Copolymer Films Of Vinylidene Fluoride Research Articles

The Domain and Structural Characteristics of Ferroelectric Copolymers Based on Vinylidene Fluoride Copolymer with Tetrafluoroethylene Composition (94/6).

This paper presents data on the macroscopic polarization of copolymer films of vinylidene fluoride with tetrafluoroethylene obtained with a modified apparatus assembled according to the Sawyer-Tower Circuit. The kinetics of the polarization process were analyzed taking into consideration the contributions of both bound and quasi-free (impurity) charges. It was shown that an "abnormal" decrease in conductivity was observed in fields near the coercive fields. This could be associated with the appearance of deep traps of the impurity charge carriers formed by the polar planes of β-phase crystals. The conductivity data obtained from the charge and current responses differed. It was concluded that chain segments contributing to polarization with sufficiently low fields were present in the amorphous phase. A comparison showed that the average size of β-phase crystals (crystals of X-ray diffraction reflection width) was almost one order of magnitude lower than the domain size obtained using piezoresponse force microscopy (PFM). The analysis of the fast-stage dielectric response before and after polarization indicated that as the external polarizing field increased in the ferroelectric polymer chains, conformational transitions occurred according to the T3GT3G- → (-TT-)n и TGTG → (-TT-)n types. This was accompanied by an increase in the effective dipole moment in the amorphous phase chains. The analysis of the IR spectroscopy data obtained in transmission and ATR modes revealed a difference in the conformational states of the chains in the core and surface parts of the film.

Thin films of vinylidene fluoride copolymer with tetrafluoroethylene: dynamics of ferroelectric polarization switching

The vinylidene fluoride copolymer with tetrafluoroethylene P(VDF-TFE) is a typical ferroelectric polymer that has not been sufficiently studied so far in comparison with pure polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene. The purpose of this work was to reveal the features of forming ferroelectric polarization and its switching in thin films of P(VDF-TFE) under various conditions, such as the level of the applied electric field and the duration of its action within 8 orders of magnitude from 10 μs to 100 s. The sequence of voltage pulses applied during poling and polarization reversal allowed to reveal the characteristics of the polarization switching dynamics. To increase the resolution of the measurements, two complementary methods of recording electrical induction were used. The features of polarization and space charge relaxation over time have been studied by thermally stimulated depolarization method. A new phenomenon of gradual separation of the relaxation processes associated with polarization and space charge was observed. Both components were accompanied by trapped charges that compensated the depolarizing field. Several practical recommendations have been formulated regarding the desirable values of poling parameters and additional processing to increase the ferroelectric polarization stability.

Influence of Dye Molecules on the Polarization of Ferroelectric Vinylidene Fluoride Copolymer

Electrical, electromechanical, and structural–optical properties of films of vinylidene fluoride copolymer with tetrafluoroethylene TFE, doped with Rhodamine 6G dye, have been investigated. It is found that the conductivity increases in doped films; at the same time, it “anomalously” decreases with an increase in the field. The hysteresis loop of local piezoelectric response has an asymmetric shape, which is related to the difference in the local field when its polarity changes. According to the data of absorption and luminescence spectra, the dye (at chosen concentrations) exists in form of, at least, monomers and dimers. It is shown by IR spectroscopy that hydrogen bonds can be formed between a dye molecule and vinylidene fluoride units located in amorphous regions. A model is formulated, which provides a correlation between the electrical and structural properties of the doped films.

Self-polarization in ultrathin Langmuir–Blodgett polymer films

Ultrathin copolymer films of vinylidene fluoride and trifluoroethylene were deposited on Al-coated polyimide substrates, by the Langmuir–Blodgett method. A top Al electrode was evaporated onto the polymer film, to form an Al/polymer/Al structured infrared detector. The pyroelectric voltage response of the detector under various polarizing processes was characterized. The detector with only one transferred polymer layer exhibited a preferential polarization direction. This was considered to result from the self-polarization of the ultrathin polymer film. It was due to the preferred alignment of the dipoles on the Al substrates. This process can be applied for designing stable fast-response infrared detectors.

A Sharp Peak of the Differential Conductivity of P(VDF-TrFe) Films Near the Coercive Field

Polyvinylidene fluoride (PVDF) films and copolymer films of vinylidene fluoride and trifluoroethylene P(VDF-TrFE) have great potential applications in uncooled infrared detection due to their good pyroelectricities and ferroelectricities. It is important to investigate the mechanism of their electrical transport. In our present work, we found the differential conductivities of P(VDF-TrFE) films exhibit a sharp peak near the coercive field. And it is proposed that the peak is caused by the delocalization of polarons during the polarization inversion.

Haloform adsorption on crystalline copolymer films of vinylidene fluoride with trifluoroethylene

Reversible bromoform adsorption on crystalline polyvinylidene fluoride with 30% of trifluoroethylene, P(VDF–TrFE 70:30) was examined by photoemission and inverse photoemission spectroscopies. The adsorption of bromoform on this polymer surface is associative and reversible. Molecular bromoform adsorption appears to be an activated process at 120 K with enhanced adsorption following the initial adsorption of bromoform. Strong intermolecular interactions are also implicated in the presence of a weak shake off or screened photoemission final state, whose intensity scales with the unscreened photoemission final state.

Evidence for an Influence of Local Dipole Excitations in Thermal Desorption

Ferroelectric crystalline copolymer films of vinylidene fluoride with trifluoroethylene (70%:30%) strongly interact with the dipoles of adsorbed and absorbed water molecules. This interaction can be probed with laser-assisted thermal desorption techniques. The UV light enhancement of water desorption is strongly light polarization dependent. The electronic structure of the ferroelectric copolymer films of vinylidene fluoride with trifluoroethylene films is locally altered with incident UV radiation suggesting metastable excited states that may involve dipole reorientation.

Ferroelectric nanomesa formation from polymer Langmuir–Blodgett films

We report the fabrication and characterization of nanoscale ferroelectric structures consisting of disk-shaped nanomesas averaging 8.7±0.4nm in height and 95±22nm in diameter, and nanowells 9.8±3.3nm in depth and 128±37nm in diameter, formed from Langmuir–Blodgett films of vinylidene fluoride copolymers after annealing in the paraelectric phase. The nanomesas retain the ferroelectric properties of the bulk material and so may be suitable for use in high-density nonvolatile random-access memories, acoustic transducer arrays, or infrared imaging arrays. The nanomesa and nanowell patterns may provide useful templates for nanoscale molding or contact-printing.

Water absorption and dielectric changes in crystalline poly(vinylidene fluoride-trifluoroethylene) copolymer films

Crystalline Langmuir–Blodgett copolymer films of vinylidene fluoride with trifluoroethylene (70%:30% and 80%:20%) absorb water. Water absorption is accompanied by film swelling, as indicated by an increase in lattice spacing, sometimes by as much as 5%. This water absorption, between 0 and 40 °C, is a result of intercalation or occupation of interstitial sites between the layers of the film, not just water molecules filling voids and defect sites alone. An increase in the film capacitance is observed, although the polymer chains retain all trans configuration of the ferroelectric phase.

Nanoscale polarization manipulation and conductance switching in ultrathin films of a ferroelectric copolymer

We report the direct observation of induced molecular reorientation on a ferroelectric copolymer with a scanning tunneling microscope (STM). Ultrathin copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%) revealed a quasihexagonal close-packing structure with long-range polymer chain ordering. By flipping the polarity of the STM tip bias voltage, a reversal of local polarization was observed through an apparent lattice shift and was accompanied by an asymmetric “diode-like” character in tunneling current I(V). These results clearly demonstrated conductance switching behavior on nanoscale with local polarization reversal.

Changes in electron–phonon coupling across a bulk phase transition in copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%)

We present evidence for a change in electron–phonon coupling across a bulk phase transition. Below the lattice stiffening transition at around 160K, there is a change in the diffracted peak width observed by neutron and X-ray scattering techniques. Also, the electronic band structure of the copolymer is shifting in binding energy below 160K, decreasing the density of states near the Fermi level. The value of the effective Debye temperature above the transition temperature is approximately 50K, while below 160K, the value of the Debye parameter is 245K. We postulate that the coupling between electrons and phonons results in a static distortion of the lattice (below 160K), and this distortion “melts” above 160K.

Phase transition in the surface structure in copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%)

Surface structures and a surface structure phase transition are identified that are distinct from the known bulk ferroelectric-paraelectric phase transition of crystalline copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%). The temperature-dependent changes in the surface structure are accompanied by the physical rotation of the polar group ${(\mathrm{C}\mathrm{H}}_{2}{\ensuremath{-}\mathrm{C}\mathrm{F}}_{2}).$ These changes in the surface structure are compared to the bulk phase transition. We show that the bulk structural transition, while distinct from the surface, is qualitatively similar in both thick and thin Langmuir-Blodgett-grown films.

Two-dimensional ferroelectrics

The minimum size for a ferroelectric crystal has not been demonstrated experimentally because of difficulties in making thin films or small particles of sufficient quality. Langmuir-Blodgett deposition of crystalline films of vinylidene fluoride copolymers has produced high quality ferroelectric films as thin as 1 nm. These films permit the ultimate investigation of the finite-size effects on the atomic thickness scale and also reveal the fundamental two-dimensional character of ferroelectricity in these materials. They exhibit the main properties of ferroelectricity: a first-order ferroelectric-paraelectric phase transition; polarization hysteresis (switching); a jump in spontaneous polarization at the phase transition temperature; thermal hysteresis in the polarization; increase of the transition temperature:with applied field; double hysteresis above the phase transition temperature: and the existence of the ferroelectric critical point. The films also exhibit a new surface-layer phase transition.

Lattice-Stiffening Transition in Copolymer Films of Vinylidene Fluoride (70%) with Trifluoroethylene (30%)

Lattice-Stiffening Transition in Copolymer Films of Vinylidene Fluoride (70%) with Trifluoroethylene (30%)

Investigation of Surface Potential of Ferroelectric Organic Molecules by Scanning Probe Microscopy

The surface potential of ferroelectric copolymer films of vinylidene fluoride and trifluoroethylene was investigated by scanning probe microscopy. The results reveal that the as-deposited copolymer films on a graphite substrate show a negative surface potential. The surface potential is ascribed to the substrate-induced effect, which causes frozen dipoles in the copolymer films. We made locally poled areas in the films using a conductive cantilever tip and measured the piezoelectric response of the films with a modified scanning force microscope. The observed asymmetric behavior of the piezoelectric response of the thin films can be consistently explained by the frozen ferroelectric layer in the films. Both surface potential and piezoresponse images of locally polarized areas were also obtained. The results indicate that the charge transferred from the tip overcompensated for the oriented dipoles and that the surface potential of the poled regions was attributed to the excess charge.

Evidence of dynamic Jahn-Teller distortions in two-dimensional crystalline molecular films

The surface electronic structure in crystalline copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%) has been studied by photoemission and inverse photoemission as a function of alkali metal (sodium) doping. Sodium doping introduces at least two new states into the band (HOMO-LUMO) gap well away from the Fermi level. While the sodium-doped copolymer is observed to resemble an n-type semiconductor, the change in electronic structure with temperature suggests that dynamic distortions lead to a photoemission initial state splitting of the lower Hubbard-like bands. There is a decrease in the effective Debye temperature with sodium doping which may, in turn, enhance the dynamic Jahn-Teller distortion.

Lattice and electronic band structure changes across the surface ferroelectric transition

The origin of the surface ferroelectric phase transition in crystalline copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%) is explored. We report a uniaxial doubling of the surface Brillouin zone in the conduction band dispersion across the surface ferroelectric phase transition. The temperature dependent changes in the electronic structure occur primarily in the conduction band and are accompanied by a surface dipole reorientation.

Changes in Metallicity and Electronic Structure Across the Surface Ferroelectric Transition of Ultrathin Crystalline Poly(vinylidene Fluoride-Trifluoroethylene) Copolymers

The surface ferroelectric-paraelectric phase transition in crystalline copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%), at about 20 \ifmmode^\circ\else\textdegree\fi{}C, is accompanied by a dramatic change in both the electronic structure and metallicity. The temperature-dependent changes in the electronic structure occur primarily in the conduction band. This P(VDF-TrFE 70:30) material is observed to resemble an $n$-type semiconductor, and the change in electronic structure at the surface ferroelectric transition resembles a semiconductor-to-semimetal transition.