various PET/P(VDF-TFE) multilayer films. In contrast, the on-edge P(VDF-TFE) orientation induced from biaxially stretching and in-plane P(VDF-TFE) crystal orientation induced from isothermal melt recrystallization of confined P(VDF-TFE) nanolayers yielded substantial reductions in the effective oxygen permeability of the P(VDF-TFE) layers in comparison to the bulk P(VDF-TFE) control. The various confined P(VDF-TFE) crystal orientations and subsequent enhanced barrier properties are enabled by the hard confinement of the PET nanolayers during biaxial stretching at high draw ratios and isothermal melt recrystallization at high temperatures. Finally, the water vapor transport rate (WVTR) was evaluated for these confined systems which mimicked the trends observed for oxygen permeability. The confined nanolayer morphologies, specifically the in-plane P(VDF-TFE) crystals, substantially reduced the WVTR in multilayer films opening new applications for this technology. (C) 2013 Elsevier Ltd. All rights reserved.

Forced assembly microlayer coextrusion was used to produce polycarbonate/poly(vinylidene fluoride-co-hexafluoropropylene) [PC/P(VDF-HFP)] layered films for dielectric capacitor applications. Low field dielectric spectroscopy was systematically carried out on the layered films and controls. A low frequency relaxation was found that shifted to higher frequency and decreased in intensity as the P(VDF-HFP) layer thickness was reduced. The interfacial Maxwell-Wagner polarization, being layer thickness independent, could not account for this reduced low frequency relaxation behavior as the layer thickness decreased. Charge diffusion models by Sawada and Coelho, however, satisfactorily predicted the observed layer thickness effect, indicating that the migration of impurity ions in the P(VDF-HFP) layer caused the low frequency relaxation. A new, convenient fitting procedure was developed for the Sawada model yielding an ion concentration and diffusion coefficient of 2 × 1021 ions/m3 and 2 × 10−13 m2/s, respectively, for films with layer thicknesses of 430 to 50 nm. Thicker layers of 7000 nm had significantly different diffusion parameters, which were attributed to differing crystal orientations in the P(VDF-HFP) layers. These findings show that low ion concentrations, whether from catalyst residue and processing or intentionally added, significantly affect the dielectric properties and can play a vital role in many applications (i.e., LCD displays, solar cells, light-emitting electrochemical cells, capacitors).

Micro/nanolayer coextrusion was used to fabricate polycarbonate (PC)/poly(vinylidene fluoride) (PVDF) layered films with significantly reduced dielectric losses while maintaining high energy density. The high-field polarization hysteresis was characterized for layered films as a function of PVDF layer thickness (6000 to 10 nm) and composition (10 to 70 vol % PVDF), and was found to decrease with decreasing layer thickness and PVDF content. To gain a mechanistic understanding of the layer thickness (or nanoconfinement) effect, wide-angle X-ray diffraction, polarized Fourier transform infrared spectroscopy, and broadband dielectric spectroscopy were employed. The results revealed that charge migration, instead of dipole flipping, was responsible for the hysteresis in multilayered films. The absence of PVDF dipole-flipping was attributed to the nonuniform electric field distribution in the layered structure, where the field in PVDF layers were calculated to be significantly lower than that in PC layers due to large contrast in dielectric constant (similar to 3 for PC versus similar to 12 for PVDF). The charges were likely to be impurity ions in the form of catalyst residue or surfactants from suspension polymerization. The characteristics of the dielectric spectroscopy relaxation indicated that ions mostly existed in the PVDF layers, and PC/PVDF interfaces prevented them from entering adjacent layers. Therefore, as the layer thickness decreases to nanometer scales, the amount of ion movement, dielectric loss, and hysteresis were decreased. This study provides clear evidence of the nanoconfinement effect in multilayered films, which advantageously decreases the hysteresis loss.