Abstract
Results are presented from studying the acoustic properties of polyvinylidene fluoride (PVDF) films of different thicknesses obtained using Langmuir‒Blodgett technology. The effect the number of monomolecular layers (10, 20, and 40 layers) and heat treatment have on the morphology of the resulting films is studied via atomic force microscopy. The existence of piezoelectric properties is demonstrated for films 40 layers thick. It is shown that additional polarization is not needed when obtaining Langmuir piezo-active PVDF films.
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INTRODUCTION
The problem of developing a component base for flexible electronics has become especially relevant in recent years. This is due first of all to the ongoing miniaturization of the element base, and second to the expanded functionality of electronic devices. The biomedical use of electronic devices is developing particularly rapidly, and the problems associated with creating a new generation of sensor platforms are becoming especially relevant. A promising line of development is creating and refining sensor systems based on acousto-electronic devices with deposited active layers.
In creating such systems, it is especially important to obtain new materials with the desired properties, and to develop technological approaches that can ensure their production. Creating polyvinylidene fluoride (PVDF) films is of great interest to researchers in developing flexible acousto-electronic devices [1, 2]. This material has piezoelectric properties and is now actively used to create flexible sound lines and sensor devices [3, 4].
The PVDF molecule is most commonly found in three different structural phases: α, β, and γ (Fig. 1). They differ in their spatial arrangements of the atoms of polymeric chains relative to one another. Only the β-phase has strong piezoelectric properties [5–7], since the unit cell in this state of the PVDF molecule consists of 4 monomer units has no center of symmetry. As a result, there is a redistribution of electron densities when pressure is applied to the molecule, resulting in its polarization. The β-phase is not a natural state of PVDF, since the molecule is in tension. It can therefore be obtained only by applying an external force for polarization.
One of the main tasks in creating PVDF films is thus polarizing them (i.e., changing the phase state of the PVDF molecules in a film). Polarization is normally a separate technological problem, which complicates the entire production cycle.
Polarization requires heating the film to a temperature in the area of 110–115°С that is lower than the Curie temperature for a given material, and applying an external electric field [8, 9]. Polymeric chains stretch under the effect of temperature, the molecules change their orientation under the influence of the electric field, and the piezo-active β-phase is stabilized as a result. This approach is used in creating films through vacuum or centrifugal deposition [10]. A mixture of PVDF and tetrafluoroethylene (TFE) is used as the initial material to obtain films, in which PVDF molecules are responsible for the existence of piezoelectric properties, and TFE molecules improve their stability. The disadvantages of these approaches include the existence of a relationship between the Curie temperature of a PVDF film and the ratio of weights in the PVDF/TFE mixture.
One way of simplifying the production cycle by excluding film polarization is to change the approach to creating films of PVDF molecules. For example, attempts have been made to use Langmuir‒Blodgett technology to form piezo-active PVDF films [11, 12]. The dielectric constants of such films have therefore been studied during temperature treatment near the point of the first-order phase transition [13].
The main idea of Langmuir–Blodgett technology is to create a thin ordered monomolecular surfactant film at the interface between two phases (water and air), with its subsequent transfer to a solid substrate. This approach allows us to create films of different thicknesses, depending on (a) the number of monolayers and (b) the thickness of one monolayer, which depends in turn on the geometric dimensions of the molecule [14]. In contrast to the procedures mentioned above, the main advantage of this technology for creating PVDF monolayers is that there is no need for the additional polarization of films on a solid substrate. This is explained by the polarization of PVDF molecules and their transition to the piezo-active phase occurring as the Langmuir monolayer forms on the water’s surface. When PVDF molecules hit the interface, hydrogen bonds form between the fluorine atoms of the PVDF molecule and the hydrogen ions in the aqueous sub-phase. The PVDF molecules are thus oriented relative to the water’s surface in such a way that the fluorine atoms in the molecule are aligned in pairs in one plane, thereby forming a piezo-active β‑phase [15]. Since the PVDF molecules retain their phase state when the monolayer is transferred to a solid substrate, this allows us to form PVDF films with molecules in the piezo-active β-phase. However, the need for additional polarization of PVDF films obtained using Langmuir–Blodgett technology remains an open question [16–18]. The aim of this work was therefore to compare the piezoelectric properties of PVDF films formed on the basis of Langmuir monolayers with or without additional polarization.
EXPERIMENTAL
Materials
Our initial material was PVDF granules (Sigma Aldrich; molecular weight, 1.8 × 105; number of units, 7.1 × 104). The PVDF granules were dissolved in dimethyl sulfide oxide (DMSO; reagent grade, manufactured by Merck) to form Langmuir monolayers. The weight concentration of the solution was 0.02%. Microscope slides 6 × 6 mm2 in size were used as substrates. Thin conducting coatings of indium‒tin oxide (electrodes) were deposited on one side of each substrate via high-frequency magnetron sputtering for further study of the piezoelectric properties of the resulting PVDF films.
Preparing the Films
To obtain compression isotherms, we formed monolayers and transferred them to solid substrates using a Nima KSV LB Trough 2002 unit with a bath area of 273 cm2. Monolayers were formed by depositing an aliquot of a solution of PVDF molecules in DMSO (volume, 125 μL) onto the surface of de-ionized water with a resistivity of 18 MΩ cm. When the solution hit the water’s surface, the solvent mixed with the volume of water and some of the PVDF molecules escaped under the interface. After 10 min of the adsorption of PVDF molecules at the water‒air interface, the monolayer was compressed by movable barriers. The rate of compression remained constant during the experiment at 10 mm/min. As a result, the rate of area loss was 14 cm2/min. The PVDF monolayer was transferred from the surface of the water to that of the substrate with the deposited electrode using Scheffer’s horizontal lift. The transfer was done when the surface pressure reached 5 mN/m. Films were thus obtained with 10, 20, and 40 PVDF monolayers. To form a piezo-active phase, the films were annealed for 60 min inside a SNOL 58/350 electric furnace at a constant temperature of 115°C [10, 13].
Morphology and Piezoelectric Properties of the Films
The morphological properties of the surfaces of the obtained films were studied via atomic force microscopy on an NT-MDT Ntegra unit in the semi-contact mode.
The piezoelectric effect in the samples was determined using an electromechanical transducer brought into contact with the surface of a given film. Longitudinal acoustic waves were excited by an electric pulse that propagated along a test sample. With the piezoelectric properties in the film, it acted as an electromechanical transducer with a resonant frequency that differed from the frequency of the exciting pulse. The characteristics of the signal generated in the test film were then displayed on the screen of an oscilloscope.
RESULTS AND DISCUSSION
Figure 2 shows the dependence of the change in surface pressure π [mN/m] on an area per molecule A [Å2], obtained during the formation of a monolayer of PVDF molecules. This dependence is referred to as the compression isotherm plot. To analyze the phase transitions of a monolayer, it is convenient to use the dependence of the change in the modulus of compression or the compressibility of a monolayer on the area per molecule. The compressibility and modulus of compression of a monolayer are then calculated according to the formula
where δ is the monolayer’s compressibility, k is the modulus of compression, dA is the change in area per molecule, and dπ is the change in surface pressure. Since the rate of the rise in surface pressure changes along with the phase state of a monolayer, using the dependences of changes in compressibility or the modulus of compression is the simplest and most productive way of determining phase transitions in a monolayer. Compression isotherms were analyzed according to [19–21].
Analysis of the plot of the compression isotherm of the PVDF monolayer shows that the monolayer had three phase states: gas, liquid, and condensed. The gas phase is observed at surface pressures of no more than 1 mN/m. The interaction between molecules in it is negligible, so the surface pressure changes weakly and the modulus of compression is minimal. The start of the rise in the compression modulus at a surface pressure of 1 mN/m indicates a transition from the gas to the liquid phase of the monolayer. In the liquid phase, the distance between PVDF molecules shrinks and is comparable to their size. Intermolecular interaction of the PVDF molecules raises the surface pressure, and the molecules are oriented parallel to the water‒air interface. The maximum pressure reached in the liquid phase is 10 mN/m. Upon further compression, the monolayer transitions to the condensed phase, for which the surface pressure is greater than 10 mN/m. For monolayers of typical surfactants, the monolayer’s transition to the condensed phase is accompanied by the layering of carboxyl groups of neighboring molecules. The film on the water’s surface retains its monolayer state, since the polar parts of the surfactant molecules remain under the water–air interface. Unlike typical surfactant molecules, PVDF molecules do not have polar parts, so when a monolayer transitions to the condensed phase, there is a trend toward the formation of a multilayer structure. The piezo-active β–phase stopped forming due to this violation of the monolayer structure [13], so the monolayer of PVDF molecules was transferred to the solid substrate at a surface pressure of 5 mN/m, which corresponds to the liquid phase of the monolayer.
Typical AFM images obtained for films containing 10, 20, and 40 PVDF layers transferred onto silicon substrates are shown in Fig. 3.
A large number of molecular aggregates are noticeable in the images obtained for PVDF films not subjected to heat treatment, and the film is inhomogeneous. An increase in the number of monolayers making up the film raises both the average thickness of the coating and the average roughness of the film, due possibly to water molecules trapped by PVDF molecules during their transfer. The amount of captured water therefore grows along with the number of layers in the film. This explanation is confirmed by considering the AFM images of films subjected to annealing, in which the morphology of the films is more uniform and the number of aggregates has fallen considerably. As a result, the roughness of the films and their average thickness were reduced. On the other hand, the reduced roughness and increased uniformity of the films’ morphology could be due to the reordering of polymeric PVDF molecules during the annealing and polarization of the films. This is confirmed by the difference between the average thicknesses of the treated and untreated films diminishing as the number of layers grows. This difference would grow if there were trapped water molecules, since their number would grow along with the number of layers in the film. Its drop, however, suggests that the reason for these changes in morphology is the reordering of the units of PVDF polymeric molecules. The effect the reordering of PVDF molecules has during temperature treatment of the films is indicated by the difference between the average roughness before and after treating the films changing along with the number of layers. At the same time, the AFM images show a drop in the number of aggregates. This could indicate their fusion and thus an increase in uniformity of the film. The numerical values of roughness and average thickness of the films are given in Table 1.
In studying the piezo-active properties of the obtained films, we found that only PVDF films containing 40 monolayers contain a piezoelectric effect whether or not they are subjected to heat treatment. The piezo-constant of the films was 0.31 × 10−12 C/N, which is much lower than the piezo-constant of bulk PVDF films formed by traditional means (20‒40) × 10−12 C/N [22, 23]. This difference could be due to defects in the films. This could also explain there being no piezoelectric effect in samples containing 10 and 20 monolayers, regardless of their temperature treatment. Let us consider a film containing 10 monolayers. For an annealed sample, the film’s thickness is comparable to its roughness. For a sample not subjected to temperature treatment, its roughness is half the film’s thickness. A similar picture is observed for a film containing 20 monolayers. The thickness of the film upon an increase in the number of its constituent monolayers in this case grew negligibly (by ~3 nm). This could also indicate a great many defects associated with the capture of water molecules during film transfer, which explains the large drop in the film’s thickness upon its annealing (by 11 nm) and its high roughness.
CONCLUSIONS
Thin films containing 10, 20, and 40 monolayers were created on the basis of Langmuir monolayers of polyvinylidene fluoride (PVDF). The effect heat treatment had on the morphology and piezoelectric properties of the obtained films was studied. It was found that samples containing 40 PVDF monolayers had piezo-active properties. The absence of ferroelectric properties in the samples containing 10 and 20 PVDF monolayers was due to defects in the resulting films. However, it should be noted that using Langmuir‒Blodgett technology to create piezo-active PVDF films greatly simplifies the procedure for preparing them. Our experiments also showed that additional polarization is not needed when obtaining Langmuir piezo-active PVDF films. To obtain Langmuir PVDF films with higher piezo-constants, we must make a number of adjustments in the technological process of film formation. Adjusting the time allotted for drying the film before reapplying the next monolayer reduces the amount of water captured during the transfer process. A likely reason for the high degree of inhomogeneity of untreated samples is dust in the environment, which is deposited on the film when drying the samples. Finally, a more hydrophobic material with less developed morphology should be used as substrates. This would raise the coefficient of transferring a monolayer to the substrate while reducing its defects caused by the film rupturing against the acicular structure of glass with a deposited ITO layer.
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Funding
The work was partially funded in the framework of the State task of the Kotelnikov Institute of Radio Engineering and Electronics, Russian Academy of Sciences, and supported by the Russian Foundation for Basic Research (project nos. 20-37-70021 and 20-57-18012).
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Translated by S. Rostovtseva
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Gorbachev, I.A., Smirnov, A.V., Shamsutdinova, E.S. et al. Studying the Structural and Piezoelectric Properties of PVDF Films Obtained Using Langmuir‒Blodgett Technology. Bull. Russ. Acad. Sci. Phys. 85, 603–607 (2021). https://doi.org/10.3103/S1062873821060101
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DOI: https://doi.org/10.3103/S1062873821060101