Structure and properties of ferroelectric copolymers of poly(vinylidene fluoride)

High-Temperature Melt Stamping of Polymers Using Polymer/Nanoporous Gold Composite Stamps

Parallel lithographic deposition of polymers onto counterpart substrates is a widely applied surface manufacturing operation. However, polymers may only be soluble in organic solvents or are insoluble at all. Solvent evaporation during stamping may trigger hardly controllable capillarity-driven flow processes or phase separation, and polymer solutions may spread on the counterpart substrates. Solvent-free stamping of melts prevents these drawbacks. Here, a stamp design for the deposition of melts is devised, which intrinsically circumvents ink depletion. The stamps' topographically patterned contact surfaces with protruding contact elements contacting the counterpart substrates consist of a nanoporous gold layer with a thickness of a few micrometers. The nanoporous gold layer is attached to a molten polymer layer, which is support for the nanoporous gold layer and ink reservoir at the same time. The nanoporous gold layer in turn stabilizes the topography of the stamps' contact surfaces. As examples, arrays of submicron microdots of polystyrene and poly(vinylidenefluoride-trifluoroethylene) (PVDF-TrFE) are manufactured. The P(VDF-TrFE) microdots are partially crystalline, ferroelectric, and can be locally poled. It is envisioned that the methodology reported here can be automatized and may be extended to functional low-molecular-mass compounds, such as active pharmaceutical ingredients.

Composition-Driven Polarization Distribution in Poly(vinylidene fluoride)-Based Copolymer Blends for High Power Density Capacitors

High power density capacitors have been highly demanded in modern electronics and pulsed power systems. Yet the long-standing challenge that restricts achieving high power in capacitors lies in the inverse relationship between the breakdown strength and permittivity of dielectric materials. Here, we introduce poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) into the host poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) to produce PVDF-based copolymer blends, resulting in composition-driven 0-3 type microstructures, featuring nanospheres of P(VDF-TrFE) lamellar crystals dispersed homogeneously in a P(VDF-HFP) matrix together with crystalline phase evolution from the γ-phase to the α-phase. At the critical composition, the TrFE/HFP mole ratio is equal to 1, and the blend film achieves maximum energy storage performance with discharged energy density (U_dis) ∼ 24.3 J/cm³ at 607 MV/m. Finite element analyses reveal the relationship between microstructures, compositions, and the distribution of local electric field and polarization, which provide an in-depth understanding of the microscopic mechanism of the enhancement in energy storage capability of the blend films. More importantly, in a practical charge/discharge circuit, the blend film could deliver an ultrahigh energy density of 20.4 J/cm³, i.e., 88.3% of the total stored energy to 20 kΩ load in 2.8 μs (τ_0.9), resulting a high power density of 7.29 MW/cm³, outperforming the reported dielectric polymer-based composites and copolymer films in both energy and power densities. The study thus demonstrates a promising strategy to develop high-performance dielectrics for high power capacitors.

Ordered nanostructures arrays fabricated by anodic aluminum oxide (AAO) template-directed methods for energy conversion

Clean and efficient energy conversion systems can overcome the depletion of the fossil fuel and meet the increasing demand of the energy. Ordered nanostructures arrays convert energy more efficiently than their disordered counterparts, by virtue of their structural merits. Among various fabrication methods of these ordered nanostructures arrays, anodic aluminum oxide (AAO) template-directed fabrication have drawn increasing attention due to its low cost, high throughput, flexibility and high structural controllability. This article reviews the application of ordered nanostructures arrays fabricated by AAO template-directed methods in mechanical energy, solar energy, electrical energy and chemical energy conversions in four sections. In each section, the corresponding advantages of these ordered nanostructures arrays in the energy conversion system are analysed, and the limitation of the to-date research is evaluated. Finally, the future directions of the ordered nanostructures arrays fabricated by AAO template-directed methods (the promising method to explore new growth mechanisms of AAO, green fabrication based on reusable AAO templates, new potential energy conversion application) are discussed.

Ferroelectric Polymers Exhibiting Negative Longitudinal Piezoelectric Coefficient: Progress and Prospects

Piezoelectric polymers are well-recognized to hold great promise for a wide range of flexible, wearable, and biocompatible applications. Among the known piezoelectric polymers, ferroelectric polymers represented by poly(vinylidene fluoride) and its copolymer poly(vinylidene fluoride-co-trifluoroethylene) possess the best piezoelectric coefficients. However, the physical origin of negative longitudinal piezoelectric coefficients occurring in the polymers remains elusive. To address this long-standing challenge, several theoretical models proposed over the past decades, which are controversial in nature, have been revisited and reviewed. It is concluded that negative longitudinal piezoelectric coefficients arise from the negative longitudinal electrostriction in the crystalline domain of the polymers, independent of amorphous and crystalline-amorphous interfacial regions. The crystalline origin of piezoelectricity offers unprecedented opportunities to improve electromechanical properties of polymers via structural engineering, i.e., design of morphotropic phase boundaries in ferroelectric polymers.

A light-mediated metal-free atom transfer radical chain transfer reaction for the controlled hydrogenation of poly(vinylidene fluoride-chlorotrifluoroethylene)

A light-mediated atom transfer radical chain transfer process is proposed for the controlled hydrogenation of P(VDF-CTFE) using metal-free photocatalyst.

Nanoscale design of multifunctional organic layers for low-power high-density memory devices

We demonstrate the design of a multifunctional organic layer by the rational combination of nanosized regions of two functional polymers. Instead of relying on a spontaneous and random phase separation process or on the tedious synthesis of block copolymers, the method involves the nanomolding of a first component, followed by the filling of the resulting open spaces by a second component. We apply this methodology to fabricate organic nonvolatile memory diodes of high density. These are built by first creating a regular array of ferroelectric nanodots by nanoimprint lithography, followed by the filling of the trenches separating the ferroelectric nanodots with a semiconducting polymer. The modulation of the current in the semiconductor by the polarization state of the ferroelectric material is demonstrated both at the scale of a single semiconductor channel and in a microscopic device measuring about 80,000 channels in parallel, for voltages below ca. 2 V. The fabrication process, which combines synergetically orthogonal functional properties with a fine control over their spatial distribution, is thus demonstrated to be efficient over large areas.

Understanding crystallization features of P(VDF-TrFE) copolymers under confinement to optimize ferroelectricity in nanostructures

The successful development of ferroelectric polymer devices depends on the effective fabrication of polar ferroelectric crystalline nanostructures. We demonstrate, by scanning X-ray microdiffraction using synchrotron light, the heterogeneous character of high aspect ratio one-dimensional nanoarrays of poly(vinylidene fluoride-co-trifluoroethylene) copolymers supported by a residual polymer film. They were prepared by melt and solution template wetting, using porous anodic aluminum oxide as a template. The spatial evolution of different polymorphs from the mixture of paraelectric and ferroelectric crystal forms (residual film) to the pure ferroelectric form (nanoarray) is evidenced for the samples prepared by solution wetting. However, for samples prepared by melt wetting the ferroelectric phase is exclusively obtained in both the residual film and nanoarray. The crystal nuclei formed in the polymer film connected to the nanoarray play a key role in determining the formation of a crystallinity distribution gradient, where the crystallinity decreases along the first 5-10 microns in the nanorods reaching a steady value afterwards. The minimum decrease in crystallinity is revealed for samples prepared by melt wetting. The results reported in this work endeavour to enhance the understanding of crystallization under confinement for ferroelectric copolymers and reveal the parameters for improving the ferroelectric character of polymer nanostructures.

Curie transitions for attograms of ferroelectric polymers

Polymer systems having one, two, or three dimensions on the nanometer length scale can exhibit physical properties different from the bulk. The degree of disorder characteristic for large amounts of matter is strongly reduced and changes in symmetry are imposed by means of geometrical confinement. This could be used to induce-through orientation and order-enhancement in the material properties. Experiments on extremely small amounts of matter, however, are naturally characterized by large fluctuations in the measured signals, especially in the case of polymer objects having three dimensions on the nanometer length scale. This imposes the necessity of repeating the measurements until a statistical distribution is obtained. Here we show that investigations on statistical ensembles of attograms of material (1 ag = 10(-18) g) are possible in a single experiment by employing highly ordered arrays of identical, independent, additive nanocontainers. Phase transitions corresponding to attograms of a ferroelectric polymer are measured by this approach. As compared to one- or two-dimensional confinement, significant changes in the Curie transitions are found.

Density fluctuations and phase transitions of ferroelectric polymer nanowires

Phase transitions of polymeric materials are accompanied by changes in density as a function of temperature. Being able to measure these changes in polymeric systems in one, two or three dimensions on the nanoscopic length-scale is a challenge, but it would provide a simple route to assess phase transitions in nanoscopically confined systems. It is shown that the measurement of the dielectric permittivity in the high frequency limit (in spectral regions not affected by dielectric dispersions) offers an effective and very sensitive means to assess density fluctuations, and hence phase transitions, in nanoscopic systems. The sensitivity of this approach is demonstrated by assessing the phase transition behavior of ferroelectric polymer nanowires confined within alumina membranes. No significant shifts in the Curie transition are observed down to pore diameters as small as 15 nm.

Regular arrays of highly ordered ferroelectric polymer nanostructures for non-volatile low-voltage memories

Ferroelectric nanostructures are attracting tremendous interest because they offer a promising route to novel integrated electronic devices such as non-volatile memories and probe-based mass data storage. Here, we demonstrate that high-density arrays of nanostructures of a ferroelectric polymer can be easily fabricated by a simple nano-embossing protocol, with integration densities larger than 33 Gbits inch(-2). The orientation of the polarization axis, about which the dipole moment rotates, is simultaneously aligned in plane over the whole patterned region. Internal structural defects are significantly eliminated in the nanostructures. The improved crystal orientation and quality enable well-defined uniform switching behaviour from cell to cell. Each nanocell shows a narrow and almost ideal square-shaped hysteresis curve, with low energy losses and a coercive field of approximately 10 MV m(-1), well below previously reported bulk values. These results pave the way to the fabrication of soft plastic memories compatible with all-organic electronics and low-power information technology.