Electron-paramagnetic resonance and infrared spectroscopic research of the structure of a southfield Polyvinylidene difluoride near the percolation threshold

In this paper, the results of scientific research work are carried out to study the role of the interfacial layer in filled polymer materials using infrared and electron-paramagnetic resonance spectroscopy using the fundamentals of solid-state physics. At the same time, the authors of this work used all the rich experience of both experimenters and theoreticians working in this field of science and their own developments regarding methods for controlling the structure of these materials. It is noteworthy that the material interpreted in this work concerns composites subjected to special technological preparation, including the stage of ultrasonic dispersion of filler particles in a binder medium.


Introduction
Various branches of modern technology would benefit from a wide range of non-traditional materials, in most cases rare, expensive, and time-consuming in terms of the nature of the technological preparation. Unconventional materials with controlled properties also include soot-filled composites, the one-sided technological preparation of which, to some extent, inhibits and limits the finding of their application in new areas.
The fruitfulness of this kind of approach to studying the properties of filled polymer materials (FPM) has shown its results. Not only were the structural features of the interphase layer (IFL) revealed in the FPM, but completely new, previously unforeseen properties of these composites were also discovered. For example, almost all soot containing composites of polyvinylidene difluoride (PVDF), despite their compositions of organic origin, exhibit strongly broadened, inhomogeneous, and very stable electronparamagnetic resonance (EPR) signals, which are comparable in width and, in some cases, even exceed similar parameters of some classical "magnets. Serious attempts have been made to explain such enhancements to magnetism. The combination of optical studies of these FPMs opens up new prospects. It gives great hopes associated with the development of recording media from such composites for optical information recording and *Corresponding author: abdusalom@inbox.ru photographic image acquisition, which excludes the stage of chemical-photographic processing with silver halide layers. Based on the strong absorption capacity of fathomfilled composites PVDF infrared (IR) rays, the author has written laboratory technological regulations for new temperature sensors from these composites, replacing semimetals metal glasses in these roles. In general, it should be noted that the use of various configurations of samples from the studied composite and polymer materials will reduce the need for precious metals, reduce the cost of working parts made on their basis for computers, optical and magnetic technology, and expand their scope.

Methods
The electron paramagnetic resonance (EPR) study was performed on a radio spectrometer of the E-4 model (Varian company). The minimum spin concentration that was available for recording by this device was the value of 10 11 spins. Improving the resolution often leads to a deterioration in the sensitivity to get a gain in resolution and vice versa. To determine the area under the absorption curves, the modulation amplitude was selected as follows: − (2 − 4) Where: H PPmax is the maximum signal width depending on the power of the microwave field.
The experimental conditions were chosen when neither the standard nor the sample was saturated, and the test and standard samples were at the same temperature. The concentration of paramagnetic centers (p. m. c.) was determined by the graphical method of integration by amplitudes and areas, and was calculated by comparison with reference samples (α, α diphenyl, β picrylhydrazyl) (DPPG) according to the following formula: Where: N a is the concentration of the standard spins (spin / g); m χ is the mass of the test substance (spin/g); S χ ,S a is the area under the curved sample and the reference, respectively The error in calculating the concentration of p.m.c. is 20-30%. The value of the line width H PP was calculated directly from the sheets calibrated by the scan scale.
Calculation of the values of the g factors manufacturers by the formula: Where: g a is the reference factor, i.e., DPPG; h χ is resonant magnetic field of the sample; H a is the resonant magnetic field of the reference. The saturation factor is determined by the formula: Where: i χ , I a is the signal intensity of the reference sample at a given power (W, mBr) MICROWAVE fields; Where: (i χ ) 0 , (I a ) 0 the intensity of the EPR signals of the reference sample at zero power.
IR spectra of film carbon black-filled samples of polyvinylidene fluoride (PVDF) were recorded on Specord 75 IR spectrometers (manufactured by the GDR) IR spectra of soot powders were recorded in a compressed KVg tablet. The experimental conditions were room temperature and the following instrument data: gain-5, gap-3, filter-10, recording time-44 minutes. UV and VD spectra were captured on SF-46.

Results and Discussion
In this paper, an attempt is made to elucidate the role of interfacial layers (IFL) in composites based on polyvinylidene fluoride (PVDF) filled with soot near the percolation threshold (v i = 0.04 <0.051 = v c ) prepared by ultrasonic dispersion using analysis of its molecular vibrations and paramagnetic characteristics.
The vibrational spectra of the composite studied in the mid-infrared (IR) range are shown in Figure 1. The spectrum shown in this figure is uninformative due to the very weak sparseness of the bands and the strong absorbability of the composite. However, the absorption bands selected for analysis characterize the structural features of the composite since they have repeatability in a series of experiments. On the whole, the IR spectrum of the composite has in mind the strong absorption capacity of these materials in a wide range of medium IR wavelengths (4000-1400 cm -1 ) and the increase in transmittance when approaching the far-infrared region.
In general, the following absorption bands of the composite may be the subject of discussion -3868; 3452; 3000; 2850; 2800; 1430; 1358; 1181; 728; 523.5 and 429.3 cm -1 . As a first approximation, it would be convenient to attribute the bands revealed for the composite in the region of 4000-400 cm -1 wavelengths to various groups (carboxyl, quinone, phenolic, lactone, etc.) that are possibly available, as was established in [1], which we used in the work of soot brand DG-100. However, a more rigorous approach to the interpretation and attribution of these bands may show something else. Thus, according to Rabek Ya. [2], bands in the range 4000-400 cm -1 can touch not only various compounds based on carbon-oxygen or oxygen-hydrogen bonds but also compounds with a sulfuroxygen bond. In our case, compounds with a sulfur-oxygen bond may occur due to incomplete volatilization of the solvent in the form of dimethyl sulfoxide (DMSO) from the composite. A detailed analysis of the comparative nature of the composite bands with vibrational spectra of compounds known from [2][3][4] based on fluorinated unsaturated (Table 1) and paraffin ( Table 2) hydrocarbons, as well as on some fluorinated benzene (Table 3), shows the more or less satisfactory result with compounds such as three fluoroethylene, methyl fluoride, meta-and para-difluorobenzene. Most of the other bands, even for these compounds, do not coincide with the bands of the composite. One of the possible reasons for this can be as follows.   If we pay attention to the transmittance of soot ( Figure 1) and composite ( Figure 2) in the range of 4000-1000 cm -1 . it is easy to notice that the composite is more strongly absorbing than soot (in our experiments. the amount of soot in the composite and KBr is almost the same) According to [3. 5], this may be due to the formation of such compounds as CH 3 OH, CF 4 , and F 2 O 2 in the composite. It is such compounds that have the ability of continuous absorption in the entire IR range. At present, we have a great tendency to give preference to the possibility of formation of F 2 О 2 bonds in the composite, and we do not exclude the location of this bond in any of the fluorobenzenes. The study of the paramagnetic characteristics of the composite will have to bring some clarity regarding the validity of the arguments advanced. A strikingly strong broadening of the EPR signal of the composite, as can be seen from Figure 3, leads its cooking process, as in [6]. The EPR line width DNRR = 7273 far exceeds the signal width of soot in air DNRR = 223. Composite in line width parameter exceeds all mentioned in table 4 and 5, nickel microwave ferrites and yttrium-aluminum ferrogranates, as well as many of those given in table 6 single-crystal garnet ferrites. The reasons for such a strong broadening in the case of this composite may be due to the presence of an indirect mechanism of interaction.  We think so because, according to the data in Table 7, the second constant is ultrafine splitting (UFS) A 0 = 277 MHz, and it is between 150-400 MHz, i.e., the value of which, according to [7], divides and sets the boundary between the contributions of the direct and indirect exchange interaction of spins.
However, at first, one should be convinced of the reality of the presence of UFR in the composite. So, according to table 8 and 9, the variability of saturation parameters from the microwave field of the narrow component and the common EPR signal are approximately similar in nature, which can be used in favor of the existence of the UFR in the composite.  The calculations made for the Zeeman energy (ΔW) and measures of the interaction energies between the electron and the nucleus (hA 0 ) (see Table 10) show that "second order" UFRs cannot occur in the composite. Therefore, the contribution to the total EPR signal of the composite only from the CF 3 radical is doubtful. It seems to us unlikely that the UFR will originate from the methyl radical CH 3 and the cation of the vacant center F 2 , since their cleavage parameters, a = 887 ers and 59 ers, respectively, do not coincide with those of the composite ( Table 7). Radicals of the SF, SH, SO types cannot contribute to the EPR signal of the composite because of their short survivability [6. 7].
Analyzes concerning the conditions of spin normalization on the s orbitals C, F, O ( Table 7) show that UFR in the composite can occur either on fluorine or oxygen, but in no case on carbon. The presence of singlet oxygen in the composite seems possible since the weak band of the composite 1400 cm -1 can come precisely from such oxygen. On the other hand, according to [6][7][8][9][10][11][12], it is singlet oxygen that can be responsible for the abnormal absorption of the super high-frequency field by the composite (see Table 11). But the presence of triplet oxygen in a composite is doubtful for two reasons: 1. In the vibrational spectra of the composite, there is no band 1580.19 cm -1 , characteristic of triplet oxygen; 2. The immutability of the intensity of the EPR signal of the composite (see table 12) from the influence of illumination by visible light. According to [7], in the event of the presence of triplet O 2 in the composite, it would be practically impossible to slow its diffusion to other radicals.  One possible compound with a triplet state responsible for the UFR in the composite is, again, three fluoroethylene. The independence of the EPR signal from the influence of light: Firstly, the fact of the fulfillment of hν / hc <D = 0.72 cm-1 (see Table 13), which would lead to the observation of only some of the lines allowed by the selection rules; Secondly, it could be interpreted in favor of this compound. However, such a strong broadening of the total EPR signal of the composite due to the indirect exchange interaction with the radical of this compound is not possible [6][7][8][9][10][11][12]. Another and more plausible, in our opinion, the compound responsible for the strong broadening of the EPR signal in the composite may be the perfluoro-p-benzo-semiquinone radical because it is singlet oxygen with an ordinary C = O bond in it that can provide the presence of a non-binding molecular orbital [13][14][15]. The localization of electrons that populate this non-binding molecular orbital once [6.13.15] due to the indirect mechanism of interaction through delocalized π-electrons can lead to super-exchange interaction and, as a result, to a strong broadening of the EPR signal of the composite, research on the unique properties of composite materials has received a lot of attention, including research on magnetic and paramagnetic properties [16][17][18][19][20].

Conclusions
Summarizing the above, it should be concluded that: 1. The IR spectrum of the composite is an overlay of signals from PVDF + DMSO + IFL + carbon black; 2. IFL in the composite can be of several types; 3. The multicomponent nature of the overall EPR signal of the composite can occur from the UFR on fluorine and oxygen with isotopes 19 and 17, respectively; 4. Among the possible reasons leading to a strongly inhomogeneous broadening of the EPR line of the composite, there may be an indirect mechanism of the exchange interaction between spins ; 5. The composite is promising as a magnetic material and a thermal receiver of infrared rays.